Linear lesions

Dr Jean Watkins, a sessional GP in Hampshire

Linear epidermal naevi

Linear epidermal naevi usually occur unilaterally on the trunk or limbs. They are thought to be due to point mutations in keratin genes causing malformation in the epidermis. They may be present at birth or develop in infancy. At first they appear as flat, itchy, pigmented plaques but may increase in size and develop a thickened warty surface. Occasionally keratoacanthoma or basal or squamous cell carcinoma may develop in the lesion and the patient should be warned to watch for any changes. The lesion may require excision or laser therapy.

Lichen striatus

This man presented with a linear, scaly rash running the length of one leg. He had no symptoms but was concerned about the cause. A diagnosis of lichen striatus was made and the patient reassured that the rash should settle spontaneously within a year. If dryness was a problem he could apply emollients.

The cause of lichen striatus is unknown. Many patients with the condition will have a family history of atopy, and autoimmune factors have been mooted but not proven. It is rare in adults and occurs in females two or three times as often as in males.

Phytophotodermatitis

This man was concerned about this streaky, blistering rash on his right arm that had developed the day before. On questioning, he volunteered that he had worked on his allotment at the weekend. The appearance of the rash was typical of phytophotodermatitis from a reaction between UV radiation and skin contact with furocoumarins that are found in plants such as parsnips, hogweed, celery or beans. The typical rash usually appears about 24 hours after exposure and should settle within a few days. A topical steroid can help ease the symptoms.

Allergic contact dermatitis

There are times when the site and appearance of an eczematous rash will suggest the cause. In this man's case, the distinct linear pattern across the dorsum of his foot and toes pointed to a recently-acquired pair of sandals, where contact matched the distribution of the rash. He was referred for patch testing to identify the allergen, and found to react to mercaptochemicals often found in shoes, rubber products, glues for plastic, leather and insulation tape. The patient was advised to frequently apply emollients and a topical steroid ointment to the feet and to avoid contact with known allergens.

Flagellate erythema

It is hard to believe that this rash is not the result of scratching. The linear red streaks may be itchy and tend to appear most commonly on the back and flanks. It may occur in association with treatment with bleomycin, other chemotherapeutic agents, or dermatomyositis. In this case, the patient had had none of these problems but had recently eaten a Chinese meal that included Shiitaki mushrooms. These have been known to induce this type of rash if not thoroughly cooked. This rash settled without treatment, but irritation may be eased by antihistamines or topical steroids.

Digitate dermatitis

Digitate dermatitis is also known as chronic superficial scaly dermatitis (CSSD) or small plaque parapsoriasis. The rash develops gradually, most commonly on the lower limbs but also on the trunk or upper limbs. The linear, finger-like patches may be pink or brownish-yellow and generally cause little in the way of symptoms. It may be difficult to distinguish CSSD from cutaneous T-cell lymphoma and biopsy should therefore be taken in order to make the correct diagnosis. CSSD tends to remain static but does sometimes clear spontaneously.

Cutaneous larva migrans

Increasing foreign travel may lead to presentation of larva migrans in the surgery. Initially, the patient may notice a prickly sensation at the point of entry of the larva of a hookworm. Two or three weeks later a snake-like track appears as it moves under the skin.

These parasites are picked up by an animal host where they penetrate the skin before migrating to the lungs and reaching the trachea. From here they are swallowed and lay their eggs in the intestine.

Fortunately, in humans they are unable to penetrate the basement membrane of the skin and get no further.

Linear morphea

Morphea is a form of localised scleroderma. The cause is unknown. A unilateral linear band develops, most often on the lower or upper limbs but it may also occur in the forehead or trunk. Both the superficial and deeper layers of the dermis are involved. Atrophic changes lead to what looks like a disfiguring scar, due to the loss of subcutaneous tissue.

In this patient, the 'scar' ran the length of her leg. Potent or intralesional steroids or calcipotriol may be tried to improve the appearance of the lesion but specialist advice may be needed. A biopsy of a lesion will confirm the diagnosis.

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