Discovering a black line beneath your nail can trigger immediate concern, and rightfully so. These dark streaks, medically known as longitudinal melanonychia, represent various underlying conditions ranging from benign trauma to serious systemic diseases. Understanding the diverse causes of subungual pigmentation enables proper assessment and timely intervention when necessary. The nail unit serves as a window into systemic health, with colour changes often reflecting both local pathology and broader medical conditions affecting the body’s pigmentation processes.
Modern dermatology recognises multiple aetiologies for nail discolouration, each presenting distinct clinical characteristics that aid in differential diagnosis. From traumatic haemorrhages to infectious processes, the spectrum of causes requires systematic evaluation to distinguish between harmless cosmetic concerns and potentially life-threatening conditions. Healthcare professionals increasingly emphasise the importance of early recognition, particularly given the potential for subungual melanoma masquerading as benign pigmentation.
Subungual haematoma: traumatic blood accumulation beneath nail plates
Subungual haematomas represent the most common cause of acute black discolouration beneath nail plates, resulting from blood accumulation in the potential space between the nail bed and nail plate. These haematomas typically develop following direct trauma to the nail unit, causing capillary rupture and subsequent haemorrhage. The characteristic appearance includes a well-demarcated area of dark pigmentation that moves distally as the nail grows, distinguishing it from other forms of longitudinal melanonychia that remain fixed at the nail matrix.
Acute injury mechanisms leading to capillary rupture
Acute trauma mechanisms vary considerably in their presentation and severity. Sudden impact injuries, such as hammer strikes or door crushing incidents, create immediate pressure increases within the nail bed, causing widespread capillary disruption. The resulting haematoma typically presents as a large, dark area covering significant portions of the nail bed. Pain intensity correlates directly with the extent of underlying tissue damage, often accompanied by throbbing sensations that peak within the first 24-48 hours post-injury.
Crush injuries and direct impact trauma to nail bed
Crush injuries represent particularly severe forms of subungual trauma, often involving significant soft tissue damage beyond simple haemorrhage. These injuries frequently occur in industrial settings or during home improvement activities. The mechanism involves compression forces exceeding the nail bed’s structural integrity, causing extensive vascular disruption and potential damage to underlying bone structures. Recovery timelines for crush injuries extend considerably beyond simple haematomas, often requiring 6-12 months for complete nail regeneration depending on matrix involvement.
Sports-related microtrauma in tennis and football athletes
Athletic activities frequently cause repetitive microtrauma to nail structures, particularly in sports involving sudden directional changes or prolonged footwear pressure. Tennis players commonly develop subungual haematomas on their dominant hand’s fingernails due to racquet grip pressure and ball impact forces. Football players experience similar injuries affecting toenails from boot friction and ball contact. These sports-specific injuries often present as linear streaks rather than large haematomas, reflecting the chronic nature of repetitive trauma patterns.
Occupational hazards in construction and manufacturing environments
Construction and manufacturing environments present numerous opportunities for nail trauma through tool-related injuries and material handling incidents. Carpenters frequently experience subungual haematomas from mis-struck nails or tool slippage, whilst manufacturing workers face risks from machinery-related crush injuries. Protective equipment usage significantly reduces injury incidence, though complete prevention remains challenging given the nature of manual labour activities. Occupational safety protocols increasingly emphasise nail protection as part of comprehensive hand safety programmes.
Longitudinal melanonychia: pigmentation disorders and melanin distribution
Longitudinal melanonychia encompasses a broad spectrum of pigmentation disorders characterised by vertical dark bands extending from the nail matrix to the distal edge. These pigmentation patterns result from increased melanin production or melanocyte proliferation within the nail matrix, creating characteristic linear streaks. Understanding the underlying mechanisms enables differentiation between benign physiological variants and pathological conditions requiring medical intervention.
Benign melanocytic hyperplasia in nail matrix
Benign melanocytic hyperplasia represents increased melanocyte numbers within the nail matrix without malignant transformation. This condition commonly affects individuals of darker skin types, presenting as single or multiple longitudinal bands of varying widths. The pigmentation typically remains stable over time, showing consistent colour and width measurements. Hormonal influences, particularly during pregnancy, can trigger melanocytic activation leading to temporary or permanent band formation. Dermatoscopic examination reveals regular pigmentation patterns with uniform colour distribution, distinguishing benign hyperplasia from malignant processes.
Subungual melanoma: hutchinson’s sign recognition
Subungual melanoma represents the most serious cause of longitudinal melanonychia, requiring immediate recognition and treatment. Hutchinson’s sign, characterised by pigmentation extending beyond the nail plate onto surrounding skin, serves as a critical diagnostic marker. This aggressive malignancy typically affects the thumb or great toe, presenting as an irregular, widening band with variegated colouration. Early detection significantly improves prognosis, with five-year survival rates exceeding 90% for localised disease compared to less than 50% for advanced stages.
Early recognition of subungual melanoma through systematic nail examination protocols can dramatically improve patient outcomes and survival rates in clinical practice.
Ethnic variations in physiological melanonychia patterns
Ethnic background significantly influences melanonychia prevalence and presentation patterns. Individuals of African, Asian, and Hispanic descent demonstrate higher rates of physiological melanonychia, with studies indicating prevalence rates exceeding 70% in some populations. These variations reflect genetic differences in melanocyte distribution and activity within nail structures. Cultural considerations become important when evaluating pigmentation changes, as patterns considered normal in certain ethnic groups might raise concern in others. Clinical assessment must account for baseline pigmentation patterns when determining whether melanonychia represents pathological change.
Drug-induced hyperpigmentation from antimalarials and chemotherapy
Medication-induced melanonychia affects numerous drug classes, with antimalarials and chemotherapy agents representing the most common culprits. Hydroxychloroquine and chloroquine frequently cause nail pigmentation through melanocyte stimulation, typically developing weeks to months after treatment initiation. Chemotherapy-induced pigmentation affects up to 60% of patients receiving certain regimens, particularly those containing bleomycin or doxorubicin. These changes often resolve following treatment discontinuation, though permanent pigmentation can occur with prolonged exposure. Medication history becomes crucial in determining whether pigmentation changes represent drug effects versus other pathological processes.
Bacterial and fungal subungual infections causing discolouration
Infectious processes represent significant causes of nail discolouration, often presenting with accompanying inflammation and structural changes. Bacterial and fungal organisms create pigmentation through various mechanisms, including direct pigment production, inflammatory responses, and tissue destruction. Recognition of infection-associated discolouration enables appropriate antimicrobial therapy and prevents progression to more severe complications.
Pseudomonas aeruginosa Green-Black nail syndrome
Pseudomonas aeruginosa creates characteristic green-black nail discolouration through pyocyanin and pyoverdin pigment production. This gram-negative bacterium thrives in moist environments, commonly affecting individuals with chronic nail exposure to water or compromised nail integrity. The infection typically begins at the lateral nail folds, progressively involving the nail plate and creating distinctive blue-green to black discolouration patterns. Treatment requires topical or systemic antibiotics targeting gram-negative organisms, with ciprofloxacin representing first-line therapy for localised infections.
Onychomycosis presenting with dark streaking
Certain fungal species cause distinctive dark pigmentation patterns during nail infection processes. Trichophyton rubrum var. nigricans and Scytalidium dimidiatum commonly produce dark streaks or patches within infected nail plates. These organisms create pigmentation through melanin production or tissue destruction, often accompanied by nail thickening and brittleness. Diagnosis requires mycological examination including potassium hydroxide preparation and fungal culture. Antifungal therapy duration typically extends 6-12 months for complete resolution, with newer agents like tavaborole showing improved efficacy against resistant strains.
Candida albicans paronychia with secondary pigmentation
Candida albicans infections primarily affect the nail fold structures, creating secondary pigmentation changes through chronic inflammation and tissue damage. The organism thrives in warm, moist conditions, commonly affecting individuals with diabetes or immunocompromise. Initial presentation includes erythema and swelling of nail folds, progressing to nail plate involvement with potential pigmentation changes. Treatment combines topical antifungal agents with aggressive moisture control measures. Chronic candida paronychia may require systemic antifungal therapy and surgical intervention for persistent cases resistant to conservative management.
Systemic medical conditions manifesting as nail discolouration
Numerous systemic diseases manifest through nail pigmentation changes, reflecting underlying metabolic, endocrine, or autoimmune processes. These conditions often present with characteristic patterns enabling clinical recognition and prompting appropriate diagnostic evaluation. Understanding the relationship between systemic disease and nail changes facilitates early diagnosis and treatment of underlying conditions that might otherwise remain undetected.
Addison’s disease frequently causes longitudinal melanonychia as part of generalised hyperpigmentation resulting from elevated ACTH levels. The pigmentation typically affects multiple nails simultaneously, appearing as uniform dark bands extending from matrix to distal edge. Thyroid disorders, particularly hyperthyroidism, can trigger melanocytic activation leading to temporary or permanent nail pigmentation changes. Autoimmune conditions like lupus and scleroderma commonly present with nail fold changes accompanied by pigmentation alterations reflecting underlying vascular pathology.
HIV-associated nail changes include longitudinal melanonychia affecting up to 40% of patients, particularly those receiving certain antiretroviral medications. The pigmentation often develops gradually, affecting multiple nails with varying intensity. Nutritional deficiencies, especially B12, folate, and protein deficiency, can cause nail pigmentation changes reflecting impaired cellular metabolism and pigment production. Hepatic conditions frequently manifest through nail changes, with Wilson’s disease causing characteristic azure lunulae and chronic liver disease producing white nails with narrow distal bands.
Splinter haemorrhages: microvascular bleeding patterns
Splinter haemorrhages appear as thin, linear streaks of blood beneath the nail plate, resembling wood splinters embedded under the nail. These lesions result from capillary rupture within the nail bed, creating characteristic vertical lines that typically affect the distal portion of the nail. Understanding splinter haemorrhage patterns enables differentiation between traumatic and pathological causes, with implications for further diagnostic evaluation.
Traumatic splinter haemorrhages commonly result from minor nail injuries, fingertip trauma, or repetitive activities causing capillary stress. These lesions typically appear as isolated findings affecting single nails without accompanying systemic symptoms. Pathological splinter haemorrhages may indicate underlying cardiovascular disease, particularly bacterial endocarditis affecting up to 15% of patients with this serious infection. Multiple splinter haemorrhages affecting several nails simultaneously raise suspicion for systemic disease requiring comprehensive medical evaluation.
Recognition of multiple splinter haemorrhages as potential markers of systemic disease can lead to early detection of serious conditions like bacterial endocarditis or vasculitis.
Vasculitic conditions commonly produce splinter haemorrhages through immune-mediated capillary damage, with lesions often accompanying other manifestations of systemic vasculitis. Rheumatoid arthritis, lupus, and polyarteritis nodosa frequently present with nail fold capillary changes and associated splinter haemorrhages. Dermatoscopic examination of nail fold capillaries reveals characteristic patterns helping differentiate between various vasculitic conditions and guiding appropriate immunosuppressive therapy.
Chemical exposure and environmental staining of nail structures
Chemical exposure represents an often-overlooked cause of nail discolouration, with various substances capable of creating persistent pigmentation changes. Occupational and recreational chemical exposure can result in distinctive staining patterns that may persist for months following contact. Understanding common chemical causes enables proper diagnosis and prevents unnecessary concern about pathological processes.
Silver nitrate exposure commonly occurs in healthcare settings and photography, creating characteristic black staining that penetrates the nail plate. The pigmentation typically appears within hours of exposure and may persist until the affected nail grows out completely. Tobacco use creates yellow-brown staining affecting fingernails of heavy smokers, with pigmentation intensity correlating with smoking duration and frequency. Hair dye chemicals , particularly those containing paraphenylenediamine, can cause temporary nail staining during application, usually resolving within weeks of exposure cessation.
Industrial chemical exposure poses significant risks for persistent nail staining, with heavy metals like mercury and lead creating characteristic pigmentation patterns. Potassium permanganate exposure results in brown-black staining that may require aggressive treatment for removal. Henna application, whilst generally safe, can cause temporary orange-brown nail discolouration lasting several weeks. Photographic chemicals , including developers and fixers, commonly cause persistent nail staining in professional photographers and darkroom technicians, emphasising the importance of protective equipment usage during chemical handling procedures.