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Neuromuscular, Peripheral Nerve, and Motor Unit Disorders

Manual: General · Subject: Neurology

Cover peripheral neuropathies, myopathies, motor neuron disease, neuromuscular junction disorders, and modern diagnostic algorithms.

The Motor Unit as a Diagnostic Axis

Anatomy of Weakness

Weakness can arise from upper motor neuron, lower motor neuron, peripheral nerve, neuromuscular junction, or muscle disease. Careful pattern recognition is essential because each level has a distinct physiologic signature.

Localizing Weakness

Upper motor neuron

  • Spasticity, hyperreflexia, Babinski sign
  • Often due to brain or spinal cord disease
  • No fasciculations as a defining feature

Lower motor neuron / peripheral

  • Atrophy, fasciculations, hyporeflexia
  • Due to anterior horn cell, root, plexus, nerve, or junction disease
  • Distribution can be focal or length-dependent

Electrodiagnostic Patterns

Demyelination
Slowed conduction and prolonged distal latencies
Axonal loss
Reduced amplitude with relatively preserved velocity early
Myopathy
Short-duration, low-amplitude motor unit potentials
Junction disorder
Decrement or facilitation depending on test paradigm

Which finding most strongly suggests a myopathic process rather than neuropathy?

Why are reflexes often preserved early in primary muscle disease?

Approach to Progressive Weakness

  1. 1

    Step 1: Determine whether weakness is truly motor rather than due to pain, fatigue, or incoordination.

  2. 2

    Step 2: Localize to central, anterior horn, root, plexus, nerve, junction, or muscle.

  3. 3

    Step 3: Use reflexes, sensory findings, distribution, and cranial nerve involvement to refine localization.

  4. 4

    Step 4: Confirm with EMG, imaging, laboratory studies, and genetics as indicated.

A patient with fluctuating ptosis and fatigable proximal weakness most likely has disease of which site?

Frontier Topics

Emerging work includes autoantibody-defined neuropathies, molecular subtyping of motor neuron disease, ultrahigh-resolution nerve imaging, and gene-targeted therapies for inherited neuromuscular disease.