Manual Muscle Strength Testing

Manual Muscle Strength Testing for the muscles that are innervated by the Five Nerves that move the wrist and fingers

A guide to diagnosis, location and follow-up of peripheral nerve function impairment

Door Wim Brandsma, Ton Schreuders

Keywords: assessment – dynamometry – muscle strength testing – neurology

Appropriate manual muscle strength testing (MMST) is essential in the (differential) diagnosis and follow-up in many neurological conditions and muscle-tendon pathologies.1 In the assessment and follow-up of neuromuscular conditions, manual muscle strength testing is often considered ‘subjective,’ and is then interpreted as unreliable.2 Brandsma et al., have demonstrated that manual muscle strength of the intrinsic muscles can be reliably assessed.3-5 In our experience, therapists, surgeons and neurologist show insufficient skills and confidence in MMST and rely more on neurophysiological examinations. This article will focus on the usefulness of MMST in disorders of the peripheral nervous system affecting the function of the hand (Figure 1). Muscle strength, as a combined function of intrinsic and extrinsic muscles, can also be assessed through dynamometry, grip- and pinch strength.6,7 The authors illustrate how using MMST and dynamometry results can complement and confirm each other and are helpful techniques in monitoring and eva-

Manual muscle strength of the intrinsic muscles can be reliably assessed

Abstract

The article outlines how assessing the strength of the muscles of the wrist and fingers will help in evaluating the function of the five nerves that innervate these muscles. This is valuable in the (differential) diagnosis and follow up of nerve pathology. Two of the three primary peripheral nerves i have a separate motor branch, the median (anterior interosseus nerve) , and the radial nerve (posterior interosseus nerve). Sound knowledge of nerve-muscle innervation patterns is key to the location, possible diagnosis, and follow-up of nerve function impairment. The article describes basic muscle tests that can guide the diagnostic process, confirm a diagnosis, and aid in the follow up of nerve function impairment. Often, hand-held dynamometry can be added to quantitate changes.

luating peripheral neuro-muscular change in the upper extremity. This article will review MMST testing and adds to the MMST assessment chapter of the assessment manual of the American Society of Hand Therapists (ASHT).

The scope of MMST Manual Muscle (strength) testing can be used in the following conditions1: • Diagnosis and follow-up of neuropathies and nerve lesions. • Diagnosis or confirmation of (suspected) tendinopathies. • Diagnosis of muscle-tendon lesions and adhesions.

Findings may determine and guide possible interventions. In this paper, we limit ourselves to the role of muscle strength testing in instances of (suspected) peripheral neural problems in the upper extremity (Table 1). Nerve impairments can be categorized as follows: nerve lacerations, neuropathies, and nerve compressions.8 The latter could be further divided in external and internal causes of compression – compression from within the body, originating within or external to the nerve or compression from outside the body on the nerve. These compression neuropathies are referred to as neuropraxia, usually transient nerve conduction blocks that will resolve with time given resolution of the cause. For all nerves, imaging techniques can be helpful in determining the location of possible pathology. 9 MMST can also aid in, or confirm a diagnosis and its location. Naturally, in traumatic cases, a cut, bullet wound, or fracture is often guiding for a location of nerve pathology.

Manual Muscle Strength Testing for the five nerves that innervate the wrist and hand muscles. This assessment is based on the Medical Research Council (MRC 0-5) grading

Figure 1 Five nerves and their muscle innervation

Table 1 Muscle testing to determine motor impairment, possibly its location, of the nerves that innervate the wrist and finger muscles.

scale of which the basic principles and testing procedures are presented in the ASHT manual and Table 2. 1 , 10 For most tests, the reader is referred to these two references.

Ulnar Nerve Because of its superficial location, the most common, traumatic, and non-traumatic sites for ulnar nerve pathology are near the elbow or wrist. The ulnar nerve supplies only a few of all extrinsic hand muscles, Flexor Carpi Ulnaris (FCU) and Flexor Digitorum Profundus (FDP) of the ring and little fingers, while most of the intrinsic muscles of the fingers are ulnar innervated.11

The primary test is that of abduction of the little finger. Weakness does not differentiate between the ulnar nerve's high (proximal) and low (distal) pathology. To determine the level – high-low, it is suggested that the FDP of the little finger is tested. The patient is asked to make a strong fist and the examiner will then test for FDP strength. Alternatively, the patient can be asked to strongly grasp two fingers of the examiner which makes it easier to test the strength of FDP. The ring finger is often still strong because of the connections with the middle finger tendon or muscle belly.12 The FCU can also be tested, but grading according to the MRC scale is hard because of cocontraction of other muscles. Strength could be estimated by feeling the tension of the FCU tendon and comparison with the unaffected site.

The most common sites for ulnar nerve pathology are near the elbow or wrist

Ulnar (C8-T1)

Low Primary test Possible secondary test Additional test Abd. dig. 5 Add. dig. 5 Abd. Index

High

Median (C5-T1) Low

High

AIN

Radial (C5-T1) FDP. dig. 5

Abd. Thumb

FPL

FPL

EDC FDP- dig 2

FDP- dig 2

EIP Opp. Thumb

Abd. Thumb

Thenar muscles are innervated!

Tabletop Test Additional tests Adduction of little finger. The subject keeps all fingers adducted. The examiner tries to separate the 5th finger from the ring finger, observing how the little finger may snap back besides the ring finger when releasing resistance. When compared with the contralateral unaffected hand, this may reveal weakness. The first dorsal interosseus muscle is superficial, therefore palpable and it can be graded (MRC).It may be innervated through the Martin-Gruber, median to ulnar, anastamosis.13

Davidge suggests an innovative test for ulnar motor function.2 Again, especially useful in unilateral conditions for comparison with the unaffected hand. The patient is asked to move the fingers, with the hand flat on the table, in maximum abduction. The tip of the index and middle fingers are marked with a pencil on a piece of paper and the distance between the index and little finger is recorded. in millimeters. Alternatively, the distance that only the middle finger can move from radial to ulnar abduction can be recorded. (Figure 2).

Median nerve (low and high) and Anterior Interosseus Nerve (AIN) For suspected median nerve involvement, thumb palmar abduction test would be the test to do.1,10 A complementary test would be thumb opposition. (Table 2) In instances of (suspected) median nerve impairment in the forearm, the following tests could be performed. The flexor pollicis longus (FPL) is the sole flexor of the thumb IP joint. This makes it the ideal muscle/tendon unit to find out if there is a proximal median neve impairment. The Flexor Digitorum Superficialis (FDS-Index) is also an excellent one to test.1-10 Alternatively, FDP2 could be tested. With suspected AIN impairment, flexion of the thumb IP joint, FPL, is the ideal test. When asking for a pulp-pulp pinch, hyperextension of the DIP and hyperflexion of the PIP joint may become noticeable. Differentiating between high median and AIN paralysis is easy; with an AIN lesion there is no weakness of the median innervated thenar muscles.

Radial Nerve and Posterior Interosseus Nerve (PIN) For testing the integrity of the main nerve, testing the combined wrist extensors, is advised by asking the patient to extend the wrist (fingers relaxed). Asking resistance can then be applied. Testing combined finger extensors is a good alternative: finger MCP extension with IPs flexed. The latter are also the muscles affected in PIN paralysis. Testing isolated index finger extension with the other fingers flexed would also be an excellent test for radial nerve and PIN paralysis. Alternatively, MCP index finger extension can be assessed from the tabletop by lifting the index finger from the table, with all other fingers extended. Differentiating between radial nerve and PIN paralysis is determined as follows; with a PIN paralysis there is radial wrist extension possible, not ulnar, and there is no MCP finger extension (Figure 1).1-10 To test the integrity of all three main nerves simultaneously, the examiner could ask for thumb-little finger opposition with the wrist extended.

Table 2 MRC scale and its modifications

Medical Research Council (MRC) Scale 6 grades 5 Full range of motion;

full resistance

4 Full range of motion;

some resistance

3 Full range of motion;

no resistance

2 Decreased range of motion

1 Muscle flicker

0 Complete paralysis Modifications 9 grades 4+ Moderate resistance

3+ Minimal resistance

2+ Nearly full range

Table from ASHT manual1 Notes: a) Please note that gravity is not part of the definition.1 The authors do not consider this relevant for the forearm/ intrinsic muscles, except maybe for wrist flexion / extension. b) Restricted range or pain influence grading. Example: strength can be graded 5 with reduced range. When pain or limited range of motion influence grading, this should be noted. Rasch analysis18 (4 grades): 3-Strong; 2-Slight weakness, 1-Severe weakness, 0, paralysis. See discussion.

MMST and dynamometry Grip strength can be reliably assessed with the Jamar dynamometer.6 Pinch strength can also be reliably assessed.7 Many studies have also reported on normative values by age and gender.6 In the case of unilateral involvement, the contralateral hand could be used to monitor changes against. If all five handle positions are used, then the height of the ‘bell-shaped’ curve is an indication of the contribution of the intrinsic muscles. For quick assessment, the second handle position is recommended.1 The Rotterdam Intrinsic Hand Myometer (RIHM) could be used to assess and monitor strength of some individual intrinsic muscles/ finger movements.12,13

Discussion Muscle strength testing in the hands of experienced therapists can greatly contribute to (differential) diagnosis and follow-up of multiple nerve pathologies. The muscle strength assessments are ‘subjective’ because they depend on an examiner’s interpretation, correct execution of the test and cooperation of the subject. Subjective should not be interpreted as unreliable. Pain is the ultimate subjective phenomenon, and many studies have shown that pain can be reliably assessed.14 Reliability of MMST of the intrinsic muscles of the hand has been shown to be sufficient.3-5 For many therapists it is important to realize that for assessment and follow up of nerve function impairment it is not necessary to assess each, and every muscle innervated by a nerve. MMST is often useful when tests results can be compared and interpreted against an unaffected extremity enhancing its validity. Much experience is needed to be able to reliably assess manual muscle strength. Both authors in their work with leprosy affected people have experienced this. In acute phases of leprosy neuropathy, nerve function is often monitored every two weeks, for months even years. New staff and ‘interns’ need much time testing many patients daily over many weeks to feel confident and score grades that agreed with the results of the authors.

Much experience is needed to reliably assess MMST

There is one pitfall in diagnosis and follow-up of ulnar nerve impairments in the forearm that can be attributed to the so named Martin-Gruber anastomosis.11 A variable number of intrinsic muscles might be innervated through the median to ulnar nerve shunt in the forearm which may lead to erroneous interpretation of examination findings. There is a plethora of ulnar weakness signs documented in the literature.15 Froment’s sign is the best known of these. The clinician should be aware that these signs are nominally graded –positive/ negative– and may often be false positive. Wheras the MRC scale is an ordinal scale, the RIHM is a dynamometer which grades at the interval level. Here also, as in MMST, reliable results depend on accurate application of the RIHM. Because full range and at least some resistance is required to be able to assess strength, the RIHM will be only useful in the MRC 3+ to 5 range. The RIHM therefore, is an ideal dynamometer to quantitatively assess and monitor strength in the broad MRC 3+-5 range. A Rasch analysis of MRC grading across many neurological conditions involving nine cohorts from ten study sites involving nine neuromuscular conditions showed poor overall reliability.16 The gradings of 72 muscles / muscle groups across 8 neuro-muscular conditions were analysed. The contributing authors (23!) concluded that in practice the 6-point MRC scale should/could be reduced to a 4-point scale (Table 1). However, as stated, both authors of the current article belief that the MRC scale in the hands of experienced (hand)therapists remains a useful tool. A recent scoping review confirms what is commonly known in MMST that MRC grade 4 covers a wide range of the potential of possible muscle contraction strength from mild to near normal resistance (Figure. 3).17 McAvoy specifically studied this for the elbow and found that grade four presented 96% of potential elbow flexion strength.18 There are limitations to the MRC grading, specifically its sensitivity in follow up studies (grade 4).17-19 But there are also distinct advantages, ease of use, diagnosing specific nerve pathology and its localization, differentiating between the five forearm nerves, and ‘availability’. Nerve injuries are often missed in emergency situations.20 Physicians, rather than asking for manual grip strength in (suspected) nerve injury, could ask, when pain or other trauma is not a limiting factor for the triple nerve test: thumb-little finger opposition (ulnar-median nerves) with wrist extended (radial nerve).

Conclusion This communication discussed MMST in the (differential) diagnosis and follow-up of impairments of the peripheral nervous system in the upper extremity. When knowledgeable about innervation patterns and familiar with specific muscle strength tests, therapists can play an important role in feedback and follow up to referring physician and patient. Dynamometry can be of additional value to quantitate and objectify changes in nerve function, complementary to MMST.21,22 Dynamometry is also a useful adjuvant assessment technique in many traumatic/ orthopedic conditions, when there are no nerve impairments, and improvements can be based on reduced pain, improved tendon excursions, and increased range of motion or combinations thereof.

Figure 2 Quantitating ulnar nerve changes: MRC > 3 Modified from Davidge: Mark tip of middle finger with pencil on paper. Measure radial / ulnar deviation distance. Alternatively, measure index-little finger distance with fully abducted fingers. The hand is here withdrawn to better show markings on paper.

Figure 3 Relationship between MRC scale, Jamar and RIHM Muscle Strength Testing in Neurology Medical Research Council (MRC) 1943 Dynamotetry (Jamar) 1954 Rotterdam Instrinsic Hand Myometer RIHM 2003 Grade 3 MRC falls below 10% of maximum voluntary contraction. Statistical hierarchy: Nerve function impairment signs (nominal): MRC (ordinal): RIHM and Jamar (interval)

AUTHORS • J Wim Brandsma, PT, PhD, Hand therapist, consultant • Ton Schreuders, PT, CHT, PhD, Hand therapist, Erasmus

Medical Center, Department of Plastic Surgery,

PO Box 2040, Rotterdam

Correspondence: jwbrandsma@gmail.com

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