Electrophysiological testing
Nerve conduction studies include:
- amplitude of action potential
- conduction velocity of motor and sensory nerves
- latency
Hereditary motor sensory neuropathies mostly show marked slowing of conduction velocity due to demyelination, except in type 2 Charcot-Marie-Tooth disease, which has similar clinical features to typical CMT but no demyelination.
Hereditary spinocerebellar degenerations such as Friedrich's ataxia show reduced or absent sensory nerve action potentials with little or no slowing of conduction velocities.
Electromyography can show evidence of denervation and renervation in neuropathies: initially brief biphasic potentials becoming prolonged, high-voltage polyphasic potentials in chronic disease. Myopathies have low-voltage polyphasic potentials
Genetic testing
The molecular genetics of the hereditary neuropathies is increasingly understood. Classical Charcot-Marie-Tooth disease is due to a duplication on chromosome 17 coding for peripheral myelin protein 22. Other myelin components such as connexin-32 and myelin P0 have been identified as defects in other forms of the syndrome. Commercial genetic tests exist for PMP-22 and some of the other abnormal myelin components.
Friedrich’s ataxia is caused by duplication of a nucleic acid triplet in chromosome 9q, which codes for the iron chelation protein frataxin. This defect can be detected by a polymerase chain reaction technique.
There is a rapidly growing literature on the molecular genetics of inherited neurological diseases, which is giving increasing options for genetic diagnosis
Other tests
Serum creatine phosphokinase is markedly increased in Duchenne and Becker muscle dystrophies and less so in other muscle dystrophies and myopathies.
Nerve biopsies may be useful in making a differential diagnosis of a peripheral neuropathy