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🦾 NAIL ANATOMY & DISORDERS — Dermatology Resident Study Note

Sources: Rook's Dermatology (Chapter 93, Pasch, Richert & Iorizzo); Dermatology 5e / Bolivnia (Tosti & Piraccini, Ch. 71); Fitzpatrick's Dermatology (Haneke, Ch. 91); Andrews' Diseases of the Skin (Ch. 33); Goldman-Cecil Medicine; Goodman & Gilman's; Recent JAAD/PubMed reviews (2024-2025)

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PART 1 — ANATOMY OF THE NAIL UNIT

1.1 Gross Anatomy

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1.2 Microscopic Anatomy

• Trilaminar: dorsal plate (from ventral PNF), intermediate/main plate (germinal matrix), ventral plate (nail bed contribution)
• Contains hard "hair-type" keratins embedded in sulfur-rich, high-cysteine and high-glycine/tyrosine proteins
• No granular layer in health; granular layer appears in psoriasis, onychomycosis = pathological sign

• Epidermis 2-3 cells thick; transition from living keratinocyte to dead ventral nail plate is abrupt (1 horizontal cell layer) — resembles Henle layer of inner root sheath
• Dermis sparse, no subcutaneous fat; collagen bundles closely attached to periosteum
• Clinical implication: bacterial infections spread rapidly to bone → osteomyelitis risk

• Keratinization occurs along an oblique axis (upward and distal)
• MIB-1 (Ki-67) staining labels proliferating matrix cells
• Contains dormant melanocytes (mainly distal matrix has an active compartment); nail bed has almost no melanocytes

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1.3 Keratins

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1.4 Blood Supply

• Paired digital arteries (large palmar, small dorsal digital artery on each side)
• Palmar arteries supplied from superficial and deep palmar arcades
• Distally: extremely tortuous and coiled vessels (can distort without kinking)
• Superficial arcade: just distal to DIP joint → supplies nail fold, extensor tendon, germinal matrix
• Glomus bodies (Masson): specialized neuromyoarterial structures; regulate capillary circulation; abundant in nail bed with gradient toward distal nail bed; dilate with cold (opposite of arterioles) to maintain acral circulation

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1.5 Nerve Supply

• Paired digital nerves follow arterial blood supply
• Periunugual soft tissues innervated by dorsal branches
• Main digital nerve passes under the nail bed and innervates both nail bed and matrix
• Dual sensory innervation is common → ring block anaesthesia must address both dorsal and palmar branches

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1.6 Nail Growth and Morphology

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1.7 Immunology of the Nail

• Nail matrix exhibits immune privilege (similar to hair matrix, eye, testis)
• Contains human cathelicidin LL-37 (kills Candida albicans) and beta-defensins hβD-2 and hβD-3
• MHC class I molecules are restricted in the nail matrix, protecting it from autoimmune T-cell attack

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1.8 Signalling Pathways in Nail Formation

• Wnt/β-catenin signalling is crucial in nail embryogenesis and adult maintenance
• R-spondin 4 mutations → autosomal recessive anonychia
• Frizzled-6 (Wnt receptor) mutations → inherited nail dysplasia
• BMPs (bone morphogenetic proteins) work with Wnt signalling
• Spondins = family of proteins for intracellular communication

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PART 2 — NAIL SIGNS AND THEIR SIGNIFICANCE

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2.1 Abnormalities of Shape

Clubbing (Hippocratic fingers, acropachy, watch-glass nails)

• Respiratory: thoracic carcinoma, mesothelioma, asbestosis, cryptogenic fibrosing alveolitis, sarcoidosis, cystic fibrosis, pulmonary AVM, tuberculosis
• Cardiac: cyanotic heart disease (PDA, septal defect), infective endocarditis, hepatopulmonary syndrome
• GI: inflammatory bowel disease, carcinoma of oesophagus, liver disease, laxative abuse, chronic parasitic infestation
• Endocrine: thyroid disease (thyroid acropachy)
• Other: HIV (37%), POEMS syndrome (70%), hemiplegia, subungual tumour, lupus erythematosus

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Koilonychia (Spoon nail)

• Concave, spoon-shaped nail plate
• Most common systemic associations: iron deficiency and haemochromatosis
• Also: deficiency of cysteine-rich keratin (trichothiodystrophy), familial (AD) in adults
• Secondary: psoriasis, lichen planus, dermatophyte infection (nail bed hyperkeratosis pushes nail up distally)
• Occupational: mechanics (oil softens nail), hairdressers (permanent wave solutions)

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Pincer nail (Trumpet nail, Arched nail)

• Transverse overcurvature increasing toward the distal tip, pinching soft periungual tissues
• Three patterns:
  1. Associated with psoriasis (thumbs and big toes)
  2. Inherited form (gradient from thumbs/big toes outward, progresses with time)
  3. Isolated (independent nail fold)
• Treatment: orthopaedic braces, nail plate ablation/partial matricectomy, Z-plasty, dermal flap

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Pachyonychia

• Thickened nail plate with yellowish colour, increased transverse overcurvature, horseshoe-shaped free edge, extreme hardness
• Congenital: Jadassohn-Lewandowsky syndrome (pachyonychia congenita)
• Acquired: psoriasis, pityriasis rubra pilaris, chronic eczema, onychomycosis

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Brachyonychia (Racquet nails)

• Width of nail plate > length; usually thumb, AD with higher prevalence in females
• Premature ossification of epiphysis of distal phalanx (age 7-10 years)
• Also in patients under chronic haemodialysis (tertiary hyperparathyroidism with phalangeal tuft resorption)

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Anonychia / Micronychia

• Anonychia: absence of nail plate; mutations in R-spondin 4, Frizzled-6 or Wnt10a genes
• Nail-Patella Syndrome (hereditary osteo-onychodysplasia, Fong syndrome): triangular lunulae are pathognomonic; LMX1B gene mutation; associated with absent/hypoplastic patella, posterior iliac horns, glomerulonephritis (20% develop renal failure), "Lester iris"

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2.2 Abnormalities of Nail Attachment

Onycholysis

• Separation of nail plate from nail bed starting distally at onychodermal band, moving proximally
• Detached nail plate looks white (apparent leukonychia) due to air underneath
• Green discoloration = Pseudomonas aeruginosa
• Causes: trauma, psoriasis (30% of psoriatics), fungal infection, contact dermatitis, photo-onycholysis (psoralens, doxycycline, demethylchlortetracycline), retinoids, drugs (5-FU, captopril), hypothyroidism, hyperthyroidism, yellow nail syndrome

• Post-traumatic: regular margin, usually lateral
• Psoriatic: irregular margin, involves whole free edge, erythematous border
• Onychomycosis: ragged/spike edge, mycelium visible proximally

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Pterygium

• Dorsal (true) pterygium: fibrotic band from proximal nail fold divides nail in two; causes: lichen planus (most common), trauma, graft-versus-host disease, leprosy
• Ventral pterygium (pterygium inversum unguis): forward extension of nail bed epithelium, obscuring distal groove; causes: trauma, systemic sclerosis, Raynaud, lupus, familial

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2.3 Nail Surface Changes

Beau Lines (Transverse grooves)

• Transverse depressions = temporary interruption of proximal nail matrix mitotic activity
• Depth = extent of matrix damage; Width = duration of insult
• Migrate distally with nail growth
• Causes: manicure trauma, eczema, chronic paronychia (local); severe febrile illness, cytotoxic drugs, erythroderma (systemic — appears on all nails simultaneously)
• Beau lines → onychomadesis when full thickness groove

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Pitting

• Shallow depressions from foci of parakeratosis in proximal matrix
• Fine pits (<1mm): psoriasis (most common; >10 pits/nail or >60 total supports diagnosis)
• Coarse pits: eczema
• Geometric/geometric arranged: alopecia areata ("Elkonyxis" = single large pit = reactive arthritis, psoriasis)
• "Elkonyx": deep full-thickness single pit in nail plate

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Trachyonychia (Twenty-nail dystrophy)

• Rough, sandpaper-like, opaque surface of all 20 nails; also "shiny" variant
• Associated with: alopecia areata (most common), psoriasis, lichen planus, eczema, vitiligo, idiopathic
• Histology: spongiosis and lymphocytic infiltrate of nail matrix

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Onychoschizia (Lamellar dystrophy)

• Distal horizontal lamellar splitting of nail plate
• Causes: repeated wet-dry cycles, chemical exposure

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Longitudinal Ridging / Beading

• Common with aging; not a sign of disease; more prominent in elderly

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Median Canaliform Dystrophy of Heller

• Midline longitudinal split with fir-tree appearance of ridges angled backwards
• Thumbs most commonly affected; often self-resolving
• Distinct from habit-tic (washboard nails): habit-tic has disturbed cuticle

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2.4 Nail Colour Changes

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2.5 Nail Signs in Systemic Disease

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PART 3 — NAIL DISORDERS

3.1 Nail Psoriasis

• Salmon (oil drop) patch: characteristic yellowish-red translucent zone at leading edge of onycholysis — pathognomonic
• Acrodermatitis continua of Hallopeau (ACH): sterile pustular form; severe destruction
• POPP (Psoriatic onychopachydermoperiostitis): nail thickening, periosteal reaction, bone erosion without joint involvement
• Onychomycosis co-infection: more common in psoriatics; must treat with antifungals alongside nail psoriasis treatment (Koebner effect)

≤3 nails (matrix) → Intralesional steroid injections FIRST LINE
                  → Topical vitamin D ± topical steroids SECOND LINE

≤3 nails (nail bed) → Topical steroids or vitamin D FIRST LINE

>3 nails → Intralesional steroids ± topical vitamin D + steroids FIRST LINE
         → Systemic: methotrexate (≤15mg/week), ciclosporin (3-5mg/kg short-term),
           acitretin (0.2-0.4mg/kg, 6 months)
         → Biologics: adalimumab (only FDA-approved for nail psoriasis), infliximab,
           etanercept, certolizumab pegol, golimumab, ustekinumab, secukinumab,
           ixekizumab, risankizumab, brodalumab, guselkumab
         → Small molecules: apremilast, tofacitinib

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3.2 Lichen Planus of the Nails (NLP)

• Irregular longitudinal grooving and ridging (onychorrhexis)
• Thinning of nail plate
• Dorsal pterygium (fusion of PNF with matrix/nail bed) — most characteristic
• Nail shedding with atrophy
• Subungual keratosis
• Erythronychia (red streaks/longitudinal erythronychia)
• Subungual hyperpigmentation
• Nail degloving
• Trachyonychia (20-nail dystrophy, shiny variant)
• Idiopathic atrophy of nails = most severe variant (acute onset, rapid irreversible nail destruction)

• Intralesional triamcinolone acetonide (matrix/nail bed; digital nerve block first)
• Oral prednisone (0.5-1 mg/kg for 3 weeks) or IM triamcinolone (0.5-1mg/kg/month x 3-6 months in children)
• Combination: oral retinoids + topical corticosteroids
• Tazarotene + topical steroids under occlusion

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3.3 Onychomycosis

• Dermatophytes: Trichophyton rubrum (most common, 70-80%), T. mentagrophytes
• Yeasts: Candida spp. (mainly fingernails; associated with chronic paronychia)
• Moulds: Aspergillus, Fusarium, Scytalidium (Neoscytalidium)

• KOH direct microscopy (filaments — see Table 93.5)
• Fungal culture (gold standard); PAS histology of nail clippings
• PCR (faster, more sensitive)

• Terbinafine oral: 250mg/day × 6 weeks (fingernails), 12 weeks (toenails) — most effective
• Itraconazole oral: 200mg/day continuous OR pulse (400mg/day × 1 week/month × 2 for fingernails, 3 for toenails)
• Fluconazole: 150-450mg weekly × 3 months (fingernails), 6 months (toenails)
• Topical: ciclopirox lacquer (8%), efinaconazole, tavaborole (less effective alone; useful in mild SWO or adjunct)
• Laser therapy: limited evidence
• Nail avulsion (partial/total) as adjunct to reduce fungal mass

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3.4 Paronychia

Acute Paronychia

• Inflammation, swelling, pain of proximal and lateral nail folds ± abscess
• 3× more common in females; index finger and thumb most affected
• Causes: Staphylococcus aureus (most common); also Streptococcus, herpes simplex (herpetic whitlow), orf virus, Candida, Treponema pallidum
• Neutropenic patients: Fusarium, Aspergillus
• Artificial nails/gel polish → can harbour bacteria inaccessible to routine hand hygiene

• Superficial abscess → drain with pointed scalpel (no anaesthesia needed)
• Deep → penicillinase-resistant antibiotic + surgery if no response in 48h
• Herpetic whitlow: oral aciclovir/famciclovir/valaciclovir for frequent recurrences; topical has no benefit

Chronic Paronychia

• Inflammatory dermatosis of nail folds; retraction of periunugal tissues, loss of cuticle
• Predominantly domestic/catering workers (wet work); females; first 3 fingers of dominant hand
• Pathophysiology: Candida is NOT the primary pathogen — it is irritant reaction/contact allergy/food hypersensitivity; Candida is a secondary coloniser
• Clinical: loss of cuticle, Beau lines, cross-ridging, ± dark discoloration lateral edges

• Avoid wet work (gloves); topical imidazoles (for Candida colonization)
• Topical/intralesional steroids
• Systemic antifungals are NOT useful (Candida not primary pathogen)
• Surgical: crescent-shaped excision of PNF when refractory (heals by secondary intention in <2 weeks)

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3.5 Darier Disease of the Nails

• Multiple nails with multiple red and white bands = candy-cane appearance (longitudinal erythronychia + longitudinal leukonychia) = pathognomonic
• Free margin: V-shaped notch and fissure (fragility)
• Histology: acantholysis + multinucleate giant cells + epithelial hyperplasia in nail bed

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3.6 Eczematous Nail Dystrophy

• Prevalence: almost all hand eczema patients; ~16% in atopic dermatitis
• Features: thickened/pitted/rough/discoloured nails; Beau lines (very common); loss of cuticle → Candida/bacterial superinfection; coarse pits (distinguish from fine pits of psoriasis)
• Phototoxic dermatitis (tetracyclines, 5-FU, capecitabine) → onycholysis matching shape of proximal nail fold

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3.7 Yellow Nail Syndrome

1. Yellow, thickened, overcurved nails with absent lunula and loss of cuticle
2. Lymphoedema (legs)
3. Pleural effusion

• Also associated with: bronchiectasis, rhinosinusitis, chronic cough
• Growth arrest: nails grow extremely slowly (<0.1mm/week)
• Treatment: vitamin E (tocopherol) topically or systemically; triazoles; treat underlying condition

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3.8 Nail Tumours

Benign

• 1-2% of all hand tumours; predominantly women (up to 90%); 4th-5th decade
• Triad: Love test (point tenderness over lesion), Hildreth's sign (pain relief with tourniquet), cold sensitivity test
• Presentations: small reddish/bluish spot under nail plate; OR longitudinal erythronychia with distal notching/fissure
• Investigation: MRI (gold standard — highest sensitivity, best extent assessment); high-resolution ultrasound with colour Doppler
• Treatment: surgical excision (transungual approach preferred)
• Complication: bone erosion in 50% of cases

• Benign osteochondral outgrowth from distal phalanx
• 75% great toenail; porcelain white hue with surface telangiectasias + collarette in early stage
• Investigation: X-ray (cup-shaped bone outgrowth)
• Treatment: surgical excision

• Leakage of synovial fluid through DIP joint capsule breach (>85% confirmed on MRI)
• Types: A (nodule between DIP and PNF), B (in PNF → longitudinal groove in nail), C (under matrix → reddish/bluish lunula)
• Treatment: cryotherapy (type A); drainage + compression; methylene-blue guided surgery (94% success on fingers)

• Pedunculated fibroepithelial lesion; garlic-clove fibroma; from proximal nail fold; lies in longitudinal groove
• NO histological difference from Koenen tumours of tuberous sclerosis (but Koenen tumours have no hyperkeratotic tip)
• When multiple → rule out tuberous sclerosis

• Rare benign matrix tumour (described 1992 by Baran and Kint)
• Classic features: longitudinal xanthopachyonychia, woodworm cavities at free edge, transverse + longitudinal overcurvature, proximal splinter haemorrhages
• Nail avulsion reveals sea-anemone matrix tumour
• MRI/ultrasound/OCT helpful

• Benign longitudinal subungual tumour; presents as isolated pink longitudinal nail streak (erythronychia) with subungual keratosis and distal onycholysis

Malignant

• 0.18-2.8% of all cutaneous melanomas; higher proportion in African/Asian/Native American populations (~25% of melanomas in Japanese/African Americans)
• Most melanomas arise from matrix (75%) → longitudinal melanonychia
• Histological subtype: usually acral lentiginous melanoma
• BRAF mutations LOW; KIT and NRAS mutations HIGH
• Hutchinson's sign (pigment extending to nail fold) = strongly suggestive of melanoma; pseudo-Hutchinson's sign = benign mimic (transparent cuticle showing nail bed pigment)
• Treatment: in situ → en bloc removal with 5-10mm margins; invasive → amputation guided by balance of tumour thickness and function
• Sentinel lymph node biopsy for melanoma >1mm thickness

• Mean age 60; 75% male
• HPV-associated (up to 60% genito-digital transmission; right index/middle fingers)
• Presents: subungual hyperkeratosis, onycholysis, oozing (underrecognised), nail plate destruction
• Bowen disease = in situ SCC; mean diagnosis delay 6 years
• Standard X-ray: bone involvement in 2-30%
• Treatment: Mohs surgery first line; amputation only if bone involved

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3.9 Traumatic Nail Disorders

• Stage 1 & 2: conservative (correct cutting habit, topical steroids for granulation, antiseptics, appropriate footwear)
• Stage 3 (refractory): chemical cautery with 88% phenol (partial avulsion + 4 min phenol application = <3% recurrence; several Cochrane reviews support this); TCA 100% equally effective but same oozing time

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3.10 Nail Changes in Connective Tissue Diseases

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3.11 Drug-Induced Nail Disorders

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PART 4 — IMAGING OF THE NAIL

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PART 5 — NAIL SURGERY (Overview)

• Nail avulsion (partial preferred; total avulsion causes nail bed shrinkage and distal pulp expansion)
• Punch biopsy (3 mm for small pigmented lesion <3mm in distal matrix)
• Longitudinal ellipse excision (for longitudinal pigmented bands)
• Tangential excision (for wide pigmented areas)
• Lateral longitudinal biopsy (sigmoid-shaped; includes all lateral nail unit structures)
• Crescent-shaped PNF excision (chronic paronychia; heals by secondary intention in <2 weeks)
• Chemical cautery (88% phenol 4 min for ingrowing toenail; tourniquet mandatory for bloodless field; oozing lasts up to 5 weeks)

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PART 6 — CLINICAL PEARLS 💎

1. The onychodermal band is the Achilles heel of nail plate attachment — once breached (psoriasis, trauma), onycholysis becomes progressive and difficult to reverse. Clipping, drying, and topical antiseptics are the cornerstone of management.
2. Chronic paronychia ≠ Candida infection. Candida is a secondary coloniser. The primary aetiology is irritant/allergic dermatitis. Systemic antifungals do NOT help — treat the inflammation first.
3. Proximal subungual onychomycosis (PSO) in a young patient = think HIV. PSO is the classic nail presentation in immunocompromised patients and should trigger HIV testing.
4. Nail-fold capillaroscopy distinguishes primary Raynaud from secondary Raynaud (connective tissue disease). Megacapillaries + avascular areas = secondary (scleroderma pattern); normal capillary architecture = primary Raynaud disease.
5. Pseudo-Hutchinson's sign: Periungual pigmentation that simulates Hutchinson's sign but is actually visible nail plate pigmentation through a transparent cuticle. Seen in racial melanonychia, benign nevi. The real Hutchinson's sign = actual extension of melanin pigment into the nail fold skin.
6. Glomus tumour triad = Love test (point tenderness) + Hildreth's sign (tourniquet pain relief) + cold sensitivity. Affects predominantly women under age 40. If you feel a mismatch of pain severity to clinical exam, think glomus tumour.
7. Darier disease candy-cane nails = red + white longitudinal bands with V-notch at free edge. This is pathognomonic and can be diagnosed even when nail is the only manifestation of Darier disease.
8. Nail psoriasis is HLA-Cw6 negative — opposite of cutaneous plaque psoriasis. This suggests a genetically distinct subset with stronger association with psoriatic arthritis.
9. Koenen tumours (periungual fibromas) of tuberous sclerosis develop around puberty, histologically identical to acquired ungual fibrokeratomas, but LACK the hyperkeratotic tip.
10. Nail melanoma in dark-skinned individuals: ~25% of all melanomas in Japanese and African Americans are nail apparatus melanomas. The absolute incidence may be similar across races, but proportionately nails matter much more in these populations.
11. The free edge of the nail reveals where pigment originates: pigment in upper portion of free edge → proximal/dorsal matrix origin (higher risk of dystrophy post-biopsy); pigment in lower portion → distal matrix (safer to biopsy, less post-biopsy dystrophy).
12. SCC is the most common malignant nail tumour, not melanoma. HPV-associated (genito-digital route); right index finger is most commonly affected. Mean diagnosis delay = 6 years.
13. Onychomycosis in toenails does NOT cure with topical treatment alone in most cases. Systemic terbinafine is the most effective single agent. Always get a KOH/culture before treating — misdiagnosis rate is significant.
14. Beau lines at same level in all 20 nails = systemic cause (cytotoxics, severe illness). Beau lines on only 1-2 nails = local trauma or paronychia.
15. The distal digital block, not a ring block, is the preferred anaesthesia for nail surgery — dual innervation requires addressing both dorsal and palmar nerve branches at the lateral digit, not the classic ring block which may miss dorsal branches.

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PART 7 — VIVA QUESTIONS & ANSWERS

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PART 8 — QUIZ (Side-by-Side with Answers)

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REFERENCES / SOURCES PER SECTION

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Recent evidence note: A 2024 JAAD review by Hwang JK, Grover C, Iorizzo M provides updated diagnostic criteria and management algorithms for nail psoriasis and nail lichen planus, confirming the superiority of biologics (especially IL-17 and IL-23 inhibitors) over conventional systemics for nail psoriasis. A 2025 JAAD expert group review by Ricardo JW et al. updates the clinical evaluation approach for longitudinal melanonychia.