What causes black hardened areas on the toes?

Black Hardened Areas on the Toes: Differential Diagnosis and Management

Black hardened areas on the toes most commonly represent either hyperkeratosis (callus formation), fungal nail infection (onychomycosis), or less commonly, traumatic hemorrhage into the skin or nail. The key distinguishing features are texture (soft/friable versus hard/thickened), location (nail plate versus skin), and associated symptoms.

Primary Differential Diagnosis

Hyperkeratosis (Most Common for Skin Lesions)

Hard, thickened, yellow-to-black areas on toe tips or between toes represent hyperkeratosis, which occurs as a hyperproliferative response to friction and pressure. 1

• Appears as hard, thickened skin with yellow coloration that may darken to black when inflamed or when central keratin plugs form (corns) 1
• Located on toe tips, between toes, or under metatarsal heads 1
• Reported in 36.8% of patients across all epidermolysis bullosa subtypes, but common in general population with repetitive trauma 1
• Blisters can form under thickened tissue and painful cracks may develop 1

Onychomycosis (Most Common for Nail Lesions)

If the black hardened area involves the nail plate with thickening and friable texture, fungal nail infection is the primary consideration. 1

• Distal and lateral subungual onychomycosis (DLSO) is the most common presentation, with nail thickening, discoloration, and varying degrees of onycholysis 1
• Nails become soft and friable in fungal infection, distinguishing it from non-infectious causes where nails remain hard 1
• Toenails affected more commonly than fingernails 1
• Accounts for 15-40% of all nail diseases in adults 2

Traumatic Hemorrhage (Less Common)

• Post-traumatic intracorneal blood presents as brown or black lesions from superficial hemorrhage 3
• "Black heel" (talon noir) affects posterolateral heel in athletes, appearing as grouped punctate hemorrhages revealed by paring 4
• Typically affects adolescents/young adults in active sports 4

Bacterial Infection

Green or black nail discoloration suggests Pseudomonas infection (Green Nail Syndrome), which may coexist with fungal infection. 1, 5

• Pseudomonas causes green-to-black discoloration 1
• Treatment includes keeping area dry and applying topical povidone iodine 2% twice daily 5

Critical Diagnostic Features to Assess

Texture Assessment

• Soft and friable texture indicates fungal infection 1
• Hard, thickened texture suggests hyperkeratosis or non-infectious nail dystrophy 1, 5

Location Specificity

• Nail plate involvement: consider onychomycosis, bacterial infection, or nail trauma 1
• Skin of toe tips/between toes: hyperkeratosis most likely 1
• Posterolateral heel: consider traumatic hemorrhage (black heel) 4

Associated Findings

• Paronychia (nail fold swelling/erythema) suggests Candida infection with occupational moisture exposure 5
• Interdigital maceration, scaling, or fissuring suggests concurrent tinea pedis 1, 6
• Pain described as "walking on a pebble" indicates corn with central keratin plug 1

Diagnostic Workup

For Suspected Fungal Infection

Laboratory confirmation is mandatory before initiating systemic antifungal therapy. 1, 5

• Obtain nail clippings through entire thickness including crumbly material 1
• Direct microscopy with potassium hydroxide (KOH) preparation 5, 7
• Fungal culture on Sabouraud's glucose agar 5
• Calcofluor white staining improves visualization of fungal elements 1

For Suspected Hyperkeratosis

• Clinical diagnosis based on appearance and location 1
• Paring reveals underlying tissue characteristics 1

Management Algorithm

For Hyperkeratosis

Conservative debridement by podiatrist is first-line, avoiding overdebridement that increases blistering risk. 1

• Manual debridement or paring by trained podiatrist 1
• Patient self-management with emery board for mild cases 1
• Apply emollients and non-adherent dressings after debridement 1
• Pressure redistribution with proper footwear and cushioning 1

For Confirmed Onychomycosis

Terbinafine 250 mg orally once daily is the most effective systemic treatment: 12 weeks for toenails, 6 weeks for fingernails. 7

• Terbinafine demonstrates 70% mycological cure for toenails at 48 weeks (12 weeks treatment + 36 weeks follow-up) 7
• Fingernail mycological cure rate of 79% at 24 weeks (6 weeks treatment + 18 weeks follow-up) 7
• Mean time to overall success: 10 months for toenails, 4 months for fingernails 7
• Measure serum transaminases (ALT/AST) before starting treatment 7
• Clinical relapse rate approximately 15% at one year post-therapy 7

Alternative oral agents include itraconazole and fluconazole, though terbinafine shows superior efficacy for Trichophyton tonsurans. 1, 8

For Interdigital Complications

Examine interdigital toe spaces carefully, as treating fissuring, scaling, or maceration reduces pathogen colonization and recurrent infection risk. 1

• Toe web infection occurs secondary to chronic fungal infection in most cases 6
• Smoking and diabetes are significant risk factors (71.2% of TWI patients were smokers) 6
• 50% of cases are bilateral, 33% have recurrent infections, 20% develop secondary cellulitis 6

Preventive Measures

Daily application of topical emollients to cuticles and periungual tissues prevents nail problems. 1, 5

• Keep toenails trimmed straight across 1
• Avoid excessive moisture exposure and wear gloves during cleaning 1, 5
• Wear comfortable, well-fitting shoes and cotton socks 1
• Daily to weekly application of urea-based cream reduces nail thickness 1
• Treat predisposing conditions including edema, obesity, venous insufficiency, and toe web abnormalities 1

Critical Pitfalls to Avoid

Never assume fungal infection without laboratory confirmation—non-infectious nail dystrophies do not produce soft, friable texture. 1, 5

• Overdebridement of hyperkeratosis increases blistering and tenderness in susceptible patients 1
• Starting systemic antifungals without mycological confirmation exposes patients to unnecessary medication risks 1
• Ignoring interdigital spaces misses the source of recurrent lower extremity cellulitis 1
• Failing to assess for diabetes and smoking status overlooks major risk factors for toe web infection and treatment failure 6