What diagnostic tests does a neurologist use to assess peripheral nerve damage in an affected limb?

Diagnostic Testing for Peripheral Nerve Damage in a Limb

A neurologist primarily uses electromyography (EMG) with nerve conduction studies (NCS) as the gold standard electrodiagnostic test to assess peripheral nerve damage in a limb, complemented by a structured clinical examination including manual muscle testing, sensory assessment with a 10-g monofilament and 128-Hz tuning fork, and evaluation of deep tendon reflexes. 1, 2

Primary Electrodiagnostic Testing

Nerve conduction studies (NCS) with electromyography (EMG) serve as the definitive diagnostic tools that neurologists employ to objectively measure nerve function and localize lesions 1, 2:

• NCS measures sensory nerve action potential (NAP) amplitude, with progressive reduction indicating axonal degeneration 1
• Compound muscle action potential (CMAP) assessment quantifies motor fiber involvement and correlates with axonal damage 1
• Nerve conduction velocity impairment follows amplitude reduction and helps distinguish demyelinating from axonal pathology 1
• These tests function as an extension of clinical examination and must be interpreted within clinical context 2, 3

Clinical Examination Components

Before or alongside electrodiagnostic testing, neurologists perform a systematic bedside assessment 1:

Sensory Testing

• 10-g monofilament testing for light touch perception, starting distally at the dorsal hallux and moving proximally until threshold is detected 1
• 128-Hz tuning fork to assess vibration sense and large fiber function 1
• Pinprick testing for pain perception and small fiber involvement 1
• Proprioception assessment to evaluate joint position sense 1

Motor Testing

• Manual muscle testing graded on a 0-5 scale or using handheld dynamometry 1
• Assessment of 16 individual muscles in the upper limb or key muscle groups in lower limbs to identify patterns of weakness 4
• Deep tendon reflex evaluation, as reduction or absence indicates motor fiber involvement 1

Pattern Recognition

• Examination follows anatomical nerve distributions to distinguish focal, multifocal, or length-dependent polyneuropathy patterns 3
• Identification of muscle weakness patterns, sensory abnormalities, and nerve trunk allodynia based on topographic innervation 5, 4

Special Circumstances and Advanced Testing

When Standard Neurophysiology May Be Normal

In small fiber neuropathy (common with certain chemotherapies like bortezomib), conventional EMG/NCS may show completely normal results 1:

• Skin biopsy examined by an experienced pathologist serves as the gold standard for demonstrating degeneration of small C (heat) and Aδ (cold) fibers 1
• Consider this when patients have burning pain and dysesthesias but normal electrodiagnostic studies 1

Autonomic Nerve Testing

When autonomic neuropathy is suspected (orthostasis, resting tachycardia, gastrointestinal symptoms), specialized neurophysiology including sympathetic skin response and heart rate variability should be conducted 1

When to Refer for Electrodiagnostic Testing

Referral to a neurologist for EMG/NCS is indicated when 1:

• Clinical features are atypical or diagnosis is unclear 1
• Asymmetric pattern of symptoms exists 1
• Normal clinical examination despite patient complaints 1
• Need to distinguish between axonal and demyelinating pathology 1
• Evaluation of pre-existing subclinical neuropathy as a risk factor 1

Critical Pitfalls to Avoid

• Do not rely solely on neurophysiology to guide treatment, as test results often correlate poorly with symptoms and functional recovery may occur despite modest electrodiagnostic improvement 1
• Do not skip clinical examination in favor of testing alone, as several clinical instruments combining multiple bedside tests have >87% sensitivity in detecting peripheral neuropathy 1
• Do not order EMG/NCS for routine screening when simple clinical tests (monofilament, tuning fork, ankle reflexes) are sufficient for annual monitoring 1
• Do not forget to assess for alternative causes including neurotoxic medications, vitamin B12 deficiency, alcohol abuse, renal disease, or inherited neuropathies when features are severe or atypical 1, 3