Approach to Hypoglycaemia and Canine Insulinomas Glycaemic Control Glucose is obtained from a variety of sources including digestion and absorption of carbohydrates; breakdown of glycogen (liver and muscles); and synthesis from lactate, pyruvate, amino acids, glycerol via gluconeogenesis. Several hormones are responsible for maintaining circulating blood glucose concentrations within a narrow range (Table 1). Hormones involved in Regulation of Glucose Hormone
Released From:
Action
Effect
Insulin
Pancreatic islets
↓
Glucagon
Pancreatic islets
Adrenaline
Adrenal medulla
Thyroxine
Thyroid
Cortisol
Adrenal Cortex
Growth Hormone
Anterior Pituitary
Increases glucose entry to cells Stimulates glycogen production from glucose Enhances fatty acid synthesis from glucose Suppresses lipolysis Increases protein synthesis Suppresses proteolysis and gluconeogenesis Stimulates release of glucose from glycogen (glycogenolysis) Increases glucose synthesis from fatty acids and amino acids (gluconeogenesis) Increases lipolysis Stimulates growth hormone release Rapidly increases glucose release from glycogen Stimulates glucagon release Stimulates release of fatty acids from adipose tissue Increases glycogenolysis Enhances absorption of glucose and other simple carbohydrates from intestines Increase gluconeogenesis Chronically antagonises all aspects of insulin activity Antagonises insulin (chronically) and reduces peripheral glucose utilization Stimulates lipolysis Decreases glucagon, growth hormone and insulin secretion Decreases secretion and absorption in the gastrointestinal tract
Somatostatin Pancreatic islets
↑
↑
↑
↑ ↑
↑
The primary ‘fuel sensor’ of the body is the pancreatic β-cells, and these respond to changes in plasma levels of various energy substrates and hormones. Signals from energy substrates such as glucose, amino acids, various hormones (insulin, glucagon-like peptide, somatostatin, and adrenalin), and neurotransmitters (nor-adrenaline, acetylcholine) result in glycaemic control. In this scenario glucose is the principle stimulus for insulin release. Very simply stated, glucose metabolism within βcells results in generation of metabolic intermediates, increased cytosolic ATP/ADP and an increase in intracellular calcium ions. Ultimately this combination of events triggers exocytosis of stored insulin into the circulation. The release of insulin in response to elevated blood glucose tends to be biphasic, with an initial rapid ‘first-phase’ release of pre-formed insulin, resulting in a quick peak over several minutes, with a subsequent decline to low level secretion. This is followed by a gradual
increase in secretion rate to a plateau in a ‘second-phase’ of secretion over several hours. Longer term regulation in glycaemic control, and of insulin concentration in particular, results in up- or down-regulation of hormonal production via changes in mRNA transcription rates. The effects of insulin are varied (Table 2), and insulin release results in: -
Immediate effects (within seconds) - modulation of ion (K+) and glucose transport into cells; Early effects (within minutes) - regulation of metabolic enzyme activity; Moderate effects (minutes to hours) – regulation of enzyme synthesis; Delayed effects (hours to days) – changes in growth and cellular differentiation.
Metabolic Effects of Insulin Stimulates
-
Inhibits
Glucose uptake in muscle and adipose
-
Glucogenesis
tissue
-
Glycogenolysis
-
Glycolysis
-
Lipolysis
-
Glycogen synthesis
-
Ketogenesis
-
Protein synthesis
-
Proteolysis
-
Uptake of ions – especially K+ and PO43-
The counter-regulatory to insulin is aimed at preventing development of hypoglycaemia. As glucose concentrations fall in response to insulin, its secretion is suppressed and the mobilization of energy from stores (glycogenolysis, lipolysis, gluconeogenesis, ketogenesis) and reduction in glucose utilization are promoted. Counter-regulatory hormones including glucagon, adrenaline, growthhormone and cortisol are increased. Glucagon is a key hormone in recovery from acute hypoglycaemia and is secreted by the alpha-cells of the pancreatic islets to act on the liver in order to activate glycogenolysis and gluconeogenesis. Adrenaline also plays a major role in stimulating hepatic glycogenolysis and hepatic and renal gluconeogenesis; by providing alternative sources of fuel via muscle glycogen and stimulation of lipolysis; by mobilizing lactate, alanine and glycerol as gluconeogenic precursors; and by inhibiting glucose utilization. Cortisol and growth-hormone play a more delayed role and are important in the defence of prolonged hypoglycaemia. This is achieved by facilitation of lipolysis, promotion of protein catabolism and the conversion of amino acids to glucose and via antagonisation of the effects of insulin. Dysregulation of the balance between hormonal controls of glucose can result in hyper- or hypoglycaemia. In particular in veterinary medicine commonly recognised conditions include insulin deficiency resulting in hyperglycaemia (Type 1 or Type 2 diabetes mellitus). Antagonism of insulin via excessive secretion of growth hormone (acromegaly), cortisol (hyperadrenocorticism), or less commonly glucagon (glucagonoma) can also contribute to the development of hyperglycaemia and a diabetic state. Excessive insulin production and dysregulated release resulting in hypoglycaemia
(insulinoma, rarely nesidioblastosis) are also reported within the veterinary literature; and will be the focus of this lecture. Insulinomas Insulinomas are relatively uncommon tumours of the pancreatic β-cells in dogs, and are rare tumours in cats. These neoplasms tend to be functional, but with insulin release that is nonresponsive to the normal regulatory feedback mechanisms mentioned above when blood glucose concentrations are low. Thus the hallmark feature of an insulinoma is a finding of hypoglycaemia, which may be most apparent in the fasted state, together with an elevated, or an inappropriately normal, blood insulin concentration. In comparison to people, where insulinomas are often solitary and benign, the disease in dogs tends to represent malignant neoplastic transformation and metastatic lesions are detected at diagnosis in 50% of cases reported in the literature. Most commonly regional lymph nodes and hepatic metastases are seen, with pulmonary metastasis seen less often. Other causes of paraneoplastic hypoglycaemia may occur with other neoplastic disease. Hepatic tumours (hepatoma, hepatocellular carcinoma) or leiomyosarcomas are reported to cause hypoglycaemia, either resulting from large-scale glucose utilization, or the secretion of insulin-like hormone(s). Rare conditions such as nesidioblastosis have also been sporadically reported in the veterinary literature and result from excessive insulin secretion as a result of benign β-cell hyperplasia. Clinical Presentation Clinical signs associated with insulinomas may be present for days to months, and can be intermittent or episodic as a reflection of prolonged periods of fasting or more intensive exercise. Clinical signs are related to the effect of hypoglycaemia on the nervous system, termed neuroglycopenia. Insulin itself does not influence the uptake or use of glucose within the central or peripheral nervous system, however where hyperinsulinaemia results in hypoglycaemia there is inadequate intracellular glucose supply for cellular processes. Weakness, ataxia, collapse, behavioural change and disorientation, and even seizures are seen. However clinical effects may also relate to catecholamine release and result in muscular tremors, shaking, anxiety and hunger. Occasionally hypoglycaemia may be detected as an incidental finding on screening blood tests. In this situation artefactual change should be always be considered, and confirmation of hypoglycaemia sought ideally via repeated bed-side testing. Signalment Dogs tend to be medium and large breed dogs (Labrador or Golden retrievers, German shepherds, German pointers, Irish setters, Boxers and mixed-breed dogs) although small breeds (West Highland white terriers) are also reported. Dogs are middle-aged to older with a median age of 9-10 years; and a range of 3 – 15 years. No sex predilection is reported. Diagnosis The identification of hypoglycaemia should ideally always be confirmed, and artefactual hypoglycaemia resulting from delayed process of biochemistry results excluded. Bed-side evaluation of blood glucose, using a validated veterinary glucometer, is ideal. Occasionally detection of
hypoglycaemia requires prolonged periods of fasting (24 hour fasting). Use of a continuous glucose monitoring system may allow detection of transient hypoglycaemic episodes for some difficult to diagnose cases. The presence of hypoglycaemia paired with the finding of a concurrently elevated plasma concentration of insulin confirms a diagnosis of an insulinoma, as well as the exclusion of other potential causes of hypoglycaemia (Table 3). Occasionally insulin concentrations within the reference range are obtained, but should also be considered abnormal in the face of hypoglycaemia, where concentration should be low to non-detectable. The use of insulin:glucose ratios do not appear to improve diagnostic accuracy in veterinary patients. Provocative testing is generally not recommended due to risk and expense, as well as concerns with interpretation. Low fructosamine or glycosylated haemoglobin concentrations can be helpful in supporting a suspicion of insulinoma where chronic hypoglycaemia has persisted for sufficient time-length. Diagnostic Imaging A number of diagnostic imaging modalities can be used in evaluating for insulinomas. Radiographic studies are generally unremarkable, especially as lesions may be small and pulmonary metastatic disease is rare. These can still be useful considerations to exclude other causes of hypoglycaemia however. Ultrasound is more commonly utilized, but is dependent upon user skill and may only identify a focal pancreatic lesion in <50% of cases. Sensitivity and specificity for detection of metastatic disease within local lymph nodes and the hepatic parenchyma is also low. Intra-operative or endoscopic ultrasonography is considered more sensitive in people for the detection of focal pancreatic lesions, but has not been reported in dogs. Computed tomography (CT) may be more useful, and detected approximately 70% of primary insulinomas compared to ultrasound and single-photon emission CT (SPECT) in one canine study. A study of 19 dogs using 111In-DTPA-D-Phe1-octreotide demonstrated an overall sensitivity of 50% for correct detection and localization of the primary tumour. Dynamic or dual-phase CT angiography may prove even more sensitive, but larger scale studies havenâ&#x20AC;&#x2122;t yet been performed. Both abdominal ultrasound and CT remain useful to exclude the possibility of other causes of hypoglycaemia, including other conditions (hepatoma, leiomyosarcoma) that may cause paraneoplastic hypoglycaemia. Scintigraphic techniques against somatostatin receptors (SRS) are used in people for detection of a variety of primary pancreatic tumours. An indium (In)111-pentetreotide SRS technique has been reported in a small number of dogs, and allowed detection of an insulinoma in 5/6, with accurate localization in 1/6. Exploratory laparotomy and careful interrogation of the pancreas for focal lesions can sometimes be required where there is a high index of suspicion for an insulinoma, yet detection of a focal pancreatic lesion has not been achieved with the above techniques. This might particularly be considered following appropriate exclusion of other possible causes of hypoglycaemia, in the face of otherwise compatible clinicopathologic findings. Treatment The primary goals of treatment for insulinomas are -
Acute medical management of symptomatic hypoglycaemia;
-
-
Tumour management: o Curative intent tumour resection, where possible, versus de-bulking procedures to reduce insulin secretion; o +/- adjunctive chemotherapy . Chronic medical management of hypoglycaemia in the face of non-resectable disease or disease recurrence.
Management of acute presentations involves correction of symptomatic hypoglycaemia. The administration of dextrose / glucose should be cautious and aimed at correcting clinical signs of hypoglycaemia whilst avoiding over-correction and potential stimulation of further unregulated insulin secretion from tumour cells; which may in turn worsen hypoglycaemia. Glucagon as a CRI has been proposed as an alternative therapy and theoretically may provide a slower correction of hypoglycaemia and be less likely to provoke unregulated insulin secretion from tumour cells. Stabilisation and close monitoring and management of blood glucose should be performed perioperatively. Consideration should also be given to performing pre-operative CT-angiography for the detection of metastatic disease. Otherwise surgical intervention generally aids confirmation and localisation of focal pancreatic lesions for possible resection; as well as some assessment of regional metastatic disease. Insulinomas are usually easily identifiable at surgery and may be present with equal frequency in either limb of the pancreas. Regional lymph nodes and the liver should be visually inspected and/or biopsy performed, especially where abnormalities are detected. Partial pancreatectomy can be considered for focal lesions, but care should be taking in handling the pancreatic tissue in order to minimise peri-operative pancreatitis. Other post-operative complications include persistence of hypoglycaemia (likely due to other the presence of contributory sources of insulin from metastatic disease), and development of transient hyperglycaemia of diabetes mellitus (due to down regulation of remaining β-cells, or absolute deficiency). In the face of persistent hypoglycaemia post-operatively, or where surgery is not undertaken at all, chronic medical management of hypoglycaemia can involves strategies aimed at frequent feeding of complex carbohydrates, insulin antagonism and/or chemotherapeutic therapies. Adjunctive chemotherapy using streptozotocin as a chemotherapeutic option has been described in dogs. This agent aims to destroy pancreatic beta-cells. Early reports suggested nephrotoxicity could limit use, but saline diuresis together with administration appears to minimise this risk. Side effects including transient vomiting during administration, and the development of diabetes mellitus, hypoglycaemia and mild haematologic change have been reported. There may be no significant increase in the duration of euglycaemia maintained in dogs with insulinoma compared with those treated surgically or with alternative medical therapy. Thus, although it may provide some clinical benefits by reducing tumour size, and resolving paraneoplastic neuropathy, the risks of therapy may minimise the benefits. Diazoxide, a nondiuretic benzothiadiazine that suppresses insulin release from β-cells, may also be considered and has been described in canine insulinoma patients. Stimulation of hepatic gluconeogenesis, glycogenolysis and inhibition of cellular glucose uptake are also achieved with this non-cytotoxic therapy. There is no effect on insulin synthesis, or on tumour size reduction. Approximately 70% of canine insulinoma cases appear to respond in available studies and side
effects appear uncommon (ptyalism, vomiting, anorexia and diarrhoea). Cost and availability limit its regular use. Octreotide, may also help to achieve euglycaemia by binding somatostatin receptors and providing long-acting effects in inhibiting the synthesis and secretion of insulin, although results of available studies are conflicting. Early studies found no effect in managing hypoglycaemia in three dogs with insulinoma vs placebo alone, with no change in circulating insulin concentrations despite detectable ocretotide levels (Simpson, 1995). In another more recent study (Robben et al, 2006), suppression of plasma insulin concentrations were achievable in healthy dogs, and dogs with insulinoma. Furthermore, in dogs with insulinoma plasma glucose concentrations increased and there was no suppressive effect on counter-regulatory hormones (GH, ACTH, glucagon). Variation in somatostatin receptor expression within neoplastic tissue may account for some cases being refractory to this treatment. Further consideration and investigation of this therapy is required, although cost can be prohibitive. Medical therapy is otherwise aimed at dietary modification towards those diets that provide high fat, protein and complex carbohydrate content. Meals should be small and frequent and simple sugar should be avoided in order to avoid post-prandial ‘spikes’ in blood glucose that risk stimulating insulin release. Prednisolone can simultaneously be used to antagonise insulin effects, with dosing titrated to control symptomatic hypoglycaemia, whilst attempting to avoid typical steroidal side effects. In addition intensive exercise and excitement are best avoided. Prognosis Short-term prognosis is often quite good in managing symptomatic hypoglycaemia. Long-term prognosis is however much more guarded as malignant neoplastic transformation is more common than benign disease in dogs. Dogs that undergo surgical resection and medical management are more likely to maintain euglycaemia for longer periods of time and have longer survival times compared with those receiving medical therapy alone. Median survival times with partial pancreatectomy of 12-14 months are reported within several studies. This prognosis appears to relate to clinical stage (WHO based tumour/node/metastasis – TNM – scheme). Prognostic indicators in one recent study demonstrated an association between tumour size, and Ki67 index (a marker of proliferation) with disease free interval and survival time. In an earlier study, dogs with Stage I disease (T1N0M0) have favourable disease free intervals with 50% of dogs remaining free of apparent hypoglycaemia 14 months after surgery. Comparatively, of dogs with Stage II (T1N1M0) or Stage III (T1N1M1) disease, less than 20% were hypoglycaemia free at the same interval. For dogs with stage III disease, 50% survived till six months post diagnosis in this same study. A more recent study demonstrated an MST of 785 days for 19 dogs undergoing surgical resection, with a median disease free interval (DFI) of 496 days. For 9/19 dogs receiving surgery and medical therapy with prednisolone, the MST was 1316 days compared with an MST of 196 for dogs receiving medical therapy alone. It is important to note that those eight dogs managed medically either had evidence of metastatic disease present at initial imaging evaluation, or were considered to have un-resectable disease at the time of surgery; and thus more advanced disease was perhaps present. Still there is adequate evidence to support the adjunctive use of medical management in addition to surgical intervention, or where clinical signs recur following surgery.
References Buishand FO, Kirpensteijn J, Jaarsma AA, Speel E-JM, Kik M, Mol JA, (2013), Gene expression profiling of primary canine insulinomas and their metastases. The Veterinary Journal, 197 (2): 192-197 Polton, G. A., White, R. N., Brearley, M. J. and Eastwood, J. M. (2007), Improved survival in a retrospective cohort of 28 dogs with insulinoma. Journal of Small Animal Practice, 48: 151–156. doi:10.1111/j.1748-5827.2006.00187.x Robben JH, van den Brom WE, Mol JA, van Haeften TW, Rijnberk A, (2006), Effect of octreotide on plasma concentrations of glucose, insulin, glucagon, growth hormone, and cortisol in healthy dogs and dogs with insulinoma, Research in Veterinary Science, 80 (1):25-32 Robben, Joris H; Visser-Wisselaar, Heleen A; Rutteman, Gerard R; et al. The Journal of Nuclear Medicine; New York 38.7 (Jul 1997): 1036-42. Garden, O. A., Reubi, J. C., Dykes, N. L., Yeager, A. E., McDonough, S. P. and Simpson, K. W. (2005), Somatostatin Receptor Imaging In Vivo by Planar Scintigraphy Facilitates the Diagnosis of Canine Insulinomas. Journal of Veterinary Internal Medicine, 19: 168–176. doi:10.1111/j.19391676.2005.tb02678.x Caroline M. Goutal, Bonnie L. Brugmann, and Kirk A. Ryan (2012) Insulinoma in Dogs: A Review. Journal of the American Animal Hospital Association: May/June 2012, Vol. 48, No. 3, pp. 151-163 Leifer CE, Peterson ME, Matus RE, (1986), Insulin-secreting tumour: diagnosis and medical and surgical management in 55 dogs, Journal of the American Veterinary Medical Association, 188 (1):60-64 Simpson, K. W., Stepien, R. L., Elwood, C. M., Boswood, A. and Vaillant, C. R. (1995), Evaluation of the long-acting somatostatin analogue Octreotide in the management of insulinoma in three dogs. Journal of Small Animal Practice, 36: 161–165. doi:10.1111/j.1748-5827.1995.tb02870.x Fischer JR, SA Smith, and KR Harkin (2000) Glucagon constant-rate infusion: a novel strategy for the management of hyperinsulinemic-hypoglycemic crisis in the dog. Journal of the American Animal Hospital Association: January/February 2000, Vol. 36, No. 1, pp. 27-32. M.A. Trifonidou, J. Kirpensteijn & J.H. Robben (1998) A Retrospective Evaluation of 51 Dogs with Insulinoma, Veterinary Quarterly, 20:sup1, S114-S115, DOI:10.1080/01652176.1998.10807459 Dunn, J. K., Heath, M. F., Herrtage, M. E., Jackson, K. F. and Walker, M. J. (1992), Diagnosis of insulinoma in the dog: A study of 11 cases. Journal of Small Animal Practice, 33: 514–520. doi:10.1111/j.1748-5827.1992.tb01041.x Lester, N. V., Newell, S. M., Hill, R. C. and Lanz, O. I. (1999), SCINTIGRAPHIC DIAGNOSIS OF INSULINOMA IN A DOG. Veterinary Radiology & Ultrasound, 40: 174–178. doi:10.1111/j.17408261.1999.tb01905.x Iseri, T., Yamada, K., Chijiwa, K., Nishimura, R., Matsunaga, S., Fujiwara, R. And Sasaki, N. (2007), Dynamic Computed Tomography Of The Pancreas In Normal Dogs And In A Dog With Pancreatic Insulinoma. Veterinary Radiology & Ultrasound, 48: 328–331. doi:10.1111/j.1740-8261.2007.00251.x
Mai, W. And Cáceres, A. V. (2008), Dual-Phase Computed Tomographic Angiography In Three Dogs With Pancreatic Insulinoma. Veterinary Radiology & Ultrasound, 49: 141–148. doi:10.1111/j.17408261.2008.00340.x
Cause Artefactual Delayed Serum Separation Inappropriate Coagulatant Use Use of a hand-held glucometer Polycythemia/leukaemia Iatrogenic Excessive exogenous insulin administration Administration of oral hypoglycaemic drugs Overdose of mitotane / trilostane Insufficient Intestinal Absorption Inadequate dietary intake / prolonged starvation Intestinal malabsorption Under Production Hepatic failure / portosystemic shunt Adrenocortical insufficiency Toy-breed hypoglycaemia Neonatal hypoglycaemia Hypopituitarism Glycogen storage disease Over-Utilization of Glucose Pancreatic B-cell neoplasia (insulinoma) Extra-pancreatic neoplasia
Causes of Hypoglycaemia Pathophysiology Glucose is used by white / red cells for glycolysis As above Can under-read actual glucose concentration Increased use of glucose by red/white cells More common in cats than dogs Can occur due to error, or transient diabetes mellitus, or in anorexic diabetic patients Administration of glipizide, which increases insulin secretion Rare in animals Loss of cortisol-induced counter-regulatory mechanisms Unlikely to cause hypoglycaemia unless combined with systemic illness, or prolonged starvation and malnutrition Inadequate glucose absorption due to intestinal pathology – unlikely to cause hypoglycaemia alone Inadequate glycogen stores, dysfunction of gluconeogenesis and glycogenolysis Hypoglycaemia does not usually develop until 70% of hepatic function is lost Loss of cortisol-induced counter-regulatory mechanisms Inadequate glycogen stores easily depleted if food take inadequate As above, plus immature hepatic enzyme systems Loss of growth hormone induced counter-regulatory mechanisms Deficiency of enzymes involved in the conversion of hepatic glycogen to glucose (glycogenolysis) Inappropriate insulin release
Release of insulin-like peptides (IGF-1 or IGF-2) Excessive consumption of glucose by neoplastic cells Failure of hepatic gluconeogenesis of glycogenolysis Up-regulation of insulin receptors Increased insulin binding by m-proteins in myeloma Severe polycythemia Increased utilization by red or white cells Xylitol ingestion Stimulates inappropriate insulin release from B-cells Occurs in dogs, not reported in cats Exercise induced Also called ‘hunting-dog’ hypoglycaemia Increased glucose consumption by muscle with insufficient glucose production to meet demand Severe and prolonged seizuring Increased glucose consumption by muscle with insufficient glucose production to meet demand Under Production and Over-Utilization Toxaemia of pregnancy Rare Ketonaemia and ketonuria without glucosuria often present Cause unclear Large litters may cause increased glucose utilization, insufficient glucose uptake and poor energy stores Hormonal effects may contribute
Sepsis
Decreased glucose intake Reduced hepatic function Non-insulin-mediated consumption by inflammatory mediators Heatstroke Increased glucose utilization from increased adenosine triphosphate demand due to high body temperatures, seizures and increased respiratory effort May be septic Taken from: McBrearty A, Ramsey I, (2013), Hypoglycaemia. Companion Animal, 18 (3): 96
Hypoadrenocorticism â&#x20AC;&#x201C; the great pretender! Adrenal Physiology The adrenal glands produce a range of hormones within the medulla (catecholamines) and the cortex (mineralocorticoids, glucocorticoids and androgens). Within the adrenal cortex there are three histopathologic layers; the zona glomerulosa, the zona fasiculata and the zona reticularis. The mineralocorticoid, aldtosterone, is exclusively produced within the zona glomerulosa. Whilst the zona fasiculata and zona reticularis are capable of producing both androgens (sex-hormones) and glucocorticoids (cortisol). Effects of Mineralocorticoids Aldosterone secretion is regulated as part of the renin-aldosterone-angiotensin system (RAAS), with some effect also resulting from plasma sodium and ACTH concentrations. RAAS activation occurs when there are increased concentrations in potassium, or when alterations to circulating fluid volume / sodium concentration are detected (hypovolaemia, hyponatraemia). Renin release from juxtaglomerular apparatus within the glomerulus as a consequence of these detected changes, causes the conversion of angiotensinogen to angiotensin I (AT I). AT I is then converted to AT II in the lungs via angiotensin-converting-enzyme (ACE). This increase in AT II concentration causes an increase in aldosterone synthesis and release from the adrenal cortex. Increased potassium concentrations also stimulate aldosterone secretion from the adrenal directly. Consequently, aldosterone increases the absorption of sodium (and therefore water), as well as secretion of potassium within the distal nephron in the kidneys and to a lesser extent within other glandular tissue. Aldosterone therefore plays an important role in the bodyâ&#x20AC;&#x2122;s sodium and fluid balance, and in regulating circulating potassium concentrations. A deficiency in aldosterone results in depletion of both sodium and water and therefore significant hypovolaemia develops, and also causes potassium retention. Hypovolaemia, hyponatraemia and hyperkalaemia are therefore the hallmark findings of hypoadrenocorticism when mineralocorticoid deficiency is present. Effects of Glucocorticoids Glucocorticoids are synthesised and secreted from the zona fasiculata and zona reticularis within the adrenal cortex. Regulation of glucocorticoid production is via the hypothalamic-pituitary-adrenal axis (HPA-axis). For this, corticotrophin-releasing hormone (CRH) is produced within the hypothalamus and stimulates secretion of adrenocorticotropic hormone (ACTH) from the pituitary gland. At the level of the adrenal gland ACTH stimulates the synthesis and secretion of glucocorticoids. As mentioned above, but to a much lesser extent, ACTH also maintains the zona glomerulosa where mineralocorticoids are produced, but has minimal influence on aldosterone production and secretion otherwise. CRH concentrations increase in response to stress, hypoglycaemia and physical exercise. In turn, ACTH secretion is increased in response to this increased CRH, but may also be stimulated by release of arginine-vasopressin, AT II, cholecystokinin, atrial natriuretic peptide and vasoactive peptides. Glucocorticoids have a wide range of effects within the body, making their presence crucial to homeostasis. These effects include: -
Stimulation of hepatic gluconeogenesis and glycogenolysis
-
-
Enhanced protein and fat catabolism Permissive effects on lipolysis and calorigenesis Maintenance of vascular reactivity to catecholamines – thus playing role in blood pressure maintenance Counteraction of the effects of stress Maintenance of normal function and integrity of the gastrointestinal mucosa o Influence digestion and intestinal absorption o Increase intestinal brush border and mitochondrial enzymes Suppression of vasopressin secretion via negative feedback Mild mineralocorticoid receptor effects similar to aldosterone, but generally glucocorticoids have weak mineralocorticoid activity – due to inactivation to cortisone.
As a consequence, deficiency in glucocorticoids can have a wide range of clinical, haematologic and biochemical effects. Hypocortisolaemia can result in gastrointestinal signs (vomiting, diarrhoea, anorexia, weight loss), hypotension and inability to maintain vascular tone, hypoglycaemia, reduced mobilization of protein and fat from tissues, muscular weakness, and increased susceptibility to stress with an inability to produce a typical stress leucogram on haematologic evaluation. Gastrointestinal signs in particular are not uncommon and likely result from a combination of alterations in motility, altered vascular and mucosal permeability, and hypovolaemia and reduced tissue perfusion resulting in mucosal injury. Pathophysiologic Development of Hypoadrenocorticism Hypoadrenocorticism is much more easily recognisable when patients present in Addisonian ‘crisis’ with collapse, hyponatraemia and hyperkalaemia. However the wide range of clinical presentations can make immediate recognition of the disease syndrome more difficult and contributes to the reputation of hypoadrenocorticism as ‘The Great Pretender’. This is particularly the case where isolated ‘atypical’ hypoadrenocorticism is present (glucocorticoid deficiency without mineralocorticoid deficiency), or in early disease, as the hallmark electrolyte and fluid disturbances are generally absent. The majority of clinical cases of hypoadrenocorticism result from bilateral destruction of the adrenal cortex; generally suspected to be as a consequence of immune mediated disease. Tumour invasion, granulomatous destruction (blastomycosis, histoplasmosis and Cryptococcus), amyloidosis, and infarction are less commonly reported in the veterinary literature. Hypoadrenocorticism resultant from therapy directed at hyperadrenocorticism (mitotane, trilostane) either as a consequence of protocols deliberately directed at adrenal destruction (using mitotane), resultant from an idiosyncratic reaction, or due to therapeutic over-control can also be seen. Chronic administration of exogenous glucocorticoids can result in hypocortisolaemia upon sudden withdrawal due to adrenal axis suppression. Therefore slow withdrawal of prednisolone therapy is strongly recommended. Secondary hypoadrenocorticism as a result of reduced ACTH release from the pituitary is rare and would be expected to predominantly affect glucocorticoid synthesis and release, with lesser effect on mineralocorticoids. Although atypical hypoadrenocorticism (glucocorticoid deficiency without mineralocorticoid deficiency) is reported for dogs, these cases appear to be adrenal rather than pituitary based most commonly.
Signalment Standard Poodles, Portugese water dogs, and Nova Scotia duck tolling retrievers are recognised to have an autosomal recessive predisposition. An inherited disorder is also recognised in Bearded Collies. A variety of other breeds have also been demonstrated to have a higher prevalence of disease, although mode of inheritance is not identified. Females appear to be over-represented, except in those breeds already mentioned. Median age of onset is 4 years (range of 4 months to 14 years), therefore the majority of dogs are young to middleaged. Clinicopathologic Findings The clinical presentation can be acute where Addisonian crisis occurs. However a gradual and subtle, waxing and waning course may be detected on careful historic evaluation for many patients. Episodic illness, particularly involving gastrointestinal signs and where associated with ‘stressful’ events , should raise suspicion for hypoadrenocorticism. Otherwise clinical signs are often vague and are not considered pathognomonic. This provides a diagnostic challenge. For dogs with Addisonian crisis, classic clinicopathologic findings include hyponatraemia, hyperkalaemia, azotaemia, a non-regenerative anaemia, lymphocytosis or lack of stress leucogram. However the range of different clinicopathologic findings otherwise found can mimic a wide-variety of disease syndromes. For example the following conditions may be suspected: -
Hepatic disease – due to the presence of hypoglycaemia, hypoalbuminaemia, hypocholesterolaemia, increased ALT/ALkP Renal disease – due to the presence of anaemia, azotaemia, hypercalcaemia, hyperphosphataemia, low USG Insulinoma – due to the presence of hypoglycaemia, increased ALT/ALkP Protein losing enteropathy (PLE) – due to the presence of hypoproteinaemia, hypocholesterolaemia, non-regenerative anaemia
Although hyponatraemia and hyperkalaemia, or an alteration in the Na:K, are the classical findings of hypoadrenocorticism, it is important to remember that these may be absent in 30% of cases. Furthermore, alterations to these electrolyte concentrations may also be seen for a range of nonadrenal diseases; particularly primary gastrointestinal disease, renal disease, and those conditions resulting in third-spacing. Diagnostic Confirmation Where hypoadrenocorticism is suspected based upon clinicopathologic findings, a diagnosis is generally confirmed via identification of reduced cortisol concentrations both pre- and post-ACTH administration (ACTH stimulation testing). Mineralocorticoid deficiency is most often not specifically evaluated via aldosterone concentration measurement, and is generally assumed to be present where compatible sodium and potassium alternations are present. As already mentioned, although the majority of cases of hypoadrenocorticism typically involve both mineralocorticoid and glucocorticoid deficiency, occasionally ‘atypical’ hypoadrenocorticism (glucocorticoid deficiency without typical electrolyte disturbances) can be identified. This situation may result from a degree of preservation of the zona glomerulosa, whilst the zona fasiculata and zona reticularis are destroyed, and therefore aldosterone concentrations are maintained. Secondary
hypoadrenocorticism (pituitary dependent disease) may produce a similar effect, but within available small studies of dogs this was identified less frequently than isolated destruction of the glucocorticoid producing regions of the adrenal gland (zona fasiculata and zona reticularis). When performed alone, baseline cortisol concentrations of < 50 nmol/L raise some suspicion of hypocortisolaemia, and should be followed with ACTH stimulation testing. Most often ACTH stimulation identifies low to undetectable cortisol concentrations at both baseline and 1 hour after ACTH stimulation administration. This result requires at least 90% or greater destruction of the adrenal glands. Therefore the presence of some reserve capacity, perhaps particularly in early disease, may produce different results. In this scenario baseline cortisol concentration may be > 50 nmol/L, but reduced capacity to increase cortisol following ACTH administration would be expected and would result in subsequent poor to minimal increases in cortisol concentrations at the 1 hour mark. Measurement of endogenous ACTH and aldosterone is not generally performed to determine if disease is primary or secondary as secondary disease is considered rare and treatment is unlikely to differ. Evaluation could be considered in dogs without classical electrolyte disturbances, and may be useful to determine those dogs more at risk of progression from glucocorticoid deficiency alone, towards mineralocorticoid loss and therefore eventual need for additional supplementation. Interestingly, aldosterone has been detected to be reduced or undetectable in cases where glucocorticoid deficiency has been confirmed, but electrolyte abnormalities are not detected. Alternative mechanisms to maintain electrolyte concentrations must be occurring in these dogs. The evaluation of aldosterone-to-renin or cortisol-to-ACTH ratios has been proposed as alternative methods of detecting primary hypoadrenocorticism to an ACTH stimulation test. In this scenario renin and ACTH concentrations would be expected to be increased, with reduced aldosterone and cortisol concentrations. When each of these hormones is evaluated on their own, significant overlap exists between health and confirmed disease. However the use of ratios separates populations adequately in those studies performed. An advantage of this approach is that only a single blood sample time-point is required, and therefore this test could potentially replace an ACTH stimulation test, which can be expensive. However, limited availability of renin and aldosterone assays limits practical use of these ratios. Furthermore, studies evaluating the effect of non-adrenal illness are currently lacking. Although rare, hypoadrenocorticism may also be a component of a polyendocrinopathy, and concurrent hypothyroidism, diabetes mellitus and hypoparathyroidism have all been reported in dogs. This is not unsurprising given that for many of these endocrinopathies, a primary immune mediated aetiology is suspected. If there is a clinical suspicion for these conditions at presentation, or if response to therapy is not as good as expected, further testing should be performed. Diagnostic Imaging Although not specifically required to diagnose hypoadrenocorticism, imaging studies may be performed to exclude other causes of identified biochemical abnormalities. Radiographic findings may include microcardia, a reduction in pulmonary vasculature, and microhepatica as a reflection of hypovolaemia. Reversible oesophageal dilation or megaoesophagus has been reported and reflects muscular weakness caused by glucocorticoid deficiency. On ultrasound, a measurable reduction in the size of the adrenal glands is often present (< 3.4 mm),
although some overlap exists with normal dogs. In particular, ultrasound helps to exclude other non-adrenal illness as well as rarer causes of hypoadrenocorticism such as metastatic neoplasia or granulomatous adrenalitis. Electrocardiogram Marked hyperkalaemia may result in marked bradycardia and ECG changes. Classically, peaked Twaves, widened QRS complexes, decreased QRS amplitude and increased duration of the p-waves may be seen with moderate to severe hyperkalaemia. Complete loss of p-waves, ventricular fibrillation and even asystole may be seen with extreme hyperkalaemia. These findings are however variable and are therefore not always seen, and may more often than not be over-ridden by factors such as sympathetic stimulation. Acute Management An Addisonian crisis can be life-threatening and requires immediate intervention, particularly where severe hyperkalaemia and/or ECG changes are present. Goals of acute management should primarily include correction of hypotension and hypovolaemia; correction of electrolyte disturbances, particularly severe hyperkalaemia; and, where present, correction of hypoglycaemia and anaemia. The cornerstones of acute management therefore involve the administration of aggressive intravenous fluid therapy to correct hypovolaemia and improve general perfusion, and the administration of glucose and/or insulin where severe hyperkalaemia is present. Blood transfusion may also be considered where anaemia secondary to significant gastrointestinal blood loss is suspected or identified. A protocol for acute management is included below. Collection of samples directed at confirming a suspected diagnosis of hypoadrenocorticism should also be performed concurrent to stabilisation therapy. This should particularly be considered as administration of supportive therapy in addition to basic intravenous fluids and correction of hyperkalaemia, particularly that which includes glucocorticoid administration, can be useful in stabilisation. HOWEVER, administration of certain glucocorticoids can also hinder subsequent diagnostic testing. Where there is strong suspicion for hypoadrenocorticism, mineralocorticoid or mixed mineralocorticoid / glucocorticoid support can be administered immediately following collection of diagnostic samples. This can aid stabilisation of vascular tone, myocardial function and correction of electrolyte disturbances. Where possible, the author advocates for administration of intravenous hydrocortisone as a constant rate infusion (CRI) titrated to effect and immediately following completion of an ACTH stimulation test. This is in preference to the administration of dexamethasone, although where hydrocortisone, or alternatives, are unavailable the administration of dexamethasone intravenously can be useful. It should always be remembered that dexamethasone potency is significantly higher than that of prednisolone and doses should therefore be adjusted accordingly. The author uses 1/10th of the dose of prednisolone when calculating therapy (i.e. 1mg/kg of prednisolone is equivalent to approximately 0.1mg/kg of dexamethasone). Long-term Management Glucocorticoid therapy using prednisolone as the supplement of choice is generally prescribed. Recommended physiologic dosing is approximately 0.1 â&#x20AC;&#x201C; 0.3 mg/kg, but should be tailored to
provide the lowest effective dose that controls clinical signs, and avoids the typical side-effects of chronic prednisolone administration. At diagnosis, especially for dogs presenting acutely unwell, a dose of 3 â&#x20AC;&#x201C; 5 x the expected physiologic dose above may be best administered. This takes into account that these patients are unable to increase cortisol concentrations in response to stress and illness as dogs with normal adrenal function would otherwise be able to do. This can aid recovery from clinical illness. Similarly, in order to prevent precipitation of a crisis, prednisolone can be administered at an increased dose (3 â&#x20AC;&#x201C; 5 x physiologic / baseline dosing) when stressful situations are expected (travel, boarding kennels etc). Concurrent mineralocorticoid therapy traditionally involved daily fludrocortisone administration, either as a single, or split dose protocol. Studies suggest that increased doses of fludrocortisone may be required over time and that higher doses may be associated with clinical signs of hyperadrenocorticism due to the intrinsic concurrent glucocorticoid effects of this medication. Administration of oral hydrocortisone as an alternative can be considered. However, the ratio of glucocorticoid to mineralocorticoid activity for this drug is approximately 1:1, and the doses required to manage mineralocorticoid deficiency often also result in excessive glucocorticoid administration and therefore side-effects. Concurrent use of fludrocortisone and cortate is sometimes prescribed. More recently a long-acting mineralocorticoid only supplement has been available. Deoxycorticosterone pivalate (DOCP) is now considered the treatment of choice for dogs with hypoadrenocorticism. Its long therapeutic action means it can be administered at 25-28 day intervals, although longer intervals can sometimes be possible and dose reductions from the recommended 2.2 mg/kg IM/SC can also be attempted. Strategies that increase the dosing interval or reduce the overall dose administered, and that demonstrate maintenance of normal electrolyte concentrations, can be helpful in managing costs for owners, particularly for those with large breed dogs. Dogs without electrolyte abnormalities that would otherwise suggest mineralocorticoid deficiency do not require specific mineralocorticoid supplementation. Where further testing has not been performed to evaluate these cases for and confirm secondary disease, close monitoring of electrolyte concentrations is recommended at least every 1 â&#x20AC;&#x201C; 3 months for the first 12 months. This potentially allows earlier detection of progression to concurrent mineralocorticoid deficiency where this does eventually occur. Prognosis The prognosis for hypoadrenocorticism is generally excellent. Expense of therapy and monitoring can be cost prohibitive for some owners, especially those with large breed dogs. Complications such as the presence of mega-oesophagus, or severe gastrointestinal compromise can limit outcome for some patients and may also compromise therapy where therapies are being administered orally. Most commonly poor response to therapy is associated with insufficient supplementation, concurrent illness such as hypothyroidism or excessive glucocorticoid administration and therefore side-effects.
PROTOCOL FOR ACUTE MANAGEMENT OF SUSPECTED HYPOADRENOCORTICISM 1
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Gain IV access either via placement of a peripheral or central (jugular) catheter Initiate Intravenous fluid therapy - 10 mL/kg IV boluses over 10-15 minutes – repeated to effect – using 0.9% NaCl or Hartmanns - The use of Hartmanns may be preferred over 0.9% NaCl, despite potassium content in order to limit rapid fluctuations in sodium. - Collect blood for haematology, biochemistry, electrolytes, and basal cortisol - Collect urine for urinalysis - Consider use of short-acting insulin (0.5 IU/kg) to rapidly lower serum potassium where hyperkalaemia is identified, and especially where ECG changes are identified or suspected - Concurrent IV glucose should be administered at 2 g/unit of insulin administered – ensuring 50% glucose is diluted 1:10 - Additionally – consider use of 10% calcium gluconate IV over 10-15 minutes (2 – 10 mL/dog) to raise the depolarization threshold and protect the myocardium against hyperkalaemia. Close concurrent monitoring with ECG is preferred. - Perform an ACTH stimulation test – 5 ug/kg up to a maximum of 250 ug - Administer mixed glucocorticoid / mineralocorticoid supplementation once ACTH stimulation test complete o Consider use of hydrocortisone as a CRI and titrated to effect o Dexamethasone (0.1 – 0.3 mg/kg) can be administered as an alternative – and as this does not interfere with cortisol assays, can be administered at any point during the ACTH stimulation test. Dosing can be repeated q24h o Methylprednisolone sodium succinate – 1-2 mg/kg IV – can also be used. - Consider the correction of metabolic acidosis with administration of bicarbonate where acidosis is severe or measured bicarbonate is < 12 mEq/L - Correction of metabolic acidosis often achieved via the above intravenous fluid therapy without the need for administration of bicarbonate. - The administration of bicarbonate should always be cautious. - Consider blood transfusions and/or synthetic colloids in dogs with anaemia due to gastrointestinal blood loss - Consider IV glucose/dextrose bolus or CRIs in dogs with hypoglycaemia - Glucose/dextrose should always be diluted to 5% in saline. - Monitor o Serum / plasma electrolytes o Blood glucose o Blood pressure o Urine output – especially where severe azotaemia is identified; Central venous pressure could also be performed o ECG – if hyperkalaemic, or during administration of calcium gluconate. o Acid-base status – where possible - Followup o Continue IV fluids until oral intake improves, then taper o Continue injectable therapies until oral medications can be substituted – consider that significant gastrointestinal compromise may limit absorption of orally administered therapies.
References Scott-Moncrieff C, (2015), Hypoadrenocorticism, In: Canine and Feline Endocrinology, Ed Feldman and Nelson. Lathan P, Tyler JW, (2005), Canine Hypoadrenocorticism: Pathogenesis and Clinical Features, Compendium, 27 (2). Scott-Moncrieff C, (2010), Hypoadrenocorticism, In: The Textbook of Veterinary Internal medicine, Ed: Ettinger and Feldman, p1847. Lifton SJ, King LG, Zerbe CA, (1996), Glucocorticoid deficient hypoadrenocorticism in dogs:18 cases (1986 – 1995), Journal of the American Veterinary Medical Association, 209 (12): 2076-2081 Peterson ME, Kintzer PP, Kass PH, (1996), Pretreatment clinical and laboratory findings in dogs with hypoadrenocorticism: 225 cases (1979-1993), Journal of the American Veterinary Medical Association, 208 (1); 85-91. Lennon EM, Boyle TE, Hutchins RG, Friedenthal A, Correa MT, Bissett SA, Moses LS, Papich MG, Birkenheuer AJ, (2007), Use of basl serum or plasma cortisol concentrations to rule out a diagnosis of hypoadrenocorticism in dogs: 123 cases (2000 – 2005). Journal of the American Veterinary Medical Assocation, 231 (3): 413-416. Kintzer, P. P. and Peterson, M. E. (1997), Treatment and Long-Term Follow-up of 205 Dogs With Hypoadrenocorticism. Journal of Veterinary Internal Medicine, 11: 43–49. doi:10.1111/j.19391676.1997.tb00072.x Javadi, S., Galac, S., Boer, P., Robben, J. H., Teske, E. and Kooistra, H. S. (2006), Aldosterone-to-Renin and Cortisol-to-Adrenocorticotropic Hormone Ratios in Healthy Dogs and Dogs with Primary Hypoadrenocorticism. Journal of Veterinary Internal Medicine, 20: 556–561. doi:10.1111/j.19391676.2006.tb02896.x DiBartola SP, Johnson SE, Davenport DJ, Prueter JC, Chew DJ, Sherding RG, (1985), Clinicopathologic findings resembling hypoadrenocorticism in dogs with primary gastrointestinal disease, Journao of the American Veterinary Medical Association, 187 (1): 60-63 Thompson, AL, Scott-Moncrieff C, Anderson JD, (2007), Comparison of classic hypoadrenocorticism with glucocorticoid-deficient hypoadrenocorticism in dogs: 46 cases (1985 – 2005), Journal of the American Veterinary Medical Association, 230 (8): 1190-1194. Kintzer PP, Petersen ME, (1997), Primary and Secondary Canine Hypoadrenocorticism, Veterinary Clinics of North America, 27 (2):349-357 Melián, C. and Peterson, M. E. (1996), Diagnosis and treatment of naturally occurring hypoadrenocorticism in 42 dogs. Journal of Small Animal Practice, 37: 268–275. doi:10.1111/j.17485827.1996.tb02377.x Lathan, P., Moore, G.E., Zambon, S. and Scott-Moncrieff, J.C. (2008), Use of a Low-Dose ACTH Stimulation Test for Diagnosis of Hypoadrenocorticism in Dogs. Journal of Veterinary Internal Medicine, 22: 1070–1073. doi:10.1111/j.1939-1676.2008.0118.x
Hughes AM, Nelson RW, Famula TR, Bannasch DL, (2007), Clinical features and heritability of hypoadrenocorticism in Nova Scotia Duck Tolling Retrievers: 25 cases (1994 – 2006), Journal of the American Veterinary Medical Association, 231 (3):407-412. Oberbauer AM, Benemann KS, Belanger JM, Wagner DR, Ward JH, Thomas BS, Famula TR, (2002), Inheritance of hypoadrenocorticism in Bearded Collies, American Journal of Veterinary Research, 63 (5):643-647 Adler, J. A., Drobatz, K. J. and Hess, R. S. (2007), Abnormalities of Serum Electrolyte Concentrations in Dogs with Hypoadrenocorticism. Journal of Veterinary Internal Medicine, 21: 1168–1173. doi:10.1111/j.1939-1676.2007.tb01933.x Baumstark, M.E., Sieber-Ruckstuhl, N.S., Müller, C., Wenger, M., Boretti, F.S. and Reusch, C.E. (2014), Evaluation of Aldosterone Concentrations in Dogs with Hypoadrenocorticism. J Vet Intern Med, 28: 154–159.