Electrodiagnostic features of hereditary neuropathy with liability to pressure palsies

Objective: Because diagnosis of hereditary neuropathy with liability to pre ssure palsies (HNPP) frequently is missed or delayed, we looked for electro diagnostic features that raise suspicion of the disorder by making comparis ons with two more common diseases that mimic it electrophysiologically: chr onic inflammatory demyelinating polyneuropathy (CIDP) and diabetic polyneur opathy. Methods: A retrospective review of the neuromuscular laboratory dat abase was performed. Results: Nine HNPP subjects, 22 with CIDP and 49 with diabetic polyneuropathy. Of all the HNPP nerves studied, abnormally slow se nsory nerve conduction velocity (SNCV) was found in 93%, prolonged distal m otor latencies (DML) in 78%, slow motor nerve conduction velocity in 31%, a nd prolonged F-wave latencies in 90%. Mean SNCV for HNPP was 85.6% +/- 10.6 % of the lower limit of normal and significantly slower than for CIDP (114. 3% +/- 20.1%; p < 0.0001) or diabetes (108.1% +/- 14.8%; p < 0.0001). Exclu ding the carpal tunnel, site from the analysis did not alter this observati on: Mean DML were more prolonged in HNPP, even without median nerve data in the analysis (118.5% +/- 31.0% of the upper limit of normal), than in CIDP (103.2% +/- 31.6%; p < 0.05) or diabetes (86.3% +/- 18.3%; p < 0.0001). Me an HNPP motor nerve conduction velocity was within normal limits. Conclusio ns: According to findings, hereditary neuropathy with liability to pressure palsies (HNPP) has a distinctive background polyneuropathy independent of superimposed entrapment neuropathy. It is characterized by diffuse sensory nerve conduction velocity (SNCV) slowing and prolongation of distal motor l atencies with relatively infrequent and minor reduction of motor nerve cond uction velocities. This indicates disproportionate distal conduction slowin g in the disorder.