hyperkeratosis

Hyperkeratosis is a thickening of the stratum corneum, the outermost layer of the epidermis, due to increased keratin production or reduced shedding of corneocytes. It is a histopathological finding that can be a normal protective response, as in calluses from repeated friction, or a feature of various dermatologic and systemic conditions.

Classification of hyperkeratosis includes focal versus diffuse involvement and orthokeratotic versus parakeratotic forms. Orthokeratosis refers to a thickened outer layer without nuclei in the stratum corneum, while parakeratosis retains nuclei in that layer. Hyperkeratosis may affect skin surfaces such as palms and soles, or extend to mucosal membranes in certain disorders.

Causes and associations are diverse. Mechanical factors such as chronic friction or pressure produce focal hyperkeratosis (callus or corn). Inflammatory and immune-mediated diseases, including psoriasis and eczema, commonly show hyperkeratosis with accompanying epidermal changes. Genetic keratinization disorders, such as palmoplantar keratoderma and ichthyosis, feature widespread or patterned hyperkeratosis. Nutritional deficiencies, arsenic exposure, infections, and certain neoplasms can also be linked to hyperkeratotic changes. In the oral cavity, mucosal hyperkeratosis can arise from irritation, tobacco use, or ill-fitting dental appliances, and may be seen in conditions like leukoplakia.

Diagnosis is usually clinical, supported by histopathology when needed. Biopsy shows a thickened stratum corneum, with orthokeratosis or parakeratosis and variable accompanying epidermal changes.

Treatment targets the underlying cause and reduction of keratin buildup. This includes keratolytics such as salicylic acid or urea, emollients, and, for some conditions, topical or systemic retinoids. Management is individualized to the condition and affected site.