This parakeratotic condition of the finger tip was first described more than 50 years ago.
It usually occurs in girls of approximately 7 years of age, typically affecting only one digit, usually a finger (Figure 5.45). The lesions start close to the free margin of the nail of a finger or toe. In some cases, a few isolated pustules or vesicles may be observed in the initial phase;
these usually disappear before the patient presents to the doctor. Confluent eczematoid changes cover the skin immediately adjacent to the distal edge of the nail. The affected area is pink or of normal skin colour and densely studded with fine scales; there is a clear margin between the normal and affected areas. The skin changes may extend to the dorsal aspect of the finger or toe, but usually only the finger tip is affected. The most striking and characteristic change is the hyperkeratosis beneath the nail tip. The nail plate is lifted up, deformed and often thickened. Commonly the deformity produced is asymmetrical and limited to one corner of the distal edge, or at least more pronounced at the corners of the nail. Pitting occurs in some cases; rarely, transverse ridging of the nail plate is present. In most cases the condition resolves within a few months, but in some cases it may persist for many years, even into adult life.
Histological findings are of some value, including hyperkeratosis and parakeratosis, pustulation and crusts, acanthosis and mild exocytosis, papillomatosis and heavy cellular infiltrates composed mainly of lymphocytes and fibroblasts around dilated capillary loops. This histological picture presents many of the features common to psoriasis and eczema.
In the differential diagnosis of parakeratosis pustulosa, the following points are important:
• Pustules are rare and are only seen in the initial stage, unlike pustular psoriasis or acropustulosis.
• Patients with psoriasis develop a coarse sheet of scales and not the fine type of scaling typically seen in parakeratosis pustulosa.
• If the nail changes predominate, especially on a toe, the disorder can be mistaken for onychomycosis. Thumb sucking, which is a predisposing factor in chronic candidal paronychia, should be ruled out when a single thumb is affected.
No treatment makes any difference to the frequency of recurrence or the overall duration of parakeratosis pustulosa. Topical steroids provide some symptomatic relief.
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