The definition of dry eye disease continues to

What is dry eye disease?

Research bringing greater clarity to dry eye disease definition and classification. Roibeard O’hEineachain reports

The definition of dry eye disease (DED) continues to evolve as research reveals new aspects of its aetiology and pathophysiology, said Prof Jose M Benitez-del-Castillo MD PhD, at the 25th ESCRS Winter Meeting.

The Tear Film and Ocular Surface Society’s report from the 2007 international dry eye workshop (DEWS) defined DED as “a multifactorial disease that results in symptoms of discomfort visual disturbance and tear film instability with potential damage to the ocular surface accompanied by increased osmolarity of the tear film and inflammation of the ocular surface” said Prof Benitez-del-Castillo, Universidad Complutense de Madrid, Spain.

He pointed out that DED had previously been considered a disorder of the tear film, but this definition defines it as a disease of the ocular surface. He added that the inclusion of visual disturbance is also very important because a bare unmoistened corneal epithelium has a very rough and optically imperfect surface, as can be seen with scanning electron microscopy.

The newer DEWS II revised definition of 2017, defines DED as “a multifactorial disease of the ocular surface characterised by the loss of homeostasis of the tear film accompanied by ocular symptoms in which tear film instability, hyperosmolarity, ocular surface inflammation and damage and neurosensory abnormalities play aetiological roles”.

Prof Benitez-del-Castillo noted that this definition lacks a key element in that it does not include visual symptoms. On the other hand, the emphasis on tear film homeostasis and the aetiological implication of neurosensory abnormalities bring the definition more closely into line with clinical experience and the current scientific understanding of the condition.

HOMEOSTASIS BREAKDOWN He noted that ED results from a breakdown in the homeostasis that maintains the ocular surface as a functional unit. In eyes with dry eye disease there is an impaired sensory input, the lachrymal glands become populated with inflammatory cells and cytokines and there is ocular surface damage. That in turn leads to more tear film instability and more hyperosmolarity.

Neurosensory abnormalities are related to inflammation and confocal microscopy photographs of the ocular surface show that in the normal eye the nerve density is high with very few antigen-presenting cells, whereas in the dry eye it is the reverse, with low nerve density and a high number of Langerhans cells, he said.

DIAGNOSIS & CLASSIFICATION When diagnosing DED, patients’ responses to answers to the DEQ-5 or OSDI questionnaire will provide a good overview of their symptoms, he noted. In addition, one should look for at least one of three signs namely, non-invasive tear break-up time or fluorescein break uptime, Osmolarity higher 308mOsm/L or interocular difference of more than 8 Osm/L and ocular surface staining.

Patients presenting with dry eye can be symptomatic or asymptomatic, Prof Benitez-del-Castillo said. However, there is not always a correlation between symptoms and signs. For example, a patient may have many signs of the condition but few of the symptoms, as might be the case in an eye with neurotrophic keratopathy. Conversely, a patient can have many symptoms but few signs, as may occur among patients with neuropathic pain, a complication of dry eye disease.

Once a patient has a diagnosis DED it is necessary to differentiate between evaporative and aqueous deficient dry eye, examining the eye though the slit-lamp and looking for signs of meibomian gland dysfunction and assessing the tear volume with a tear meniscus height. However, he noted that the limit between the two DED categories is very complex and unclear and around a third of patients have a mix of signs and symptoms of both DED types.

DED confocal microscopy Normal confocal microscopy Courtesy of Prof Jose M Benitez-del-Castillo MD PhD

He added that wettability of the ocular surface is an important factor in tear film stability, so much so that poor wettability DED, a result of mucin deficiency, might be considered a third DED category alongside aqueous deficient and evaporative dry eye. The different types of DED can be distinguished by their characteristic tear break-up patterns.

MGD IN THE 21ST CENTURY Prof Benitez-del-Castillo noted that meibomian gland dysfunction is a chronic diffuse abnormality of the meibomian glands characterised by terminal duct obstruction and changes in the quantity and quality of meibum secreted. This may result in the alteration of the tear film and clinically apparent inflammation and ocular surface disease.

The initiating factors of MGD can include poor blinking habits and infections. He noted that long hours before a computer screen not only reduces the frequency of blinking, but also causes people to blink in an abnormal, partial way, resulting in reduced meibum secretion.

He added that the COVID-19 pandemic has not only led to people having increased screen time, but the wearing of masks may also be contributing to an increased incidence of MGD-related infectious dry eye, in addition to increased rates of other MGD-related conditions such as chalazion and hordeolum. He noted that masks tend to direct exhaled air across the ocular surface, both drying the eye and exposing it to a multitude of bacteria.