Neurological Examination for Suspected Peripheral Neuropathies

The main objective of a neurological examination in the context of suspected peripheral neuropathies is to identify loss of action potential conductivity. This manifests as hypoesthesia, muscle weakness or reduced reflexes (loss of function). Signs of loss of function are characteristics of peripheral neuropathies. In contrast, gain of function (e.g., hyperalgesia) is not specific to neuropathies, but is also common in musculoskeletal conditions (e.g., osteoarthritis). Gain of function is not the primary goal of peripheral neurological examinations.

Although neurological examinations provide essential diagnostic information and have direct implications for decisions, there is substantial variation in what is included in a neurological examination and how it is performed. The paucity of evidence about the most valid and reliable implementation prevents strict rules.

From this point of view, this work uses neuroscientific principles to offer suggestions on how doctors can improve the way they perform and interpret the neurological examination within a framework of clinical reasoning. Six common challenges are highlighted and six recommendations are made on how to overcome them in practice. The central focus will be the peripheral nervous system (sensation, muscle strength, reflexes), however, clinicians should consider testing for proprioception or the central nervous system as necessary.

Clinical Challenge 1: Rethink Your Approach to Dermatomal Testing

Clinicians typically detect sensory changes in a dermatomal pattern using points of least overlap optimized to recognize nerve root involvement. Although the dermatomal spot test is rapid, it has limitations.

1. First, dermatomal maps vary, which may limit interpretation. The altered sensation near the anterior knee is attributed to L2, L3 or L4 nerve roots depending on the dermatomal map used.
    
2. Second, neuropathies affecting distal nerve trunks (eg, carpal tunnel syndrome, peroneal neuropathy) are more common than nerve root disorders (eg, radiculopathies). The innervation territories of the peripheral nerve trunks do not always coincide with dermatomal points.

Recommendation 1:

Test somatosensation in a circumferential pattern rather than using point test maps of dermatomes and peripherally innervated territories.

Based on innervation maps (but with the caveat that validity studies are required), two circles around the upper and lower arms/legs followed by single digit tests (palmar/plantar and dorsal) will cover most of the dermatomes and territories of peripheral innervation. If changes are identified, map in more detail using star-shaped delineation of sensory changes. Use this approach to help differentiate dermatomal innervation patterns from peripheral ones.

Clinical Challenge 2: Large and Small Nerve Fiber Testing

In patients with suspected peripheral neuropathies, neurological screening traditionally focuses on light touch, muscle strength, and reflex testing. These tests provide information about the function of large diameter nerve fibers. However, they do not evaluate small Aδ and C fibers, which are usually affected in peripheral neuropathies. Small fiber involvement may be present even in the absence of large fiber deficits. Relying solely on integrity testing of large nerve fibers is insufficient.

Recommendation 2:

Low-cost tools consisting of mechanical pain thresholds and thermal sensing have been developed, suggested for time-efficient assessment of small fiber function. It is recommended that clinicians include circumferential pinprick testing as part of a standard neurologic screening examination, followed by thermal testing in the area of ​​maximal pain (e.g., with coins) if the pinprick test is normal.

Clinical Challenge 3: Challenges with Contralateral Comparison

Clinicians often compare the sensory and motor responses (muscle strength, reflexes) of the affected side with contralateral asymptomatic sites to correct for normal variation in skin innervation and motor function along the extremities. Recent research using sensitive laboratory equipment suggests that ipsilateral sensory changes appear to be mimicked on the contralateral side, although to a lesser extent, even in patients with neuropathies. It is unclear whether these subtle contralateral changes can mask ipsilateral deficits on clinical examination.

It is important to remember that many peripheral neuropathies are bilateral (e.g., diabetic polyneuropathy, lumbar spinal stenosis, most carpal tunnel syndromes) and may mask sensory and motor deficits in contralateral comparison. Bilateral changes can also occur even in healthy people : approximately 1 in 4 healthy adults over 60 years of age have bilateral absence of ankle reflexes.

Recommendation 3:

When interpreting changes in the presence of bilateral motor or sensory deficits, contralateral comparison is no longer useful or sensible.

For motor testing, testing other muscle groups and reflexes can provide additional information. For sensory testing, testing an adjacent (unaffected) proximal area (e.g., abdomen, chest, proximal limb) can add information to help you interpret the results.

Clinical Challenge 4: Things to Consider for Myotome Testing

Motor assessment conventionally involves manual myotome testing of baseline muscles that may be complemented by functional assessment (e.g., heel (L4) or toe walking (S1)) and identification of atrophies. However, baseline muscle testing alone does not help differentiate between nerve root or distal nerve trunk involvement. Additionally, nerve roots innervate a wide range of muscles that can also vary between patients. Muscle testing may underestimate the severity of motor deficits, particularly in patients with relatively well-preserved strength.

Recommendation 4:

To establish whether a weakness identified on myotome testing is consistent with distal nerve root or trunk involvement, further evaluation is required. For example, weakness in the extensor pollicis longus could be due to an injury to the C8 nerve root or radial nerve. When motor symptoms are more related to fatigue than maximal strength, clinicians may consider evaluating motor function after physical exertion or using repeated testing. Myotome test results should be interpreted in the context of other findings.

Clinical Challenge 5: Interpretation of Reflex Findings

While reflex testing is essential to complement a clinical picture, there are some things to consider.

1. First, common tendon reflexes do not comprehensively cover all segments.
    
2. Second, reflexes may not provide accurate information about the affected level: the patellar tendon reflex is reduced in 30% of patients with L5 radiculopathy.

Recommendation 5:

Reflexes are not susceptible to interpretation by patients and are not directly influenced by pain: reflexes can, therefore, provide important objective information. However, it is important to interpret the findings in the context of a comprehensive neurological examination and a sound framework of clinical reasoning.

Clinical Challenge 6: Tracking Progress: A Call for Quantification

Quantification of the extent of neurological deficits is not standard in clinical practice, although it is common in other musculoskeletal examinations (e.g., shoulder range of motion). If not measured, it is impossible to monitor progress (or deterioration) over time.

Recommendation 6:

Carefully measure neurological deficits to ensure that you are monitoring changes objectively over time. Quantification of deficits can inform treatment recommendations (e.g., referral for surgical opinion for progressive neurological deficits in people with radiculopathy) and can help patients maintain motivation (e.g., track degree of neural regeneration over time).

There are practical solutions to help clinicians quantify the extent of sensory deficits: measuring the intensity of sensory perception compared to a control area, mapping the size of the area of ​​sensory loss, or using specialized equipment.

While manual muscle testing is easily implemented, a hand-held dynamometer allows for more objective quantification.

Although, the validity of dynamometry in peripheral neuropathies is not clear. In healthy people, dynamometry has a moderate to very high correlation with isokinetic testing and is superior to manual muscle testing in detecting minor differences between sides and changes over time. However, its reliability is influenced by the testing position or strength of the examiner.

For reflex testing, two scales can be used to quantify the extent of reflex change. To our knowledge, the validity or reliability of these reflex scales in patients with mono- or polyneuropathies is unknown.