A bump under the nail is an elevation or mass developing beneath the transparent nail plate, originating from the underlying nail bed or nail matrix tissue. This area is highly sensitive, and a structural change can produce noticeable symptoms, ranging from pressure and discoloration to intense pain. Determining the cause requires understanding the specific location and nature of the growth, which can range from common, benign injuries to rare, serious conditions.
Causes Related to Minor Trauma and Inflammation
The most frequent causes of a bump under the nail stem from mechanical injury or localized inflammatory responses, often presenting as temporary or self-resolving issues. A subungual hematoma, commonly known as a blood blister, occurs when direct trauma crushes the nail, causing blood vessels in the nail bed to rupture and bleed. This pooling of blood is trapped between the nail plate and the bed, resulting in a dark, reddish-purple or black discoloration and intense, throbbing pain due to pressure buildup. The bump will naturally grow out as the nail advances, typically resolving completely over several months as the damaged portion is replaced.
Another benign yet rapidly growing lesion is a pyogenic granuloma, which is a highly vascular tumor-like growth of small capillaries, not an infection. This condition frequently develops after minor trauma or chronic irritation, appearing as a soft, reddish mass that bleeds easily upon contact. If the growth extends beneath the nail plate, it can lift the nail and create a noticeable bump, sometimes mimicking a more serious condition due to its size. Periungual warts, caused by the human papillomavirus (HPV), can also manifest as a bump when they occur under the nail (subungual). These warts are difficult to treat due to their location and can be painful, causing the nail surface to become raised, ridged, or deformed.
Specific Nail Bed Growths and Infections
Some bumps arise from specific biological processes within the nail unit, including chronic infections and rare benign tumors. Onychomycosis, a fungal infection of the nail, is a common cause of thickening and apparent elevation due to the accumulation of subungual hyperkeratosis, a debris-like material under the nail plate. The infection changes the nail structure, leading to discoloration, brittleness, and a buildup that creates a bump as the nail lifts off the bed. This condition is often chronic and requires antifungal treatment to resolve the structural changes.
One rare, benign tumor originating in the nail matrix is the onychomatricoma, which uniquely affects the growth and appearance of the nail plate. Clinically, this tumor presents with a thickened, yellow-discolored nail plate that often displays a distinct transverse overcurvature. Examination of the free edge may reveal multiple small, channel-like cavities or a “woodworm-like” appearance, corresponding to the tumor’s finger-like projections extending into the nail plate.
A glomus tumor is a small, benign growth that arises from the glomus body, a specialized structure involved in temperature regulation, and is frequently found beneath the nail. While often small, this tumor causes disproportionately intense, localized pain that is sensitive to cold temperatures or direct pressure. The bump may present as a subtle blue or purple discoloration beneath the nail, and this extreme tenderness is often the primary symptom prompting medical consultation.
Identifying Urgent Warning Signs
While most subungual masses are benign, certain features signal the potential for malignancy and require immediate professional evaluation. The most concerning growth is subungual melanoma, a rare but aggressive form of skin cancer that originates in the nail matrix. This condition typically appears as a longitudinal brown or black streak on the nail plate, which may become wider or darker over time and often has irregular borders.
A significant warning sign is Hutchinson’s sign, which describes the spread of pigment from the nail plate onto the surrounding skin of the cuticle or lateral nail folds. This outward migration of color suggests that pigment-producing cells are active in the nail matrix and are extending into the adjacent skin, which is a strong indicator for melanoma. Clinicians also use the ABCDEF mnemonic to assess pigmented lesions, looking for factors such as a broad band (B), a change in size or color (C), and extension of pigment (E).
Another potential malignancy is subungual squamous cell carcinoma, which may present as a persistent, non-healing sore, a rough, wart-like growth, or a chronic infection that does not respond to standard treatment. General red flags that necessitate a biopsy include any bump that is rapidly growing, spontaneously bleeding, or causing persistent pain without a history of significant trauma. A definitive diagnosis requires a full-thickness biopsy of the nail bed and matrix, which is the only way to distinguish a benign growth from cancer.