What is the recommended EMG (electromyography) and nerve‑conduction study work‑up, including history, physical exam, baseline labs, and selection of at least four muscle groups, for a patient presenting with unexplained weakness, sensory changes, and reflex abnormalities?

EMG and Nerve Conduction Study Work-Up for Unexplained Weakness

For patients presenting with unexplained weakness, sensory changes, and reflex abnormalities, perform combined EMG and nerve conduction studies together as an integrated electrodiagnostic consultation, examining at least 4-5 muscles per limb across multiple nerve distributions, with both upper and lower extremities tested. 1, 2

Essential Pre-Test Clinical Assessment

History Elements to Obtain

• Pattern and distribution of weakness: Determine if proximal versus distal, symmetric versus asymmetric, and temporal progression (acute, subacute, chronic) 3
• Associated sensory symptoms: Numbness, paresthesias, pain, and their distribution (length-dependent versus patchy) 4
• Reflex changes: Hyperreflexia suggests upper motor neuron involvement; hyporeflexia/areflexia suggests peripheral nerve or root pathology 4
• Bulbar symptoms: Dysphagia, dysarthria, or respiratory difficulty may indicate motor neuron disease or myasthenia gravis 5, 3
• Recent critical illness or ICU stay: Raises suspicion for ICU-acquired weakness 4, 5
• Systemic disease markers: Autoimmune conditions (especially SLE), diabetes, or exposure to neurotoxins 4

Physical Examination Specifics

• Manual muscle testing (MMT): Test at least 12 muscle groups bilaterally using Medical Research Council (MRC) scale; composite MRC score <48/60 (or <80% of maximum) defines significant weakness 4
• Sensory examination: Map out sensory deficits to distinguish polyneuropathy (stocking-glove) from mononeuropathy multiplex or radiculopathy 4
• Deep tendon reflexes: Document presence, absence, or asymmetry 4
• Gait assessment and functional testing: Observe for foot drop, steppage gait, or proximal weakness patterns 4
• Cranial nerve examination: Especially cranial nerves III, IV, VI, VII, and VIII if cranial neuropathy suspected 4

Baseline Laboratory Studies

• Complete blood count and metabolic panel: Screen for systemic illness 4
• Creatine kinase (CK): Elevated in myopathy or rhabdomyolysis 5
• Thyroid function tests: Hypothyroidism can cause myopathy 5
• Hemoglobin A1c and glucose: Diabetic neuropathy is common 1
• Vitamin B12 and folate levels: Deficiency causes neuropathy 6
• Antiphospholipid antibodies and autoimmune markers: If SLE or vasculitis suspected 4
• CSF analysis: Reserved for suspected inflammatory demyelinating polyradiculoneuropathy (AIDP/Guillain-Barré) showing elevated protein with normal cell count 4

Electrodiagnostic Testing Protocol

Nerve Conduction Studies (NCS)

NCS should always be performed in conjunction with needle EMG, not in isolation, as NCS alone provides incomplete diagnostic information and may miss critical diagnoses. 2

• Test at least 4 sensory and 4 motor nerves spanning both upper and lower extremities 3
• Stimulate at least 3 sites in 3 different nerves when searching for conduction blocks that distinguish motor neuropathy from motor neuron disease 3
• Measure conduction velocity (CV) and amplitude: CV reflects myelin integrity; amplitude reflects axonal integrity 6, 7
• Distinguish axonal versus demyelinating patterns: Demyelinating shows slowed CV with preserved amplitude; axonal shows reduced amplitude with normal/near-normal CV 4, 6
• Identify distribution: Generalized (polyneuropathy), multifocal (mononeuropathy multiplex), or focal (entrapment) 6

Needle Electromyography (EMG)

Needle EMG is essential and must be combined with NCS; when used alone, EMG has only 50% positive predictive value for weakness diagnosis. 1

• Examine at least 4-5 muscles per limb in both upper and lower extremities, totaling minimum 8-10 muscles 3
• Include proximal and distal muscles: This distinguishes myopathy (proximal) from neuropathy (distal) 3
• Test muscles from multiple nerve and root distributions: Ensures detection of non-myotomal patterns in motor neuron disease 3
• Examine paraspinal muscles: Abnormalities indicate radiculopathy rather than peripheral nerve lesions 3
• Include bulbar muscles if clinically indicated: Tongue and facial muscles for suspected motor neuron disease 3
• Document spontaneous activity: Fibrillations and fasciculations indicate denervation; their presence and distribution guide diagnosis 3
• Assess motor unit potentials: Myopathic changes (short duration, low amplitude, polyphasic) versus neurogenic changes (long duration, high amplitude, reduced recruitment) 5, 6

Timing Considerations

Critical timing caveat: Electrodiagnostic studies performed within the first week of symptom onset are normal in 30-34% of patients with active demyelinating disease. 1

• If initial studies are normal but clinical suspicion remains high, repeat testing in 2-3 weeks to capture evolving abnormalities 1
• For ICU patients: Test early (Day 2-10) in uncooperative/sedated patients; test later (after 2-7 days of persistent weakness) in cooperative patients with abnormal MMT 4

Common Diagnostic Patterns

Polyneuropathy

• NCS show diffuse, relatively uniform abnormalities across multiple tested nerves 1
• EMG reveals length-dependent patterns of abnormal motor unit activity, typically worse distally 1
• Classic diabetic neuropathy with stocking-glove distribution and reduced ankle reflexes can often be diagnosed clinically without electrodiagnostic testing 1

Radiculopathy

• NCS may be completely normal because nerve root compression occurs proximal to the dorsal root ganglion 7
• Needle EMG is essential: Shows denervation in myotomal distribution including paraspinal muscles 7, 2

Motor Neuron Disease (ALS)

• NCS primarily exclude treatable motor neuropathy that mimics ALS 3
• Needle EMG demonstrates widespread non-myotomal denervation (both acute and chronic) with fibrillations and fasciculations in at least 3 limbs plus bulbar/thoracic regions 3

ICU-Acquired Weakness

• Both EMG (90% of studies) and NCS (84% of studies) are standard for this diagnosis 4, 1
• Testing can detect critical illness polyneuropathy/myopathy before clinical examination is possible in sedated patients 5

Myopathy

• NCS are typically normal as sensory nerves are unaffected 5
• Needle EMG shows myopathic motor unit potentials: Short duration, low amplitude, early recruitment, polyphasic potentials 5, 6

Critical Pitfalls to Avoid

• Never perform NCS alone without needle EMG except in rare circumstances; this leads to missed diagnoses of myopathy, radiculopathy, plexopathy, motor neuron disorders, and most peripheral nerve disorders 2
• Do not repeat electrodiagnostic studies routinely in stable patients: Serial clinical examinations are preferred; repeated studies do not improve management and increase cost 1
• Reserve repeat testing for clinical uncertainty about new or worsening neurological processes 1
• Ensure the same physician supervises and interprets both NCS and EMG: Interpretation requires comprehensive neuromuscular knowledge and correlation with clinical context 8, 2
• Recognize that early testing may be falsely negative: If suspicion remains high despite normal initial studies, repeat in 2-3 weeks 1