Despite extensive investigation, the cause of chronic neuropathy cannot be identified in 30% of such cases.
The following conditions require exclusion before a chronic neuropathy is classified as idiopathic or of unknown aetiology: diabetes, uraemia, deficiency states, connective tissue disorders, paraproteinaemias, underlying malignancy, drugs and toxins. Hereditary disease can sometimes be excluded by clinical examination and genetic analysis of relatives.
The cause of acute or subacute neuropathy can usually be defined. Here, CSF examination may prove a useful diagnostic investigation, e.g. Guillain-Barre syndrome.
An electrical stimulus is applied at points along a nerve (20-100 V for 0.05-0.1 ms) and the evoked muscle response recorded. By applying a stimulus at various points along a nerve and recording the latency between stimulus and muscle response the motor conduction velocity of a particular nerve may be measured.
,, , Distance between two stimuli
Difference in conducuon time between the two sites
Motor conduction velocity can be measured in most motor peripheral nerves from the brachial plexus in the upper limbs and sciatic and femoral outlets in the lower limbs.
These studies not only aid in the diagnosis of generalised neuropathies but also in entrapments, e.g. ulnar nerve at elbow or median nerve at wrist (carpal tunnel syndrome).
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