A Thumbnail Sketch

My patient, a 33-year-old man who works with his hands, presented with a growthunder his left thumb nail (Figure). The thumb nail is slightly more sensitive thanthe other nails at times, but otherwise the growth does not bother him. A fungalculture was negative. The medical history and a review of systems were both noncontributory.I would appreciate your input.
-Rafael Olivares, MD
  Bergenfield, NJ

This man has an acquired nail plate dystrophy and an associated subunguallesion restricted to a single digit. The evaluation of thisabnormality begins with a detailed history.¹ For example, trauma to adigit might have either directly injured the nail matrix or preceded thedevelopment of a subungual exostosis; both scenarios could resultin a defect in the nail plate.

Causes of an abnormal nail plate include:

• A localized problem of 1 or more components of the nail unit, which includesthe underlying bone, soft tissue, andnail plate. Enchondromas, glomus tumors,and onycomycosis are examplesof such problems.
• A primary dermatologic conditionin which nail involvement is eitherthe sole clinical feature or one of severalcutaneous manifestations. Examplesinclude psoriasis vulgaris andlichen planus.
• A systemic disorder with secondaryonychodystrophy-such as koilonychia(concave, spoon-shaped nailplates) in patients with iron deficiencyanemia or distorted nail plates in patientswith cancer and subungualmetastases.

Workup. The initial evaluation includesinspection of the nails of all thefingers and toes.¹ Additional workupmay be necessary to determine thecause of an acquired nail plate dystrophy.A positive fungal culture of thedystrophic nail plate or underlying nailbed may establish a diagnosis of onychomycosis (when the organism is a dermatophyte,such as Trichophyton rubrum) or suggest the possibility of secondaryfungal involvement of a previously abnormal nail plate (when a saprophytic organismis identified). Radiographs of the involved digit can help determinewhether the nail plate dystrophy is secondary to either a lesion or an infection ofthe subungual bone or soft tissue. However, in some patients, partial or total nailplate avulsion and biopsy of either the nail bed, nail matrix, or both may be necessaryto determine the cause of the dystrophy.

Differential diagnosis. The photograph of this patient's thumb shows asubungual lesion that emerges from beneath the free edge of the nail plate;dystrophy of the nail plate is localized to the portion of the nail plate that overliesthe subungual lesion (see Figure). The morphology of this lesion is similar to that of lesions identified by Baran and Haneke² as "distal subungual keratoses."The authors describe the distal subungual keratosis as "a small hornylesion originating from the hyponychium, resembling a forme fruste of thesubungual filamentous tumor." They note that this lesion "may be traced clinicallyand histologically as far as the lunula."²

In addition to distal subungual keratosis, subungual filamentous tumor,and (acquired digital) fibrokeratoma, the differential diagnosis for a distally locatedsubungual hyperkeratosis includes:

• An ungual manifestation of certain dermatologic or systemic conditions.These include lichen planus, psoriasis vulgaris, keratosis follicularis (Darier-White disease), and incontinentia pigmenti (Bloch-Sulzberger syndrome).
• Tumors-both benign and malignant-of the nail apparatus and subungualexostoses. Benign subungual tumors that can present as hyperkeratotic lesionsinclude verrucae and fibromas (angiofibromas and dermatofibromas);malignant tumors that can originate beneath the nail plate and whose morphologycan appear hyperkeratotic include keratoacanthomas, squamous cell carcinomasin situ (Bowen's disease), and squamous cell carcinomas.

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Philip R. Cohen, MD
   Clinical Associate Professor
   Department of Dermatology
   University of Texas Houston Medical School

References:

REFERENCES:1. Cohen PR. How to nail down a diagnosis for a patient with "dirty" nails. Consultant. 1999;39:968.
2. Baran R, Haneke E. Tumours of the nail apparatus and adjacent tissues. In: Baran R, Dawber RPR, eds.Diseases of the Nails and Their Management. 2nd ed. Oxford, England: Blackwell Scientific Publications;1994:417-497.