Characteristics of Leprosy Neuropathy

Asymmetric sensory loss is the hallmark of leprosy neuropathy. Nerve hypertrophy is also often present, although not always detected on examination. Nerve ultrasonography is effective in detecting nerve hypertrophy and should be considered in patients with idiopathic asymmetric neuropathy.

There are some characteristics which together are extremely suggestive of leprosy neuropathy:

• 1.
  

  There are two patterns of nerve involvement in leprosy. Dermal nerve injury does not follow a specific nerve territory distribution, and affects thermal, pain, tactile, and autonomic fibers of the skin in a mosaic distribution. Nerve trunk injury compromises sensation and muscle strength in the territory of specific nerves. Superposition of both patterns results in a very suggestive clinical presentation.

  
  
• 2.
  

  M. leprae has a predilection to involve initially the dermal nerves and the unmyelinated and small myelinated nerve fibers of the nerve trunks. Consequently, abnormalities of proprioception, vibration, motor function, and reflexes are usually seen late in the disease course.

  
  
• 3.
  

  M. leprae has a predilection to invade and proliferate in nerves localized in the coolest areas of the body, such as the elbow, dorsal regions of the forearm, dorsal hand, knees, lateral aspects of the legs, earlobes, and malar region, resulting in a temperature-dependent distribution, in clear contrast to the length-dependent pattern observed in most polyneuropathies. Even in advanced disease, a careful sensory examination will demonstrate that the warmest body parts have intact (or at least partially preserved) sensation. The confluent neuropathy seen in advanced cases may superficially simulate a length-dependent symmetrical polyneuropathy, but a careful examination will not reveal a glove-and-stocking distribution, but rather a very asymmetrical pattern when involvement of the coolest and the warmest regions of the lower limbs, upper limbs, trunk, and face are compared.

  
  
• 4.
  

  The preference of M. leprae to invade and multiply in the coldest and superficial regions probably explains the preferential sites of involvement of the nerve trunks in leprosy, resulting in focal nerve damage. The ulnar nerve is typically involved 10–15 cm above the olecranon and 2–4 cm proximal to the wrist, the median nerve 5–7 cm proximal to the carpal tunnel, the radial nerve 4–6 cm proximal to the elbow, the fibular nerve at the head of the fibula, and the posterior tibial nerve at the ankle and above the gastrocnemius muscle. The facial branches are asymmetrically involved. Affected nerve segments may be enlarged and tender. Neurophysiologic studies usually show focal slowing of nerve conduction at these sites with distal axonal degeneration.

  
  
• 5.
  

  Although leprosy neuropathy mainly results in negative sensory and motor manifestations, positive sensory symptoms—such as neuropathic pain—may be debilitating.

  
  
• 6.
  

  The final pattern of nerve injury depends on the balance between body temperature and the immunological competence of the host, which is at least partially genetically determined. Individuals who are highly resistant to the bacilli (tuberculoid pole of leprosy) will have a mononeuropathy or a restricted mononeuritis multiplex, with involvement of only a few nerves, which will usually be severely damaged. In contrast, in subjects with no resistance to M. leprae (lepromatous and borderline-lepromatous patients), there will be a diffuse and slowly progressive neuropathy with late nerve destruction. In these patients, early diagnosis and treatment is essential to avoid a diffuse and incapacitating neuropathy. Those patients with intermediate resistance to M. leprae (borderline leprosy) may present with a less discrete neuropathy, as their immune response to M. leprae is not strong enough to prevent dissemination of the bacilli but is sufficient to mount a marked local response resulting in severe damage to the affected nerves.

Intradermal sensory loss in leprosy neuropathy is most often apparent at the elbows, knees, and ears. The ulnar, superficial peroneal, radial, median, and sural are the most frequently involved sensory nerves. Weakness is most severe in the distributions of the ulnar, median, peroneal, and tibial nerves. Subjects in the tuberculoid pole generally present with a mononeuropathy or mononeuritis multiplex, while those in the lepromatous pole have a more diffuse mononeuritis multiplex, with both nerve trunk and dermal involvement.

The absence of protective sensation predisposes affected individuals to soft tissue injuries and other trophic manifestations. As proprioception and motor function are involved late in the disease course, injury to insensate areas may not be recognized as being due to neuropathy, especially in children and those with low educational background. Trophic ulcers are most commonly plantar, but ulcers may also develop over the knees, hands, ears, nose, and eyes. Persisting infection may be complicated by bone resorption.

Some clinical variants of leprosy neuropathy have been described but most are unconfirmed, do not affect children, or are just a variation of the classic phenotype. Silent neuritis is progressive neural damage that occurs in the absence of symptoms.

Lepromatous reactions are episodes of acute change in the immunological host status, usually (but not always) in response to treatment. The episodes may result in acute, severe nerve damage. The reversal reaction (type 1 reaction) occurs mainly in subjects with borderline and tuberculoid leprosy, due to an increase in cell-mediated response. There is effective destruction of the bacilli, but this response may also damage the peripheral nerve. Erythema nodosum leprosum (type 2 reaction) occurs in lepromatous and borderline lepromatous patients, usually in association with death of bacilli. It is a systemic reaction, caused by a vasculitis affecting medium and small arteries and arterioles, which should be promptly treated to avoid permanent damage.