ABPMR Neuromuscular Medicine cheat sheet

ALS requires upper and lower motor neuron dysfunction in at least two regions (bulbar, cervical, thoracic, lumbosacral) with progression; sensory, bowel, and bladder findings are atypical and should prompt an alternative diagnosis.

Needle EMG in ALS shows active denervation (fibrillation potentials, positive sharp waves) plus chronic reinnervation (large-amplitude, long-duration, polyphasic MUAPs) in at least two myotomes, while sensory nerve conduction studies remain normal.

Spinal muscular atrophy: about 95% have a homozygous SMN1 exon 7 deletion; SMN2 copy number inversely correlates with severity (type 1 usually 2 copies, type 3 usually 3-4 copies).

Kennedy disease (spinobulbar muscular atrophy): X-linked CAG repeat expansion in the androgen receptor gene; bulbar and limb weakness, gynecomastia, facial fasciculations, and a sensory axonal neuropathy on NCS.

Hirayama disease: adolescent males with asymmetric distal upper limb atrophy (C7-T1); flexion cervical MRI shows anterior displacement of the posterior dura with cord compression.

Syringomyelia: suspended cape-like loss of pain and temperature, lower motor neuron signs at the cavity level, and spasticity below; commonly associated with Chiari I malformation.

Anterior spinal artery infarction: bilateral weakness and spinothalamic sensory loss below the level with preservation of proprioception and vibration (posterior columns).

Brown-Séquard syndrome: ipsilateral spastic weakness and proprioceptive loss with contralateral pain and temperature loss beginning one to two segments below the lesion.

Poliomyelitis: asymmetric febrile lower motor neuron paralysis with preserved sensation; chronic electrodiagnostic testing shows enlarged, stable motor units in affected segments without active denervation.

Radiculopathy: sensory nerve action potentials are typically normal because the lesion lies proximal to the dorsal root ganglion; needle EMG is the primary localizing test.

To localize a root, needle EMG requires abnormal muscles in the same myotome innervated by different peripheral nerves, plus paraspinal (multifidus) fibrillation potentials.

C5-C6 and C7 are the most frequently affected cervical roots; L5 and S1 are the most frequently affected lumbosacral roots.

Root avulsion: preserved SNAPs despite dense denervation in the corresponding myotome; MRI or CT myelography may show a pseudomeningocele.

Parsonage-Turner syndrome: abrupt severe shoulder pain followed by patchy weakness; long thoracic, suprascapular, and anterior interosseous nerves are most often involved.

Upper trunk (C5-C6) Erb palsy weakens shoulder abduction and elbow flexion; lower trunk (C8-T1) Klumpke palsy weakens hand muscles and may produce Horner syndrome.

Pancoast tumor compresses the lower plexus (C8-T1), producing hand weakness, medial forearm numbness, and Horner syndrome.

Radiation plexopathy is usually painless and progressive with myokymic discharges on EMG; neoplastic infiltration causes severe pain, rapid progression, and no myokymia.

CMT1 is demyelinating with uniform conduction slowing (median motor velocity typically below 38 m/s); CMT2 is axonal with normal or mildly reduced velocities and reduced CMAP and SNAP amplitudes.

HNPP results from PMP22 deletion; NCS shows mild generalized slowing plus focal conduction slowing or block at common entrapment sites (carpal tunnel, elbow, fibular head).

Guillain-Barré syndrome: albuminocytologic dissociation (elevated CSF protein without pleocytosis); early NCS may be normal, so look for absent H reflexes, prolonged F waves, conduction block, and prolonged distal latencies.

Vasculitic neuropathy: painful, asymmetric, stepwise, non-length-dependent axonal sensorimotor neuropathy; nerve biopsy shows transmural inflammation with axonal degeneration.

Diabetic distal symmetric polyneuropathy: length-dependent, sensory-predominant axonal loss; the sural SNAP is reduced early and abnormalities follow a stocking-glove gradient.

Ulnar neuropathy at the elbow: conduction slowing or block across the elbow segment; a normal dorsal ulnar cutaneous SNAP argues against a lesion at or below Guyon canal.

Common peroneal neuropathy at the fibular head: foot drop with preserved ankle inversion (tibialis posterior) and a normal ankle jerk, distinguishing it from L5 radiculopathy and sciatic neuropathy.

Carpal tunnel syndrome: prolonged median sensory distal latency compared with the ulnar or radial response, or prolonged median motor distal latency; thenar atrophy indicates axonal loss and a worse prognosis.

Myasthenia gravis: fatigable weakness, often ocular or bulbar at onset; anti-AChR antibodies in about 85% of generalized disease, anti-MuSK in 5-8%, anti-LRP4 in a small minority, and some patients are seronegative.

Repetitive nerve stimulation at 2-3 Hz: a decrement greater than 10% between the first and fourth CMAP is diagnostic and is most often found in proximal muscles.

Single-fiber EMG shows increased jitter and blocking and is the most sensitive test for neuromuscular junction failure, but it is not specific and can be abnormal in ALS and myopathy.

Anti-MuSK myasthenia gravis: prominent bulbar, facial, and respiratory weakness with frequent atrophy and poor response to thymectomy; thymoma occurs in about 10-15% of anti-AChR disease.

Lambert-Eaton syndrome: presynaptic VGCC antibodies, proximal weakness, hyporeflexia with post-exercise facilitation, autonomic symptoms, and small-cell lung cancer association in older patients.

Lambert-Eaton electrodiagnosis: low baseline CMAP amplitude with a marked increment (often over 100%) after about 10 seconds of exercise or 20-50 Hz stimulation; sensory studies are usually normal.

Botulism: descending flaccid paralysis with early bulbar and autonomic symptoms, mydriasis, low CMAP amplitudes, and an incremental response to high-frequency stimulation.

Congenital myasthenic syndromes are genetic (CHRNE, RAPSN, DOK7, COLQ), antibody-negative, and begin in infancy or childhood; some subtypes respond to ephedrine or salbutamol rather than pyridostigmine.

Myopathic EMG: short-duration, low-amplitude, polyphasic MUAPs with early full recruitment at low effort; fibrillation potentials and positive sharp waves indicate active inflammation or necrosis.

Dermatomyositis: perifascicular atrophy, perivascular inflammation, and capillary membrane attack complex deposition; anti-Mi2, TIF1-gamma, NXP2, and MDA5 antibodies (MDA5 predicts rapidly progressive interstitial lung disease).

Polymyositis: endomysial CD8-positive T cells invading non-necrotic fibers; anti-synthetase antibodies such as Jo-1 accompany myositis, interstitial lung disease, mechanic's hands, arthritis, and Raynaud phenomenon.

Immune-mediated necrotizing myopathy: sparse inflammation with abundant necrosis; anti-SRP and anti-HMGCR (statin-associated) forms require immunosuppression and do not resolve with statin withdrawal alone.

Inclusion body myositis: after age 50, asymmetric finger flexor and quadriceps weakness, mildly elevated CK, rimmed vacuoles, mixed myopathic and neurogenic EMG; poorly responsive to immunotherapy.

Duchenne and Becker muscular dystrophy: X-linked dystrophinopathy with Gowers sign, calf pseudohypertrophy, and markedly elevated CK; cardiomyopathy is a major cause of death in Duchenne.

Myotonic dystrophy type 1: CTG expansion in DMPK with distal weakness, grip and percussion myotonia, cataracts, cardiac conduction disease, and diabetes; EMG shows myotonic discharges.

Metabolic myopathies: McArdle disease (myophosphorylase deficiency) causes exercise cramps, myoglobinuria, and second-wind phenomenon with a flat lactate rise on forearm ischemic exercise; CPT II deficiency causes episodic rhabdomyolysis.

Steroid myopathy: proximal weakness with normal CK and normal or mildly myopathic EMG; biopsy shows type II fiber atrophy.

Sensory NCS may be performed antidromically (median, ulnar) or orthodromically (sural, superficial radial); amplitudes and latencies are compared with age-matched normative values and the contralateral side.

Reduced CMAP amplitude indicates motor axonal loss or conduction block; reduced SNAP indicates sensory axonal loss or ganglionopathy; a normal SNAP with abnormal CMAP suggests a preganglionic or motor-axon lesion.

H reflex (S1, soleus) is absent or prolonged in S1 radiculopathy and demyelinating polyneuropathy; F waves assess proximal motor segments but may be normal in radiculopathy.

Conduction block: amplitude or area drop greater than 50% across a nerve segment with normal distal amplitude and no excessive temporal dispersion, indicating focal demyelination.

Needle EMG timing: fibrillation potentials and positive sharp waves appear 2-3 weeks after acute nerve injury, peak over the following weeks, and diminish as reinnervation occurs.

Motor unit remodeling: reinnervated units become larger in amplitude and longer in duration with reduced recruitment, whereas myopathic units are small and recruit early.

Myokymic and neuromyotonic discharges suggest demyelination, radiation plexopathy, or nerve hyperexcitability; complex repetitive discharges are nonspecific findings of chronic injury.

For suspected neuromuscular junction disease, test a weak, warm muscle with repetitive stimulation; if nondiagnostic, proceed to single-fiber EMG or exercise testing.