Dark spot under a toenail: what it may mean

The discovery of a dark spot beneath a toenail can trigger immediate concern, particularly given the well-publicised cases of celebrities like Bob Marley, who tragically succumbed to subungual melanoma. However, whilst such fears are understandable, the vast majority of dark nail discolorations stem from benign causes that require minimal intervention. Understanding the various aetiologies behind these pigmented lesions enables both healthcare professionals and patients to approach such findings with appropriate clinical perspective rather than unwarranted alarm.

Dark spots under toenails, medically termed subungual pigmentation , represent a diverse spectrum of conditions ranging from simple traumatic bleeding to complex malignant transformations. The key lies in recognising the distinctive clinical features that differentiate each condition, as proper identification directly influences management strategies and patient outcomes. Modern diagnostic approaches, including advanced dermoscopic techniques and targeted biopsy protocols, have significantly enhanced our ability to distinguish between benign and concerning lesions.

Subungual haematoma: Post-Traumatic nail bed bleeding

Subungual haematomas represent the most prevalent cause of dark discoloration beneath toenails, accounting for approximately 85% of all cases presenting to podiatric clinics. These lesions result from acute or chronic trauma that disrupts the delicate vascular network within the nail bed, leading to blood accumulation in the subungual space. The clinical presentation typically manifests as a well-demarcated dark red, purple, or black area that may occupy varying portions of the nail plate surface.

Acute Trauma-Induced capillary rupture mechanisms

The pathophysiology of subungual haematoma formation involves the mechanical disruption of capillaries within the highly vascularised nail bed matrix. Direct impact injuries, such as dropping heavy objects onto the foot or experiencing crush injuries during sporting activities, create immediate hydrostatic pressure increases that exceed the tensile strength of capillary walls. This mechanism explains why even seemingly minor impacts can produce significant bleeding when the force is concentrated over the small surface area of the nail bed.

Repetitive microtrauma represents another common mechanism, particularly prevalent amongst athletes and individuals wearing ill-fitting footwear. The constant friction and pressure from shoes that are too tight in the toe box create cumulative damage to the nail bed vasculature. This process may occur gradually over weeks or months, with patients often unable to identify a specific precipitating event when questioned about the onset of their nail discoloration.

Blood accumulation patterns beneath nail plate structure

The anatomical constraints of the nail unit significantly influence how blood distributes following capillary rupture. The nail plate acts as a rigid cover that prevents outward expansion of accumulated blood, instead forcing lateral and proximal spread within the confined subungual space. This containment effect explains the characteristic appearance of haematomas, which often present as crescentic or irregular patches with well-defined borders that correspond to the limits of blood spread.

Fresh haematomas typically display bright red coloration due to the presence of oxygenated haemoglobin. As cellular breakdown progresses, the chromatic evolution follows predictable patterns, transitioning through purple, brown, and eventually black phases as haemoglobin degrades into haemosiderin and other iron-containing compounds. This temporal colour progression serves as a valuable diagnostic tool for estimating the age of traumatic lesions.

Pressure-related pain symptomatology and nail deformation

The pain associated with acute subungual haematomas results from the mechanical pressure exerted by trapped blood against pain-sensitive structures within the nail bed. The severity of discomfort correlates directly with the volume of accumulated blood and the rate of accumulation. Rapidly forming haematomas following acute trauma typically produce intense, throbbing pain that may persist for several days until pressure equilibrium is established.

Chronic haematomas, developing gradually through repetitive microtrauma, often present with minimal pain despite potentially extensive discoloration. The nail plate may exhibit subtle deformational changes, including longitudinal ridging or transverse depression lines, reflecting the underlying structural damage to the nail matrix. These morphological alterations can persist even after the haematoma has resolved, serving as permanent markers of previous trauma.

Conservative management versus trephination procedures

Management decisions for subungual haematomas depend primarily on the timing of presentation, extent of involvement, and severity of associated symptoms. Conservative management remains the preferred approach for most cases, particularly those presenting more than 48 hours after initial trauma or occupying less than 50% of the nail surface area. This approach relies on the natural processes of blood resorption and nail growth to gradually eliminate the discoloration over several months.

Trephination procedures, involving the creation of small drainage holes through the nail plate, offer rapid symptom relief for acute, painful haematomas. The technique requires careful consideration of sterile conditions and appropriate patient selection to minimise complications such as secondary bacterial infection. Success rates exceed 95% when performed within the first 24-48 hours of trauma, with immediate pressure relief typically providing dramatic pain reduction.

Contemporary evidence suggests that early intervention through nail trephination not only provides immediate symptom relief but may also reduce the likelihood of permanent nail deformity by preventing prolonged pressure-induced matrix damage.

Melanonychia striata: pigmented nail matrix disorders

Melanonychia striata presents as longitudinal pigmented bands extending from the nail matrix to the free edge, representing a distinct clinical entity from the more commonly encountered subungual haematomas. These pigmented streaks result from increased melanin production within the nail matrix, creating characteristic linear patterns that can vary significantly in width, intensity, and multiplicity. Understanding the underlying mechanisms of melanocyte activation helps differentiate between physiological and pathological causes of nail pigmentation.

Melanocyte hyperactivation and longitudinal banding patterns

The development of melanonychia striata involves complex interactions between melanocytes residing within the nail matrix and various stimulating factors that promote increased melanin synthesis. These specialised cells, normally quiescent in individuals with lighter skin pigmentation, can become activated through mechanical trauma, inflammatory processes, or hormonal influences. The resulting melanin deposition follows the natural growth pattern of the nail plate, creating the characteristic longitudinal banding that distinguishes this condition from other forms of subungual pigmentation.

The width and intensity of pigmented bands correlate with the extent and degree of melanocyte activation within specific regions of the nail matrix. Single, narrow bands typically indicate focal stimulation of a small population of melanocytes, whilst multiple or broad bands suggest more extensive involvement. The pigmentation intensity can range from light brown to deep black, with variations often reflecting differences in melanin concentration and the depth of pigment deposition within the nail plate structure.

Hutchinson’s sign: periungual pigmentation extension

Hutchinson’s sign represents a critical diagnostic feature characterised by the extension of pigmentation from the nail plate onto the surrounding periungual skin, including the cuticle, nail folds, and hyponychium. This finding holds particular significance as it may indicate malignant transformation, distinguishing potentially dangerous lesions from benign melanonychia striata. The mechanism underlying this pigmentation extension involves the spread of activated melanocytes beyond the confines of the nail matrix into adjacent dermal structures.

Recognition of true Hutchinson’s sign requires careful examination under adequate illumination, as pseudo-Hutchinson’s sign can occur when pigmentation appears to extend onto periungual skin but actually represents visualisation of subungual pigment through thin, translucent cuticle tissue. This distinction proves crucial for clinical decision-making, as true Hutchinson’s sign mandates urgent dermatological evaluation and potential biopsy procedures.

Ethnic predisposition factors in benign melanonychia

Ethnic background significantly influences the prevalence and characteristics of benign melanonychia striata, with individuals of African, Asian, and Hispanic descent demonstrating markedly higher rates of physiological nail pigmentation. Studies indicate that up to 77% of African Americans over age 20 exhibit some degree of melanonychia, compared to less than 1% of Caucasian populations. This ethnic predisposition reflects genetic differences in melanocyte density and activity within nail matrix tissues.

The clinical presentation of ethnic melanonychia typically involves multiple nails with bilateral symmetry, narrow band widths (usually less than 3mm), and stable appearance over time. These characteristics help distinguish benign ethnic variants from pathological processes, which more commonly present as solitary lesions with progressive changes in width, colour intensity, or morphology. Understanding these ethnic variations prevents unnecessary anxiety and inappropriate interventional procedures in individuals with physiological pigmentation patterns.

Dermoscopic evaluation techniques for pigmented lesions

Dermoscopy has revolutionised the evaluation of pigmented nail lesions by providing magnified visualisation of structural details not apparent to naked-eye examination. This non-invasive diagnostic technique enables detailed assessment of pigmentation patterns, including the regularity of longitudinal lines, presence of colour variations, and identification of specific morphological features that may indicate malignant potential. Modern dermoscopic criteria for nail evaluation have significantly improved diagnostic accuracy whilst reducing the need for unnecessary biopsy procedures.

Key dermoscopic features suggesting benign melanonychia include regular parallel lines with uniform spacing, consistent brown coloration throughout the band, and sharp demarcation at the lateral borders. Concerning features that warrant further investigation include irregular line patterns, colour variations within the same band, micro-Hutchinson’s sign visible only under magnification, and progressive band widening documented through serial examinations.

Subungual melanoma: malignant transformation indicators

Subungual melanoma represents the most feared cause of dark nail discoloration, despite its relatively rare occurrence, accounting for only 0.7-3.5% of all melanomas globally. This malignant condition predominantly affects the thumb and great toe, with a marked predilection for individuals aged 50-70 years. The clinical challenge lies in early recognition, as delayed diagnosis significantly impacts prognosis, with five-year survival rates dropping from 95% for localised disease to 15% for metastatic cases. Understanding the subtle early manifestations and applying systematic evaluation criteria proves essential for optimal patient outcomes.

ABCDEF criteria application in nail unit assessment

The traditional ABCDE criteria for melanoma evaluation have been adapted specifically for nail unit assessment, with the addition of an ‘F’ criterion to create the comprehensive ABCDEF framework. This systematic approach provides clinicians with a standardised method for evaluating suspicious pigmented nail lesions. The ‘A’ criterion addresses Age (peak incidence 50-70 years) and anatomical location (thumb, great toe, or index finger), whilst ‘B’ evaluates Brown to Black coloration with Breadth greater than 3mm and irregular Borders.

The ‘C’ criterion encompasses Change in the nail band, including alterations in Colour, width, or morphology, whilst ‘D’ identifies Digit involvement, typically affecting a single nail in most cases. The ‘E’ criterion recognises Extension of pigmentation onto periungual skin (Hutchinson’s sign), and the ‘F’ criterion acknowledges Family or personal history of melanoma or dysplastic naevus syndrome. Application of these criteria enables systematic risk stratification and guides decisions regarding the need for tissue sampling.

Amelanotic melanoma presentation and diagnostic challenges

Amelanotic subungual melanoma presents unique diagnostic challenges due to the absence of characteristic pigmentation, instead manifesting as flesh-coloured or reddish nodular lesions that may mimic benign conditions such as pyogenic granulomas or viral warts. These non-pigmented variants account for approximately 25-30% of all subungual melanomas and often undergo delayed diagnosis due to their atypical appearance. The lack of pigmentation results from absent or minimal melanin production by the malignant melanocytes.

Clinical features suggestive of amelanotic subungual melanoma include rapidly growing nodular lesions, ulceration, bleeding, nail dystrophy, and failure to respond to conventional treatments for presumed benign conditions. The diagnosis requires a high index of suspicion, particularly in elderly patients presenting with solitary nail abnormalities that demonstrate progressive changes over time. Dermoscopic evaluation may reveal atypical vascular patterns and architectural distortion that provide valuable diagnostic clues.

Histopathological staging using breslow thickness measurements

Breslow thickness measurement represents the most critical prognostic factor for subungual melanoma, providing precise quantification of tumour depth from the granular layer to the deepest point of invasion. This measurement system, expressed in millimetres, directly correlates with metastatic potential and guides staging classification according to the American Joint Committee on Cancer (AJCC) criteria. Tumours measuring less than 1mm in thickness carry excellent prognosis, whilst those exceeding 4mm demonstrate significantly increased mortality rates.

The unique anatomical features of the nail unit complicate standard Breslow measurements due to the absence of a well-defined granular layer beneath the nail plate. Pathologists must adapt measurement techniques to account for these structural differences, often measuring from the nail plate surface or the level of the nail bed depending on the specific location of tumour involvement. Additional histopathological features, including mitotic rate, ulceration, and lymphovascular invasion, provide supplementary prognostic information for treatment planning.

Sentinel lymph node biopsy protocols for advanced cases

Sentinel lymph node biopsy (SLNB) protocols for subungual melanoma follow established melanoma guidelines, with specific considerations for the unique drainage patterns of nail unit anatomy. The procedure involves preoperative lymphoscintigraphy to identify the primary drainage basin, followed by intraoperative sentinel node localisation using radioactive tracers and blue dye techniques. For digit melanomas, drainage typically occurs to axillary nodes for upper extremity lesions and inguinal nodes for lower extremity involvement.

Patient selection for SLNB depends on tumour thickness, with current guidelines recommending the procedure for melanomas exceeding 1mm in Breslow depth or thinner lesions with adverse histological features such as ulceration or high mitotic rates. The information obtained from sentinel node evaluation provides crucial staging data that influences adjuvant treatment decisions and enables more accurate prognostic counselling for patients and families.

Early recognition and appropriate surgical management of subungual melanoma remain the most critical factors determining patient survival, emphasising the importance of maintaining clinical vigilance when evaluating any persistent or changing nail abnormality.

Fungal onychomycosis: Dermatophyte-Induced discolouration

Fungal nail infections, medically termed onychomycosis, can occasionally manifest with dark discoloration beneath the nail plate, challenging the conventional association of fungal disease with yellow or white nail changes. This variant presentation occurs in approximately 10-15% of onychomycosis cases and results from specific fungal species or secondary bacterial colonisation of the infected nail environment. The dark pigmentation typically develops gradually over months, often accompanied by characteristic nail thickening, brittleness, and structural deformity that helps distinguish fungal causes from traumatic or malignant aetiologies.

Certain dermatophyte species, particularly Trichophyton rubrum var. nigricans and Scytalidium dimidiatum , demonstrate inherent melanin production capabilities that directly contribute to nail darkening. Additionally, the altered nail environment created by fungal infection promotes secondary bacterial growth, particularly anaerobic species that produce dark pigments as metabolic byproducts. The combination of fungal structural damage and bacterial pigment deposition creates the characteristic appearance of darkly discoloured, dystrophic nails that may be confused with other subungual pathologies.

Diagnostic confirmation requires mycological examination through potassium hydroxide (KOH) preparation and fungal culture techniques. The presence of fungal hyphae and spores on KOH examination, combined with positive culture results identifying the specific causative organism, establishes the diagnosis and guides appropriate antifungal therapy selection. Treatment typically requires systemic antifungal agents such as terbinafine or itraconazole for 6-12 months, with cure rates varying depending on the causative organism and extent of nail involvement. Response monitoring through periodic mycological testing ensures treatment adequacy and helps detect potential resistance patterns.

Bacterial pseudomonas infections and chloronychia

Pseudomonas aeruginosa infections of the nail unit produce characteristic green-black discoloration, a condition termed chloronychia, which can be mistaken for other causes of dark nail pigmentation. This gram-negative bacterium thrives in moist environments and commonly affects individuals with compromised nail integrity, such as those with chronic paronychia, nail trauma, or immunocompromised states. The distinctive colo

uration stems from pyocyanin and pyoverdin pigments produced by the organism as part of its normal metabolic processes.

The infection typically develops following minor nail trauma or in the presence of chronic moisture exposure, conditions that compromise the nail’s natural barrier function. Healthcare workers, food handlers, and individuals with occupational water exposure demonstrate increased susceptibility due to frequent hand washing and prolonged moisture contact. The organism’s ability to form biofilms on nail surfaces enhances its persistence and resistance to topical antimicrobial treatments.

Clinical diagnosis relies on the characteristic green-black discoloration pattern, often accompanied by a distinctive sweet, fruity odour emanating from the affected nail. The pigmentation typically begins at the nail fold and progresses distally, creating irregular patches rather than the linear patterns seen in melanonychia striata. Bacterial culture confirmation identifies the specific Pseudomonas species and determines antibiotic sensitivity patterns for targeted therapy.

Treatment requires systemic antibiotic therapy, commonly ciprofloxacin or levofloxacin for 7-14 days, combined with topical antiseptic measures to eliminate surface colonisation. Resolution involves gradual fading of the green discoloration as the nail grows out, typically requiring 3-6 months for complete clearance. Preventive measures focus on maintaining nail hygiene, avoiding prolonged moisture exposure, and promptly addressing any nail trauma that might predispose to bacterial invasion.

Clinical differential diagnosis and biopsy indications

Establishing an accurate diagnosis for dark subungual lesions requires systematic evaluation incorporating clinical history, physical examination findings, and appropriate diagnostic testing modalities. The differential diagnosis encompasses a broad spectrum of conditions ranging from benign traumatic lesions to life-threatening malignancies, necessitating a structured approach that prioritises patient safety whilst avoiding unnecessary interventions for benign conditions.

Clinical history provides crucial diagnostic clues, with particular attention to the timeline of lesion development, presence of inciting trauma, occupational exposures, and family history of skin cancer. Acute onset following identifiable trauma strongly suggests subungual haematoma, whilst gradual development over months may indicate fungal infection, benign melanonychia, or malignant transformation. The presence of pain typically correlates with acute traumatic lesions or bacterial infections, whereas painless progressive lesions raise greater concern for malignant processes.

Physical examination should encompass detailed assessment of lesion morphology, including size, shape, colour uniformity, border characteristics, and any associated nail plate changes. Dermoscopic evaluation enhances diagnostic accuracy by revealing structural details invisible to naked-eye examination, particularly useful for distinguishing between haematomas and pigmented lesions. The presence of Hutchinson’s sign demands immediate attention, as this finding significantly elevates malignancy risk and mandates urgent specialist referral.

Biopsy indications for subungual pigmented lesions include any lesion demonstrating progressive change in size, shape, or colour intensity over time. New pigmented bands appearing after age 50, solitary lesions in Caucasian individuals, bands wider than 3mm, or any lesion associated with Hutchinson’s sign require tissue sampling for definitive diagnosis. The ABCDEF criteria provide a systematic framework for identifying high-risk lesions that warrant histopathological examination.

Contemporary consensus guidelines recommend tissue sampling for any subungual pigmented lesion that cannot be definitively diagnosed as benign based on clinical and dermoscopic features, as early detection remains the most critical factor determining outcomes in cases of subungual melanoma.

Biopsy techniques for nail lesions require specialised expertise due to the unique anatomical constraints and functional considerations of the nail unit. Punch biopsy through the nail plate allows sampling of the nail bed and matrix whilst preserving nail structure, though complete nail avulsion may be necessary for comprehensive tissue evaluation in cases with high malignancy suspicion. The choice of biopsy technique depends on lesion characteristics, anatomical location, and the degree of clinical concern for malignant transformation.

Laboratory investigations may support clinical diagnosis in specific scenarios, with fungal culture and KOH examination proving valuable for suspected onychomycosis cases. Bacterial culture helps confirm Pseudomonas infections whilst guiding antibiotic selection based on sensitivity patterns. However, tissue sampling remains the definitive diagnostic method for excluding malignancy in cases where clinical features raise concern for subungual melanoma.

The management of patients with dark subungual lesions requires balancing appropriate clinical vigilance with judicious use of invasive diagnostic procedures. Establishing clear follow-up protocols for lesions deemed clinically benign ensures ongoing monitoring for any changes that might indicate previously unrecognised malignant potential. Patient education regarding warning signs and the importance of prompt medical attention for any concerning changes empowers individuals to participate actively in their ongoing nail health surveillance.