Electrodiagnostic features of hereditary neuropathy with liability to pressure palsies

Objective: Because diagnosis of hereditary neuropathy with liability to pressure palsies (HNPP) frequently is missed or delayed, we looked for electrodiagnostic features that raise suspicion of the disorder by making comparisons with two more common diseases that mimic it electrophysiologically: chronic inflammatory demyelinating polyneuropathy (CIDP) and diabetic polyneuropathy. Methods: A retrospective review of the neuromuscular laboratory database was performed. Results: Nine HNPP subjects, 22 with CIDP and 49 with diabetic polyneuropathy. Of all the HNPP nerves studied, abnormally slow sensory nerve conduction velocity (SNCV) was found in 93%, prolonged distal motor latencies (DML) in 78%, slow motor nerve conduction velocity in 31%, and 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). Excluding the carpal tunnel, site from the analysis did not alter this observation: 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). Mean HNPP motor nerve conduction velocity was within normal limits. Conclusions: 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 latencies with relatively infrequent and minor reduction of motor nerve conduction velocities. This indicates disproportionate distal conduction slowing in the disorder.