Subungual hyperkeratosis refers to the accumulation of scale or debris beneath the nail plate, causing the underlying tissue to thicken. The term is descriptive: “sub” means under, “ungual” refers to the nail, and “hyperkeratosis” signifies an excessive production of keratin. This condition is a sign of an underlying process, resulting from the body’s reaction to chronic irritation or inflammation of the nail bed. The thickening pushes the nail plate upward and away from the nail bed, a process known as onycholysis.
Physical Appearance of Subungual Hyperkeratosis
The visible effect is a noticeable thickening and elevation of the nail plate, often beginning at the tip. The material that builds up beneath the nail is typically chalky, crumbly, and appears yellowish or white. This accumulation of keratinous debris can be slight, moderate, or severe; thicknesses exceeding three millimeters are considered serious.
While subungual hyperkeratosis can affect both fingernails and toenails, it is frequently more pronounced and problematic in the toes. Pressure from footwear on the elevated nail can cause pain and discomfort, potentially making walking difficult. The presence of this elevated, discolored material also creates an environment conducive to secondary infections. Since the appearance can be similar across different causes, physical features alone are insufficient for a definitive diagnosis of the root problem.
Underlying Conditions That Cause Subungual Hyperkeratosis
Subungual hyperkeratosis occurs because of a heightened rate of cell turnover in the nail bed, often caused by inflammation or infection. The most frequent cause is a fungal infection of the nail, medically known as onychomycosis, which accounts for a large number of cases. Onychomycosis is usually caused by dermatophytes, such as Trichophyton rubrum, which invade the nail unit and trigger the hyperkeratosis as a host response.
Inflammatory skin conditions are another major category of causes, most notably nail psoriasis. When psoriasis affects the nail bed, it causes skin cells to reproduce too quickly, leading to the characteristic buildup of scale underneath the nail. Psoriatic nail disease may also be associated with psoriatic arthritis, and it is possible to have both psoriasis and a fungal infection simultaneously, which can complicate the clinical picture. Other dermatological conditions, like eczema or Pityriasis rubra pilaris, can also trigger this excessive keratin production.
Chronic trauma and friction represent a non-infectious, non-inflammatory cause. Repeated mechanical stress, such as that from ill-fitting shoes or activities that constantly jar the nail, can irritate the nail bed and lead to a defensive hyperkeratosis. This chronic injury can initiate the process, sometimes creating an entry point for secondary fungal or bacterial invaders. Identifying the precise underlying cause is important because the treatment strategy for a fungal infection differs significantly from that of an inflammatory disease.
How Doctors Diagnose Subungual Hyperkeratosis
The diagnostic process begins with a thorough physical examination and medical history to assess the nail’s appearance and identify potential risk factors. Since subungual hyperkeratosis is a symptom rather than a diagnosis, the primary goal of testing is to distinguish between fungal and non-fungal causes, particularly psoriasis, as the visual signs can be nearly identical for both conditions.
The most common initial test involves collecting a sample of the keratinous debris from under the nail for microscopic examination. This sample is often subjected to a Potassium Hydroxide (KOH) preparation, which dissolves human cells to reveal fungal elements like hyphae. While the KOH prep is a quick screening tool, a fungal culture may also be performed to definitively identify the specific species of fungus, though this process can take several weeks.
If the diagnosis remains unclear or an inflammatory cause like psoriasis is suspected, a nail biopsy may be necessary. A biopsy involves removing a small piece of the affected nail bed tissue for detailed histopathological analysis. A Periodic Acid-Schiff (PAS) stain performed on the biopsy provides a highly sensitive method for detecting fungal elements, even when KOH or cultures yield negative results.
Management and Treatment Options
Treatment for subungual hyperkeratosis is always directed at resolving the identified underlying cause, whether it is an infection or an inflammatory condition. For cases confirmed to be onychomycosis, antifungal medications are the standard approach. Topical antifungals, such as lacquers containing ciclopirox or efinaconazole, may be prescribed for milder cases.
More severe or persistent fungal infections often require systemic, or oral, antifungal drugs to clear the infection from the nail matrix and bed. Oral medications are generally taken for several months, reflecting the slow growth rate of the nail, and may be used in combination with topical treatments for enhanced efficacy. This dual approach helps ensure the drug reaches the entire affected nail unit.
When the hyperkeratosis is caused by nail psoriasis, treatment focuses on reducing the inflammation and slowing the excessive cell production. First-line therapies frequently involve high-potency topical corticosteroids or Vitamin D analogs, such as calcipotriol, massaged into the nail fold and cuticle. For widespread or severe psoriatic nail disease, systemic therapies like methotrexate or newer biologic drugs may be used to suppress the immune response. Regardless of the cause, mechanical reduction of the thickened material, known as debridement, can be performed to relieve pressure and allow topical medications to penetrate the nail bed more effectively.