NCV study

Axonal Demylinating

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2. Peripheral nerve diseases

This encompasses a vast number of diseases and only a cursory overview will be attempted. 
 
Clinical features 

Damage to the peripheral nervous system results in motor, sensory and autonomic dysfunction.  A neuropathy is any disease of the nerves.  There are a number of different classes of neuropathies, but we will consider only one of them here.

Distal polyneuropathy: All the nerves are affected distally in the extremities.  Clinically the patients have sensory loss in a glove and stocking distribution, weakness and absent tendon reflexes in distal extremity muscles (e.g. ankle jerk).  Longer nerves are affected more severely and thus the changes predominate in the legs.  Most distal polyneuropathies are purely sensory or affect the sensory and motor nerves together.  Pure motor distal neuropathies are rare.  Depending on the etiology, the neuropathies can be axonal (axis cylinder), demyelinating, or show features of both.  Diseases that cause distal polyneuropathies include diabetes, toxins, and vitamin deficiency/alcohol abuse.  Many of these neuropathies are familial. 

EMG findings 

a) Predominantly axonal disease: Normal motor and sensory nerve conduction velocities with low or absent CMAP and SNAP amplitudes.  Needle examination shows large MUPs that result from denervation and subsequent re-innervation.  Fasciculations. 

b) Predominantly demyelinating disease: Relatively normal CMAP and SNAP amplitudes with slowed nerve conduction velocities.  Needle examination reveals normal MUPs as the axons are not damaged and the muscle fibers are not denervated.  In practice pure demyelination is rare and some associated axonal damage is common. 

Histology 

Type grouping and group atrophy only if there is axonal (axis cylinder) damage. 

3.  Neuromuscular transmission defects 

Only myasthenia gravis will be discussed further.  This disease is characterized by abnormal fatigue with exercise.  Myasthenia gravis commonly affects young woman and has a predilection for ocular, facial, masticator and proximal upper extremity muscles.  Typically the patients recover to some degree after rest.  Thus they feel much better in the morning, but become weaker as the day progresses.  When the extraocular eye muscles are affected, diplopia (double vision) and ptosis (drooping of upper eyelid) are common and bothersome signs.  This is an auto-immune disease with antibodies destroying the acetylcholine receptors (a postsynaptic defect). 

 

continued to Neuromuscular transmission defects