What is the role of Nerve Conduction Studies in Hansen's Disease?

Leprosy is one of the most significant infectious neuropathies, widespread in 91 countries, with India carrying 50% of the global leprosy burden. The bacterium, M. Leprae, targets the cold areas of sensory nerve and skin, beginning with distal nerve endings, then progressively involving dermal cutaneous sensory and mixed nerves. Its propensity for these areas often results in a diagnostic clinical picture.

In lepromatous leprosy, the bacteria multiply within macrophages and Schwann cells, leading to inflammation and increased bacterial load in the epineurium and endoneurium. The involvement of intradermal nerves may result in asymptomatic cases or distinct clinical manifestations, ranging from an anesthetic skin patch to acute neuropathy in lepra reactions.

Recently, a study by Chaudhary et al. was published in Neurology India, which explored the role of Nerve Conduction Studies (NCS) in diagnosing, documenting, and prognosticating leprosy. The study found sensory NCS abnormalities in 46 (43.6%) and motor NCS abnormalities in 38 (32.2%) nerves, resembling mono-neuritis multiplex due to axonal changes. Though these abnormalities corresponded with respective sensory or motor impairments, they failed to predict disability or outcome.

In cases where the clinical diagnosis was suggested by thickened nerves and maculo-anesthetic patches, the NCS results did not always align with clinical findings. For instance, 11 patients had thickened nerves, but their NCS results were normal. This discrepancy can be attributed to the limitation of NCS in evaluating small nerves, small-diameter, and unmyelinated nerve fibres.

In lepra reactions, notable nerve conduction abnormalities were observed due to increased immune activity and nerve damage from cytokine release. Patients with these reactions exhibited nerve conduction abnormalities across multiple nerves. Despite this, data on Nerve Conduction Velocity (NCV) studies during lepra reactions remain sparse.

Notably, leprosy may result in painful neuropathy. However, NCS are unlikely to detect abnormalities in these cases, as pain signals are conducted by small-diameter fibres, which are not evaluated by standard NCS. This limitation is exemplified in a patient case with a nerve abscess in the peroneal nerve, despite receiving Multi-Drug Therapy (MDT) for a year. This case underscores the importance of regular monitoring using NCS and ultrasonography.

Though NCS correlated with respective sensory or motor abnormalities, there was no significant association with the World Health Organization (WHO) disability grade or the disease outcome. Disabilities affecting daily living may not significantly alter NCS, but they can severely impact the patient's quality of life.

In conclusion, the study underscores the importance of NCS in objectively documenting nerve involvement in leprosy and confirming the nature of nerve involvement (axonal or demyelinating multiplex mononeuritis). However, NCS were not found to be indicative of disability grade or disease outcome. The early diagnosis and prompt treatment of leprosy remain critical in minimising nerve damage and mitigating disease progression.

Reference

Role of Nerve Conduction Studies in Hansen's Disease

Sarvesh K Chaudhary, Jayantee Kalita, Usha K Misra

Neurology India, Vol. 71, No. 3, May-June 2023, pp. 458-462

DOI: 10.4103/0028-3886.378653