Hereditary neuropathy with liability to pressure palsies (HNPP) is an inherited peripheral nerve disorder characterized by recurrent mononeuropathy usually triggered by minor physical activities.
The actual prevalence is unknown due to under-diagnosis but estimates range between 1/50,000-1/20,000. In Finland the prevalence is reported to be 1/6250. The disease affects males and females equally.
Disease onset usually occurs in the 2nd to 3rd decade of life, but rarely can present in childhood. Some patients are asymptomatic and are never diagnosed. The most common presenting symptom is the sudden onset of painless focal sensory loss and muscle weakness in the distribution of a single nerve. In many cases, these acute focal symptoms are triggered by mechanical stresses to the nerve, such as compression, repetitive movement or stretching of the affected limbs. Commonly affected nerves include the peroneal nerve at the fibular head, ulnar nerve at the elbow, median nerve at the wrist, brachial plexus and radial nerve. Clinical manifestations caused by these mononeuropathies include foot drop, hand numbness and weakness, arm weakness, and sensory loss over the index finger and thumb or lateral aspect of the hand. In 50% of cases, patients recover from these episodes within a few days to months but others have incomplete recovery and suffer from recurrent focal sensory and motor deficits. In rare cases, brachial plexopathy with unilateral painless arm paralysis and sensory loss may be a presenting symptom. Occasionally, mild impairments of cranial nerve functions may be seen. Absent deep tendon reflexes and pes cavus foot deformity are seen in some patients but not in others. The phenotype of HNPP often evolves into a symmetric sensory motor polyneuropathy in aged patients.
HNPP is due to a mutation in the PMP22 gene (17p11.2-12), encoding the peripheral myelin protein-22 (PMP22) that is predominantly expressed in the compact myelin of the peripheral nervous system. In 80% of cases a 1.4Mb deletion at 17p11.2-12 that includes the PMP22 gene is found, and in the remaining 20%, patients carry a point mutation or small deletion in PMP22 or mutations in as yet unidentified genes.
Diagnosis of HNPP is typically suggested by the presence of recurrent focal mononeuropathies and the evidence of a family history. Electrophysiologic testing shows prolonged distal latency at the sites susceptible to mechanical stress. Conduction velocities out of these sites are often either normal or only mildly slowed. Tomacula (focal thickening of the myelin sheath) is the characteristic histological finding in sural nerve biopsies. It may however be seen in other types of neuropathies. A PMP22 mutation confirms diagnosis of HNPP.
Differential diagnosis includes both compression induced mononeuropathies and generalized polyneuropathies. Hereditary neuralgic amyotrophy (see this term) can sometimes be mistaken for HNPP.
Prenatal testing is theoretically possible in families where the disease causing gene has been identified.
HNPP is inherited autosomal dominantly and genetic counseling is beneficial.
In some, treatment is unnecessary as recovery can be spontaneous. Those with foot drop or wrist drop may benefit from an ankle-foot orthosis or a wrist splint. Protective pads for the elbows and knees can alleviate nerves from mechanical stresses. Activities that involve prolonged sitting with crossed legs, leaning on elbows and repetitive movements of the wrists, as well as rapid weight loss should be avoided. Vincristine should equally be avoided.
HNPP is not life threatening. Prognosis is good as half of patients recover fully after an episode and half with an incomplete recovery. Residual symptoms are usually relatively mild and do not result in severe disability. However, in some, severe and prolonged limb paralysis can occur, particularly after strenuous physical activities.
Last update: September 2013