Terminal Latency Index (TLI) Abnormalities in Hereditary Neuropathy withLiability to Pressure Palsies (HNPP)

Introduction: The diagnosis of Hereditary Neuropathy with Liability to Pressure Palsy (HNPP) is frequently missed or delayed because of its clinical and electrophysiological heterogeneity. Nonetheless, electrophysiological criteria were published 15 years ago. Our aim was to highlight the Terminal Latency Index (TLI) disorders by describing the electrophysiological characteristics in patients with HNPP. Methods: All the consecutive patients with the clinical and genetic diagnosis of HNPP due to the 1.5Mb deletion of PMP22 or to others mutation or deletion were included. Motor and sensory conductions were studied using surface electrodes placed near the median, ulnar, fibular, tibial, superficial fibular and sural nerves in accordance with standard techniques. The results were compared with our own laboratory standards. Results: All of the 22 included patients had low TLI in the median, ulnar and peroneal nerves, whereas 6% of the patients had low tibial TLI. The other frequent electrophysiological characteristics were increased distal motor latency and low sensory conduction velocity, especially of the median nerve. Conclusions: TLI abnormalities are almost constant in HNPP, which is a great argument to define HNPP as a distally accentuated myelinopathy. TLI disorders should be added to HNPP electrophysiological criteria to improve their diagnosis accuracy.