Leprosy | 11 Differential diagnosis

11 Differential diagnosis

Initially the differential diagnosis must take into account a large number of other diseases. Fixed drug eruption, morphea (localised scleroderma), dermatophytosis, dermal filariasis, eczema, scars, nodular cutaneous leishmaniasis, post-kala azar dermatitis and keloids may exhibit clinical similarities.

Lepromatous leprosy can be very similar to diffuse cutaneous leishmaniasis and even to cutaneous lymphoma (mycosis fungoides).

Keloidal blastomycosis ( = lobomycosis or Lobo’s disease) is very rare and occurs almost exclusively in the Amazon and Orinoco basins, although some cases have been known from Surinam and Central America. The disease, also known as, is caused by a fungus, Loboa loboi, and may be clinically very similar to lepromatous leprosy. The diagnosis is easy with a skin biopsy.

Systemic lupus erythematosus (SLE) may be taken for leprosy. Skin and mucosal lesions of lupus erythematosus discoides, necrobiosis lipoidica (check for hyperglycaemia) and of porphyria cutanea tarda (lesions chiefly on the hands and face, where exposed to the light) may pose diagnostic problems.

Neurofibromatosis (Recklinghausen’s disease) sometimes causes a problem in differential diagnosis. [In neurofibromatosis type 1, 100% of the children have café au lait patches before they are 2 years old, 70% have freckles in the skin folds (axilla) and 90-100% of patients also have hamartomas in the iris (Lisch’s nodules) as well as neurofibromas by the time they are 20 years old. In the rarer type 2 (NF2) café au lait patches only occur in 1% and the freckles are absent. In NF2 the peripheral nerves may develop schwannomas, but in these patients acoustic neurinomas are the most common].

Annular skin lesions which are similar to tuberculoid leprosy may also occur in cutaneous sarcoidosis (lupus pernio), granuloma annulare, granuloma multiforme, syphilis, actinic granulomas and Jessner-Kanof ’s lymphocytic skin infiltration (pseudolymphoma; aetiology unknown). A Sutton’s naevus is generally easy to recognise (ring-shaped depigmentation with central hyperpigmentation).

Annular psoriasis is characterised by the presence of thick scales which usually exhibit symmetrical distribution, with enlarged blood vessels in the dermis. There may be pustules or pitting of the nails and/or arthropathy. Köbner’s phenomenon may occur.

Granuloma annulare is more difficult to differentiate. It is a benign skin disorder characterised by a granulomatous inflammatory process, which manifests itself in a ring or annular configuration of papules. The lesions usually occur in the region of a joint (the hands, elbows), but may also occur elsewhere. There is no neural dysfunction. An aetiological association with sunlight is assumed, but this is only one hypothesis. Most lesions (75%) heal spontaneously in 1-2 years. It is possible that granuloma multiforme is a variant of granuloma annulare. Biopsy is usually central to the diagnosis. In view of the strong similarity to leprosy and since granuloma multiforme is regularly seen and treated as leprosy, it is advisable to study a number of photographs of people with this disorder, or better still the patients themselves, in order to become familiar with the clinical picture.

Light-coloured skin spots also occur in pityriasis alba (slightly scaly, on skin exposed to the light), Gibert’s pityriasis rosea, pityriasis versicolor (Gr. "pityron" = bran; refers to the light skin scaliness). Pityriasis versicolor is a very common skin infection with a fungus: Pityrosporum ovale (yeast stage) or Malassezia furfur (mycelium stage). This lipophilic fungus forms the tyrosinase inhibitor azelaic acid from sebaceous fats, a substance which inhibits melanin synthesis. This explains the white appearance of the skin spots. Account must be taken of the fact that depigmented skin spots can also be caused by damage to the melanocytes (pigment cells) after an ordinary infection, wound or burn (post-inflammatory hypopigmentation), due to vitiligo (the texture of the skin with this condition is otherwise normal), as a residual lesion in endemic treponematosis and syphilis (the differential diagnosis is often difficult here). It is important to know that people with leprosy often have a false positive VDRL (screening for syphilis). TPHA [the Trepanoma pallidum haemagglutination test] permits differentiation.

There are not many neuropathies where temperature and pain sensation are diminished, while sparing vibration and position sense, as well as sparing deep tendon reflexes. One should consider in these cases primary amyloidosis and syringomyelia (lesion of the crossing fibers of the central grey matter of the spinal cord) in the differential diagnosis. Less than 10% of the leprosy cases develop secondary amyloidosis. Patients with primary / hereditary amyloidosis usually have pronounced autonomic neuropathy from the onset, with episodic diarrhoea, impotence, decreased sweating, postural hypotension and other evidence of impaired vasomotor control.