Hereditary Motor Neuropathy With Liability to Pressure Palsies (HNPP)
In hereditary motor neuropathy with liability to pressure palsies (HNPP), nerves become increasingly sensitive to pressure and stretch.
(See also Overview of Peripheral Nervous System Disorders.)
In HNPP, nerves lose their myelin sheath and do not conduct nerve impulses normally. Inheritance is usually autosomal dominant. In 80%, the cause is loss of one copy of peripheral myelin protein-22 gene (PMP22), located on the short arm of chromosome 17. Two copies of the gene are needed for normal function.
Incidence of HNPP is estimated to be 2 to 5/100,000.
Usually, symptoms start during adolescence or young adulthood, but they may start at any age.
Peroneal nerve palsy with footdrop, ulnar nerve palsy, and carpal tunnel syndrome commonly develop. The pressure palsies can be mild or severe and last from minutes to months. Numbness and weakness occur in affected areas.
After an episode, about half of affected people completely recover, and symptoms are mild in most of the rest.
HNPP should be suspected in patients with any of the following:
Electrodiagnostic testing and genetic testing aid in diagnosis; rarely, biopsy is required.