Skin & Subcutaneous Tissue

Onychomycosis: diagnosis and treatment

Clinical Overview and When to Suspect Onychomycosis

— Age >60, male sex

— Diabetes mellitus (3× risk; high-yield association)

— Peripheral vascular disease, peripheral neuropathy

— Tinea pedis (concurrent in 30–40% — "one foot, two hand" syndrome possible)

— Immunosuppression (HIV, transplant, biologics)

— Occlusive footwear, communal showers, athletes, hyperhidrosis

— Family history (autosomal dominant susceptibility to T. rubrum described)

— Thickened, discolored (yellow-brown-white), brittle, or onycholytic nail

— Subungual debris, distal nail lifting

— Coexisting interdigital scaling or moccasin-distribution tinea pedis

— In diabetics, onychomycosis is a portal for bacterial cellulitis, foot ulcers, and lower-extremity amputation — treat aggressively.

— Cosmetic complaint is also a legitimate indication, but always confirm diagnosis before systemic therapy because oral antifungals carry hepatotoxicity risk.

Definition: Fungal infection of the nail unit (plate, bed, matrix) caused most often by dermatophytes (Trichophyton rubrum >> T. mentagrophytes), less commonly non-dermatophyte molds (Scopulariopsis, Fusarium, Aspergillus) or yeasts (Candida, especially fingernails).

Epidemiology: Affects ~10% of US adults, up to 50% of those >70. Toenails involved 10× more than fingernails. Hallux is the most common nail affected.

Risk factors to elicit on Step 3 stems:

When to suspect:

Why it matters for primary care (Step 3 framing):

Board pearl: Only ~50% of dystrophic nails are actually fungal — psoriasis, trauma, and lichen planus mimic onychomycosis. Empiric oral terbinafine without confirmation is a common wrong answer.

Step 3 management: Confirm with laboratory testing before initiating oral antifungals; topical therapy alone may be reasonable empirically only in low-risk patients with mild disease.

Presentation Patterns and Key History

— Distal lateral subungual onychomycosis (DLSO): Most common. Starts at hyponychium, spreads proximally. Yellow-white discoloration, onycholysis, subungual hyperkeratosis. Usually T. rubrum.

— White superficial onychomycosis (WSO): Chalky white islands on dorsal nail plate, scrape off easily. Usually T. mentagrophytes in adults; molds (Fusarium, Acremonium) more often in children and HIV.

— Proximal subungual onychomycosis (PSO): White discoloration begins at proximal nail fold. Red flag — strongly associated with HIV/immunosuppression; offer HIV testing.

— Endonyx: Milky-white plate without subungual hyperkeratosis or onycholysis; T. soudanense, T. violaceum.

— Total dystrophic onychomycosis: End-stage; entire nail thickened, crumbling.

— Duration, progression, prior treatments (and adherence/duration)

— Tinea pedis, tinea cruris, tinea manuum elsewhere

— Trauma history (running, ill-fitting shoes — mimics onychomycosis)

— Diabetes control, peripheral arterial symptoms

— Hepatic disease, alcohol use, hepatotoxic meds — relevant to oral therapy

— Medications: statins, warfarin, SSRIs, TCAs (terbinafine inhibits CYP2D6); calcineurin inhibitors, sulfonylureas, warfarin (itraconazole inhibits CYP3A4)

— Cosmetic concerns, occupational impact, pain with shoes

Five clinical subtypes — recognize the pattern, infer the likely organism:

Candida onychomycosis: Predominantly fingernails, often with chronic paronychia, in wet-work occupations (dishwashers, bartenders, healthcare).

History to obtain:

Key distinction: Proximal subungual disease in a previously healthy adult should prompt HIV testing before you write any prescription — this is a classic Step 3 trap.

Board pearl: A patient with bilateral moccasin tinea pedis and unilateral hand involvement = "two feet–one hand" syndrome, pathognomonic for T. rubrum dermatophytosis, often with toenail onychomycosis.

Physical Exam Findings and Vascular/Neuropathic Assessment

— Onycholysis (separation of plate from bed) starting distolaterally

— Subungual hyperkeratosis (debris under the nail)

— Dyschromia: yellow, brown, white, or "dermatophytoma" — a dense yellow-orange longitudinal streak representing a fungal mass; predicts poor response to oral therapy alone

— Nail plate thickening (pachyonychia) and crumbling

— Splinter hemorrhages can occur but are nonspecific

— Interdigital maceration/scaling (especially 4th web space)

— Moccasin-distribution scale on plantar surface

— Tinea cruris, tinea corporis (rule out reservoir)

— Pedal pulses (dorsalis pedis, posterior tibial); consider ABI if diminished

— 10-g monofilament at 10 sites + 128-Hz tuning fork for neuropathy

— Inspect interdigital spaces, heel fissures, callus

— Document any ulceration, cellulitis, or paronychia — these change urgency

— Psoriasis: pitting, oil-drop sign, salmon patches, scalp/elbow plaques, distal interphalangeal arthritis

— Lichen planus: longitudinal ridging, pterygium formation, mucosal Wickham striae

— Trauma/onychogryphosis: single nail, ram's-horn deformity, history

— Subungual melanoma: single-digit longitudinal pigmented band, Hutchinson sign (pigment extending onto proximal nail fold) — biopsy, do not treat as fungus

Nail unit inspection — look for:

Examine ALL 20 nails — multifocal involvement supports fungal etiology over trauma.

Skin exam — high-yield concurrent findings:

Diabetic/vascular foot exam (Step 3 staple):

Differential clues at the bedside:

CCS pearl: In a diabetic with thickened toenails, document pulses, monofilament, and skin integrity on initial exam — these orders flag the patient as higher-risk and justify more aggressive therapy and podiatry referral for routine nail care.

Board pearl: A pigmented longitudinal streak with periungual pigment spread is melanoma until proven otherwise — punch biopsy of the matrix, not antifungal therapy.

Diagnostic Workup — Confirmatory Mycology Before Systemic Therapy

— Clean nail with alcohol to reduce contaminants

— Clip nail back to the most proximal area of onycholysis

— Use a curette to obtain subungual debris from the leading edge of disease (where viable hyphae live)

— For WSO: scrape the dorsal plate surface

— For PSO: punch or drill through proximal plate

— KOH preparation with chlorazol black or DMSO: Quick, in-office; shows septate hyphae. Sensitivity ~60–80%, specificity ~70%. Operator-dependent.

— Fungal culture (Sabouraud agar): Identifies organism (dermatophyte vs mold vs yeast), guides therapy. Slow (2–6 weeks), sensitivity only ~50–60% due to nonviable organisms.

— PAS stain of nail clipping (histopathology): Highest sensitivity (~80–90%), fast turnaround, identifies hyphae but not species. Often the single best test if available.

— PCR: Sensitivity >90%, species-level ID, expensive, increasingly used.

— Baseline AST/ALT for terbinafine or itraconazole

— Consider CBC (rare neutropenia with terbinafine)

— Pregnancy test in reproductive-age women (both are teratogenic)

— Review medication list for interactions

Why confirm? Only ~50% of dystrophic-appearing nails are fungal. Oral antifungals carry hepatotoxicity, drug interactions, and cost; confirmation is standard of care before systemic therapy (and often required by payers).

Sample collection:

First-line diagnostic options (any one is acceptable):

Recommended strategy: PAS + culture combined maximizes yield. If PAS positive but culture negative, treat as dermatophyte.

Pre-treatment labs before oral therapy:

Board pearl: PAS histopathology of nail clippings is the most sensitive single test and does not require viable organisms — preferred when prior topical use may suppress culture.

Step 3 management: Order KOH + PAS + culture before prescribing oral terbinafine; do not skip confirmation in a "looks fungal" toenail.

Diagnostic Workup — Advanced and Confirmatory Studies

— Repeat sampling from a more proximal/active site

— Send nail clipping for PAS if not already done

— Consider PCR-based dermatophyte assays (commercially available; results in 1–3 days)

— Discontinue any topical antifungal for 2 weeks before resampling to improve yield

— Culture must show same NDM on repeat sampling (≥2 specimens) to be considered pathogen vs contaminant

— Direct microscopy must also show hyphae

— Jagged proximal edge with spikes of onycholysis ("aurora borealis" pattern)

— Longitudinal striae, ruin appearance

— Distinguishes from traumatic onycholysis (linear, sharp border)

— Single pigmented streak → matrix punch biopsy to exclude subungual melanoma

— Atypical features, failed treatment, or suspicion of lichen planus/psoriasis

— Suspicion of squamous cell carcinoma (verrucous periungual lesion in HPV-exposed patient)

— PSO pattern → HIV test (and consider CD4 count)

— Recurrent/widespread tinea → screen for diabetes (HbA1c), check immunosuppression history

— Persistent Candida onychomycosis with mucocutaneous candidiasis → consider chronic mucocutaneous candidiasis or endocrinopathy (APECED)

When initial KOH/PAS is negative but suspicion remains high:

Identifying non-dermatophyte molds (NDM): Important because NDMs (Scopulariopsis brevicaulis, Fusarium, Aspergillus, Acremonium) respond poorly to terbinafine and require itraconazole or combination therapy.

Dermoscopy (onychoscopy) findings supportive of onychomycosis:

When to biopsy:

Workup for underlying conditions:

Key distinction: Dermatophyte vs non-dermatophyte mold identification changes drug choice — terbinafine works well for dermatophytes; itraconazole has broader coverage including most NDMs and Candida.

Board pearl: A negative culture does not rule out onychomycosis — PAS is more sensitive. Don't withhold treatment from a clinically classic case with positive PAS just because culture was negative.

Treatment Decision Logic — Who, When, and What

— Strong indications: diabetes, peripheral vascular disease, immunosuppression, prior cellulitis, pain/functional impairment, occupational concerns

— Relative indications: cosmetic distress, psychosocial impact

— Reasonable to defer: asymptomatic, low-risk patient who declines after counseling on risks of progression

— Mild (<50% distal involvement, no matrix involvement, ≤2 nails): topical therapy reasonable

— Moderate–severe (>50% involvement, matrix involvement, multiple nails, dermatophytoma, total dystrophy): oral therapy preferred

— Lateral nail involvement, longitudinal streaks, thickness >2 mm, or yellow spikes predict poor response — consider combination therapy or debridement

— Oral terbinafine — first-line for dermatophyte onychomycosis (highest cure rates)

— Oral itraconazole — alternative, especially for Candida or NDM

— Topical efinaconazole 10%, tavaborole 5%, ciclopirox 8% — for mild disease, contraindications to oral therapy, or adjunctive use

— Mechanical/chemical debridement — adjunctive; improves penetration and outcomes

— Laser therapy — FDA-cleared for "temporary increase in clear nail"; not curative; poor evidence

— Surgical avulsion — reserved for single severely affected nail, dermatophytoma not responding to systemic therapy

— Mycologic cure: negative KOH and culture

— Clinical cure: normal-appearing nail (often <10% residual abnormality acceptable)

— Complete cure: both — achieved in only 35–50% even with oral terbinafine

Decide first: does this patient need treatment at all?

Severity assessment guides modality:

Treatment modalities:

Cure definitions (set expectations!):

Step 3 management: Counsel patients that toenails grow ~1 mm/month — visible improvement takes 6–12 months even after mycologic cure, and recurrence rates are 20–25%.

Board pearl: Choose oral over topical when matrix is involved, when >50% of plate is affected, or when multiple nails are diseased — topicals don't reliably penetrate to the nail bed.

Pharmacotherapy — First-Line Oral Regimens

— Dose: 250 mg PO daily

— Duration: 6 weeks for fingernails, 12 weeks for toenails (continuous)

— Complete cure rate: ~38–50% (highest of oral agents for dermatophytes)

— Mechanism: Squalene epoxidase inhibitor → blocks ergosterol synthesis, fungicidal

— Adverse effects: GI upset, headache, taste disturbance/dysgeusia (can be permanent), rash, rare hepatotoxicity and Stevens-Johnson syndrome, neutropenia, lupus-like reaction

— Monitoring: Baseline AST/ALT; recheck at 4–6 weeks if prolonged course or risk factors. Stop if ALT >3× ULN or symptoms.

— Interactions: CYP2D6 inhibitor → increases levels of TCAs, SSRIs (especially fluoxetine, paroxetine), beta-blockers (metoprolol), antiarrhythmics. Rifampin lowers terbinafine levels.

— Continuous: 200 mg PO daily × 6 weeks (fingernails) or 12 weeks (toenails)

— Pulse: 200 mg BID × 1 week per month × 2 months (fingernails) or 3 months (toenails) — same efficacy, lower cost, fewer total mg exposure

— Complete cure rate: ~14–28% for toenails (lower than terbinafine for dermatophytes)

— Absorption: Requires acidic gastric pH — take with food/cola; avoid PPIs/H2 blockers

— Black box warnings: Congestive heart failure (negative inotrope — contraindicated in HF), hepatotoxicity, and QT prolongation

— Major interactions (CYP3A4 inhibitor): statins (rhabdo with simvastatin/lovastatin — contraindicated), warfarin, sulfonylureas, digoxin, calcineurin inhibitors, some DOACs, ergot alkaloids, methadone

Terbinafine — drug of choice for dermatophyte onychomycosis

Itraconazole — alternative; preferred for Candida and most NDMs

Fluconazole — 150–300 mg weekly × 6–12 months; off-label, less effective, useful when other agents contraindicated.

Griseofulvin — largely obsolete (low efficacy, long duration); occasional pediatric use.

Board pearl: Terbinafine + simvastatin = ↑ statin levels (mild); itraconazole + simvastatin = contraindicated (rhabdomyolysis). Always reconcile statins before starting an azole.

Step 3 management: For uncomplicated dermatophyte toenail onychomycosis in a healthy adult — terbinafine 250 mg daily × 12 weeks with baseline LFTs is the textbook answer.

Pharmacotherapy — Topicals, Adjuncts, and Device-Based Therapy

— Mild distal disease (<50% nail, no matrix involvement)

— ≤4 nails involved

— Pediatric patients (thinner nails → better penetration)

— Contraindications to oral therapy (hepatic disease, polypharmacy, pregnancy)

— Adjunct to oral therapy for severe disease

— Apply once daily × 48 weeks

— Complete cure rate ~17–18% (better than ciclopirox)

— No occlusion needed; penetrates well

— Minimal systemic absorption; no LFT monitoring

— Once daily × 48 weeks

— Complete cure ~6–9%; boron-based, novel mechanism (leucyl-tRNA synthetase inhibitor)

— Daily application, weekly removal with alcohol, × 48 weeks

— Complete cure ~5–9%; requires nail debridement; lowest efficacy but cheapest

— Mechanical debridement of thickened nail every 2–3 months (or self-filing)

— Treat concurrent tinea pedis with topical terbinafine/clotrimazole × 4 weeks — reduces reinfection

— Footwear hygiene: rotate shoes, dry interiors, antifungal powders (miconazole), launder socks hot, discard or treat old shoes with UV/antifungal sprays

— Avoid barefoot walking in communal areas

— Manage hyperhidrosis (aluminum chloride, moisture-wicking socks)

— Nd:YAG and diode lasers FDA-cleared for "temporary clearance"

— Not recommended as primary therapy — low cure rates, high cost, often not covered by insurance

— Useful for single severely dystrophic nail or dermatophytoma

— Always combine with antifungal — avulsion alone is not curative

Topical antifungals — when to choose:

Efinaconazole 10% solution

Tavaborole 5% solution

Ciclopirox 8% nail lacquer

Adjunctive measures (improve outcomes regardless of modality):

Laser and photodynamic therapy:

Surgical/chemical nail avulsion (40% urea):

Key distinction: Topicals have mycologic cure rates around 30–55% but complete cure rates of only 6–18% — set realistic expectations and avoid wasting 48 weeks on inappropriate cases.

Board pearl: Pairing oral terbinafine with debridement or topical efinaconazole improves complete cure in severe disease and dermatophytoma — combination therapy is a high-yield answer for refractory cases.

Special Populations — Elderly, Renal, and Hepatic Impairment

— Highest prevalence (up to 50% over age 70) due to slow nail growth, reduced peripheral circulation, trauma, comorbidities

— Polypharmacy risk — meticulous interaction check before terbinafine or itraconazole

— Reduced ability to perform daily topical application — consider caregiver support

— Slow nail growth means even longer time to visible improvement (12–18 months)

— Podiatry referral for routine debridement is often more practical than aggressive antifungal therapy in frail elderly

— Terbinafine: avoid in chronic or active liver disease (per label) — labeling cautions against use if baseline ALT/AST elevated or in cirrhosis

— Itraconazole: contraindicated in significant hepatic dysfunction and carries black-box hepatotoxicity warning

— In these patients, prefer topical therapy (efinaconazole, tavaborole, ciclopirox) — negligible systemic absorption

— Always check baseline LFTs; recheck if symptoms (nausea, RUQ pain, jaundice, dark urine, fatigue)

— Terbinafine: label suggests avoiding if CrCl <50 mL/min due to limited data; some experts use reduced dose (125 mg daily) — generally well tolerated

— Itraconazole oral solution contains cyclodextrin — avoid in CrCl <30 mL/min; capsules may be used cautiously

— Fluconazole requires dose adjustment for CrCl <50 mL/min

— Itraconazole contraindicated in CHF (negative inotrope, black box)

— Caution with QT-prolonging drugs

— Terbinafine generally cardiac-safe

— Treat onychomycosis to reduce risk of cellulitis, foot ulceration, and amputation

— Counsel on daily foot inspection

— Annual diabetic foot exam and podiatry co-management

— Terbinafine is first-line; monitor for hypoglycemia if on sulfonylureas + itraconazole

Elderly patients:

Hepatic impairment:

Renal impairment:

Cardiac disease:

Diabetes (high-yield Step 3 population):

Board pearl: In a cirrhotic patient with painful onychomycosis, the right answer is topical efinaconazole or ciclopirox, not oral terbinafine — and document shared decision-making.

Step 3 management: Always check baseline LFTs and reconcile statins, warfarin, and SSRIs before any oral antifungal in older adults.

Special Populations — Pregnancy, Lactation, and Pediatrics

— Onychomycosis is never urgent — defer treatment until after delivery (and breastfeeding) in nearly all cases

— Terbinafine: former Category B; limited human data, animal data reassuring — generally avoid; defer

— Itraconazole: contraindicated in pregnancy for onychomycosis (teratogenic in animals; associated with skeletal defects); pregnancy must be excluded before starting and contraception continued for 2 months after

— Fluconazole: high-dose (≥400 mg/day) is teratogenic; avoid

— Griseofulvin: contraindicated (teratogenic; male partners advised to avoid conception during and 6 months after)

— Topicals: minimal systemic absorption, but data are limited — still typically deferred unless symptomatic

— Terbinafine and itraconazole are excreted in breast milk — manufacturers recommend avoiding during breastfeeding

— Defer cosmetic-indication treatment until weaning

— Less common than in adults but increasing prevalence (~0.4%)

— Higher response rates due to thinner, faster-growing nails and lower fungal burden

— Investigate for tinea capitis in the household — common reservoir

— Terbinafine dosing (off-label for onychomycosis but FDA-approved for tinea capitis ≥4 yr):

— <20 kg: 62.5 mg daily

— 20–40 kg: 125 mg daily

— >40 kg: 250 mg daily

— Duration: 6 weeks fingernails, 12 weeks toenails

— Itraconazole pulse: 5 mg/kg/day × 1 week/month

— Topical ciclopirox or efinaconazole ≥6 years are reasonable for mild cases

— Baseline LFTs recommended before oral therapy

Pregnancy:

Lactation:

Pediatrics:

Key distinction: Itraconazole is contraindicated in pregnancy and CHF; terbinafine is preferred when oral therapy is necessary in non-pregnant adults. In pregnancy, the correct Step 3 answer is almost always defer treatment.

Board pearl: A child with toenail onychomycosis often has a household source of tinea capitis — examine and treat family members and pets to prevent recurrence.

Complications and Adverse Outcomes

— Cellulitis and erysipelas of lower extremity — onychomycosis and tinea pedis are well-established portals of entry; treating them reduces recurrence

— Bacterial paronychia — especially with Candida onychomycosis

— Onychogryphosis (ram's-horn deformity) from chronic untreated disease

— Permanent nail dystrophy if matrix is destroyed by chronic infection

— Pain, difficulty with footwear, gait alteration

— Foot ulceration under thickened, deformed nail

— Osteomyelitis from deep infection

— Lower-extremity amputation — onychomycosis is an independent risk factor in diabetic patients

— Charcot foot progression masked by chronic nail/skin changes

— Embarrassment, avoidance of swimming/sandals, intimate-relationship effects

— Functional impairment in athletes, dancers, service members

— Terbinafine:

— Hepatotoxicity (1 in 50,000 to 1 in 120,000) — usually within 4–6 weeks

— Taste loss (1–3%) — may be permanent; counsel before prescribing

— Smell disturbance

— Cutaneous drug reactions: SJS/TEN, DRESS, AGEP, subacute cutaneous lupus

— Hematologic: neutropenia, thrombocytopenia

— Itraconazole:

— Heart failure precipitation (negative inotrope)

— Hepatotoxicity — black box

— Drug-drug interactions causing rhabdomyolysis (statins), bleeding (warfarin), hypoglycemia (sulfonylureas)

— Peripheral neuropathy, edema, hypokalemia

— All oral antifungals: rash, GI upset

— 20–25% within 2–3 years even after complete cure

— Higher in diabetics, immunosuppressed, and those with untreated tinea pedis

Local complications:

Diabetic foot complications (high-yield):

Psychosocial:

Medication-related adverse outcomes:

Recurrence:

Board pearl: A patient on simvastatin who develops myalgia and dark urine after starting itraconazole — diagnose rhabdomyolysis; this interaction is contraindicated and a classic Step 3 wrong-prescription pitfall.

Step 3 management: Treating tinea pedis and onychomycosis in a diabetic with recurrent lower-extremity cellulitis is a guideline-supported secondary prevention strategy.

When to Escalate Care — Specialist Referral and Inpatient Triage

— Diagnosis uncertain after KOH/PAS/culture

— Failed first-line oral therapy

— Suspicion of psoriasis, lichen planus, melanoma, SCC

— Need for nail unit biopsy

— Recurrent disease despite adequate treatment

— Pediatric or pregnant patient with strong indication for therapy

— Non-dermatophyte mold infection requiring tailored therapy

— Diabetic foot — for routine nail care, debridement, callus management

— Significant pain, mechanical/footwear issues

— Need for surgical nail avulsion

— Concurrent orthopedic foot pathology

— Underlying immunosuppression with atypical organism (Fusarium in neutropenic patient — can disseminate)

— Suspected systemic mycosis

— Acute cellulitis with systemic signs (fever, tachycardia, hypotension) — IV antibiotics, blood cultures

— Necrotizing soft tissue infection in diabetic foot — surgical emergency

— Osteomyelitis — MRI, bone biopsy, IV antibiotics, often surgical

— Severe drug reaction (SJS/TEN, DRESS, fulminant hepatitis) from antifungal — stop drug, admit, supportive care, consult dermatology/hepatology

— Proximal subungual onychomycosis in absence of trauma → test for HIV

— If positive and CD4 <200, consider ID co-management

Onychomycosis itself is virtually never an inpatient diagnosis — but its complications and comorbid conditions may require escalation.

Refer to dermatology when:

Refer to podiatry when:

Refer to infectious disease when:

Escalate urgently / consider admission when:

HIV testing and ID referral:

CCS pearl: A diabetic with onychomycosis presenting with fever, erythema tracking up the leg, and a tender lymph node — admit, draw blood cultures, start IV cefazolin or vancomycin (if MRSA risk), elevate limb, mark borders, consult podiatry/vascular surgery if foot involvement.

Board pearl: "Refer to podiatry for ongoing nail care" is a frequently correct Step 3 answer in diabetics — emphasizes the longitudinal, team-based outpatient model.

Key Differentials — Other Nail Dystrophies (Same Category)

— Pitting (irregular depressions on nail surface) — classic

— Oil-drop sign (yellow-brown subungual discoloration)

— Onycholysis with erythematous border (vs the white border in fungal)

— Subungual hyperkeratosis (overlaps with onychomycosis)

— Look for skin/scalp psoriasis, DIP arthritis, dactylitis

— Up to 30% of psoriatic nails are co-infected with dermatophytes — KOH/PAS still indicated

— Longitudinal ridging, thinning, pterygium formation (scarring connecting proximal fold to nail bed)

— Can cause permanent nail loss (anonychia)

— Wickham striae on oral mucosa, violaceous papules on flexor wrists

— Early treatment with intralesional or systemic steroids to prevent scarring

— Thick, curved, opaque nail

— Usually elderly, neglect, single hallux

— Trauma/pressure etiology — not infection (though may coexist)

— Runners, ill-fitting shoes

— Single nail, distal-lateral, often hallux

— Subungual hematoma (acute, painful, dark — distinguish from melanoma with dermoscopy / clip and observe progression)

— Triad: yellow thickened nails + lymphedema + respiratory disease (bronchiectasis, pleural effusion, chronic sinusitis)

— Slow nail growth, absent lunula

— Sandpaper appearance of all 20 nails; associated with alopecia areata, atopic dermatitis, lichen planus

— Beau lines (chemotherapy, severe illness), Mees lines (arsenic), Muehrcke lines (hypoalbuminemia), photo-onycholysis (tetracyclines, psoralens)

Psoriatic nail disease (most important mimic):

Lichen planus of the nail:

Onychogryphosis ("ram's horn"):

Trauma-induced onychodystrophy:

Yellow nail syndrome:

Twenty-nail dystrophy (trachyonychia):

Drug-induced nail changes:

Key distinction: Pitting + DIP arthritis = psoriasis; pterygium = lichen planus; single longitudinal pigmented band with Hutchinson sign = melanoma. None of these need terbinafine — they need the correct diagnosis.

Board pearl: When KOH/PAS is repeatedly negative in a dystrophic nail, the answer is rarely "treat empirically" — it's biopsy or dermatology referral.

Key Differentials — Other-Category Causes of Nail Discoloration

— Single-digit longitudinal melanonychia with variegated brown/black band

— Band width >3 mm, asymmetric pigmentation, recent change

— Hutchinson sign: pigment extending onto proximal or lateral nail fold — highly suggestive

— Higher incidence in Black, Asian, and Hispanic patients (Bob Marley)

— Punch biopsy of the matrix is mandatory — do NOT treat as fungus

— Verrucous, persistent periungual lesion, often HPV-associated

— May mimic chronic paronychia or warts; biopsy if not resolving

— Painful, blue-red subungual lesion, cold sensitivity, pinpoint tenderness

— MRI helpful; surgical excision curative

— Greenish-black discoloration under onycholytic nail

— Wet work, paronychia

— Treat: clip onycholytic plate, dry the area, dilute acetic acid soaks (1:4 vinegar:water) or topical gentamicin/ciprofloxacin drops

— Erythematous, tender proximal nail fold with loss of cuticle; chronic course

— Treat: keep dry, topical antifungal ± topical steroid, address wet-work exposure

— Clubbing: lung disease (cancer, IPF, bronchiectasis), cyanotic heart disease, IBD

— Splinter hemorrhages: endocarditis, trauma, vasculitis

— Koilonychia (spoon nails): iron deficiency anemia

— Half-and-half nails (Lindsay): chronic kidney disease

— Terry nails (white proximal, distal pink band): cirrhosis, CHF, diabetes

— Beau lines: transient growth arrest (severe illness, chemo, COVID)

Subungual melanoma (acral lentiginous melanoma):

Subungual squamous cell carcinoma:

Glomus tumor:

Pseudomonas nail infection ("green nail syndrome"):

Candida paronychia:

Systemic disease nail signs:

Key distinction: Greenish nail = Pseudomonas, not fungus — soaks and clipping, not terbinafine.

Board pearl: A single nail with a widening dark longitudinal band and periungual pigmentation in a 55-year-old is melanoma until biopsy proves otherwise — antifungal therapy is the wrong-answer trap.

Secondary Prevention and Long-Term Plan

— Tinea pedis with topical terbinafine, clotrimazole, or butenafine for 4 weeks, even if minimal

— Tinea cruris, tinea corporis, tinea capitis (in household)

— Treat affected family members; screen pets if zoophilic species suspected

— Discard or disinfect old shoes (UV shoe sanitizers, antifungal sprays/powders, freezing 48 hours)

— Antifungal foot powders (miconazole, tolnaftate) in shoes daily

— Moisture-wicking socks; change midday if hyperhidrosis

— Rotate shoes to allow drying

— Avoid barefoot in locker rooms, pool decks, hotel rooms — wear shower sandals

— Keep nails short and dry; do not share clippers

— Dry interdigital spaces thoroughly after bathing

— Treat hyperhidrosis (aluminum chloride 20% nightly; iontophoresis; botulinum toxin in refractory cases)

— Efinaconazole, ciclopirox, or tavaborole 1–2× weekly may reduce recurrence

— Topical terbinafine cream to soles 1–2× weekly during high-risk periods (humid months, athletes)

— New discoloration, onycholysis, or scaling between toes → retreat tinea pedis early

— Annual self-checks; reconfirm diagnosis if recurrence (PAS) before reinitiating oral therapy

— Annual comprehensive foot exam with monofilament and pulses

— Routine podiatric nail care (every 8–12 weeks)

— Optimize glycemic control (HbA1c target individualized)

Counsel that recurrence is common (20–25%) — prevention is a long-term project, not a one-time prescription.

Treat reservoirs of infection:

Footwear and hygiene measures:

Skin care:

Prophylactic topical antifungals after cure:

Patient education on relapse signs:

Special diabetic considerations:

Step 3 management: After successful treatment, prescribe weekly topical antifungal prophylaxis and treat tinea pedis aggressively — this is the right way to answer "how do you prevent recurrence?"

Board pearl: Untreated tinea pedis is the #1 reason onychomycosis recurs; treating it is non-negotiable secondary prevention.

Follow-Up, Monitoring, and Counseling

— Toenails grow ~1–1.5 mm/month; fingernails ~3 mm/month

— Visible clearance takes 9–12 months for toenails even after mycologic cure

— Complete cure rates 35–50% with oral terbinafine; lower with topicals

— Baseline: AST/ALT, pregnancy test, medication reconciliation

— Week 4–6: check LFTs if at risk or symptomatic; assess adherence and side effects (especially taste disturbance)

— End of treatment (week 12): clinical assessment; no need to repeat mycology yet

— Month 6: clinical reassessment — proximal nail should appear clear

— Month 9–12: assess complete cure; if persistent disease, repeat KOH/PAS before retreating

— Reassess at 3 and 6 months for adherence and partial response

— Continue full 48 weeks before declaring failure

— Take terbinafine with or without food; itraconazole with food and acidic beverage

— Stop drug and call provider for: jaundice, dark urine, RUQ pain, persistent nausea, rash, mouth sores, fever, severe taste loss

— Avoid alcohol excess during therapy

— File thickened nails to improve topical penetration

— Apply topicals to clean, dry nail; clean lacquer weekly with alcohol (ciclopirox)

— Photograph nails at baseline and follow-up

— Track % nail involvement

— Confirmed mycologic relapse → second course of oral terbinafine often effective

— Consider combination therapy (oral + topical + debridement) for second-line attempts

Realistic timeline counseling (set at first visit):

Follow-up schedule for oral terbinafine:

Follow-up for topical therapy:

Patient counseling points:

Adherence is the leading cause of treatment failure — confirm fills, dose, duration

Documenting outcomes:

Retreatment criteria:

CCS pearl: In the CCS module — order LFTs at baseline, advance the clock 6 weeks, recheck LFTs and assess symptoms, then advance to 12 weeks and stop the drug. Schedule a 6-month follow-up visit for outcome assessment.

Board pearl: Patients who "fail" terbinafine often actually have inadequate adherence or incorrect diagnosis (psoriasis) — recheck diagnosis with PAS before re-prescribing.

Ethical, Legal, and Patient Safety Considerations

— Discuss hepatotoxicity, permanent taste loss, severe cutaneous reactions, and drug interactions explicitly

— Document modest cure rates (~35–50%) and recurrence risk so the patient can weigh cosmetic vs medical benefit

— In purely cosmetic indications, shared decision-making is critical — a serious adverse event from a cosmetic prescription has both ethical and medicolegal weight

— Itraconazole + simvastatin = rhabdomyolysis risk (contraindicated)

— Itraconazole + warfarin = bleeding (frequent INR checks needed)

— Itraconazole in CHF = decompensation (black box; ask about EF before prescribing)

— Terbinafine + metoprolol or TCA = increased levels via CYP2D6 inhibition

— Always run an interaction check; this is a classic root cause analysis topic in Step 3 patient safety

— Always document negative pregnancy test before itraconazole; counsel on contraception for 2 months after itraconazole completion

— Defer treatment in pregnancy/lactation unless compelling indication

— When a primary care clinician hands off long therapy (12 weeks), ensure LFT monitoring is scheduled and results are reviewed — dropped monitoring is a recognized safety gap

— Closed-loop communication with pharmacy and patient

— Confirming diagnosis before systemic therapy is both ethically sound and consistent with antimicrobial stewardship — prevents unnecessary drug exposure and resistance pressure

— Cost of efinaconazole and tavaborole (high) versus ciclopirox or oral terbinafine (generic, cheap) — counsel on options and insurance coverage

— Access to podiatry varies by insurance — important for diabetic foot care

— In elderly with cognitive impairment, prefer simpler regimens or caregiver-administered topical therapy

Informed consent for oral antifungals:

Medication reconciliation as patient safety:

Pregnancy safety:

Transitions of care:

Diagnostic stewardship and antimicrobial stewardship:

Health equity considerations:

Vulnerable populations:

Step 3 management: A Step 3 vignette where a patient on simvastatin is prescribed itraconazole and develops muscle pain — the correct quality/safety answer is medication reconciliation failure, and the corrective system change is automated EHR interaction alerts and pharmacist double-check.

Board pearl: "Cosmetic" does not equal "low-risk" when the treatment can cause hepatic failure — informed consent and shared decision-making are tested.

High-Yield Associations and Rapid-Fire Clinical Facts

Most common organism overall: Trichophyton rubrum (>70%)

Most common toenail involved: hallux (great toe)

Most common pattern: distal lateral subungual onychomycosis (DLSO)

PSO pattern → think HIV; offer testing

White superficial onychomycosis in a child → think non-dermatophyte mold (Fusarium, Acremonium)

Candida onychomycosis → fingernails, wet workers, chronic paronychia

"Two feet–one hand" syndrome → T. rubrum dermatophytosis, often with onychomycosis

Best single diagnostic test: PAS-stained nail clipping (sensitivity ~85%)

Confirmation required before oral therapy (standard of care; payer requirement)

First-line oral therapy: terbinafine 250 mg daily × 12 weeks for toenails, 6 weeks for fingernails

Itraconazole pulse: 200 mg BID × 1 week/month × 3 months for toenails

Itraconazole contraindications: CHF, pregnancy, significant hepatic disease, strong CYP3A4 substrate interactions

Black-box statin interaction: itraconazole + simvastatin/lovastatin = rhabdomyolysis

Permanent side effect to counsel about with terbinafine: dysgeusia / ageusia

Topical first-line: efinaconazole 10% daily × 48 weeks (highest cure of topicals)

Complete cure rates: terbinafine ~38–50%, itraconazole ~14–28%, efinaconazole ~17%, ciclopirox ~5–9%

Recurrence rate: 20–25% at 2–3 years

Most important secondary prevention: treat tinea pedis + footwear hygiene + weekly topical antifungal

Diabetic onychomycosis is a risk factor for cellulitis, ulceration, and amputation — treat actively

Greenish nail discoloration → Pseudomonas, not fungus → acetic acid soaks

Single pigmented longitudinal band + Hutchinson sign → subungual melanoma → biopsy

Nail pitting + DIP arthritis → psoriasis, not onychomycosis

Yellow nail syndrome: yellow nails + lymphedema + respiratory disease

Half-and-half nails → CKD; Terry nails → cirrhosis; koilonychia → iron deficiency

Pediatric onychomycosis → check for tinea capitis in household

Toenail growth rate → 1–1.5 mm/month → visible clearance takes ~12 months

Board pearl: Memorize the three "contraindication" landmines — itraconazole + statin (rhabdo), itraconazole + CHF (decompensation), and itraconazole/terbinafine + pregnancy (defer treatment).

Board Question Stem Patterns

Stem 1 — Classic dermatophyte: 58-year-old man with thickened, yellowed great toenails for 2 years and scaling between toes. KOH shows septate hyphae. Next step → oral terbinafine 250 mg daily × 12 weeks, after baseline LFTs.

Stem 2 — Statin trap: 65-year-old on simvastatin started on itraconazole for onychomycosis develops muscle pain, dark urine, CK 12,000. Diagnosis → rhabdomyolysis from CYP3A4 interaction. Best preventive intervention → medication reconciliation / interaction alerting.

Stem 3 — CHF trap: 70-year-old with EF 30% has dystrophic toenails. Best therapy → topical efinaconazole (itraconazole contraindicated in HF; terbinafine acceptable but topical is safest if mild).

Stem 4 — Pregnancy: 28-year-old pregnant woman with cosmetic complaint of yellowed toenails. Best management → defer treatment until after delivery and lactation.

Stem 5 — HIV clue: 35-year-old man with proximal white nail discoloration. Next step → HIV testing.

Stem 6 — Melanoma trap: 60-year-old with single dark longitudinal band on thumbnail, pigment extending onto cuticle. Best next step → nail matrix biopsy, not antifungal therapy.

Stem 7 — Psoriasis trap: Patient with "fungal nails" has DIP joint swelling, scalp scaling, pitting. KOH negative. Diagnosis → psoriatic nail disease; refer dermatology/rheumatology.

Stem 8 — Pseudomonas: Greenish-black under-nail discoloration in a hair stylist with chronic wet hands. Diagnosis → green nail syndrome; treatment → clip onycholytic nail, dilute vinegar soaks.

Stem 9 — Diabetic high-risk: Diabetic with onychomycosis and recurrent cellulitis. Best secondary prevention → treat onychomycosis + tinea pedis, optimize foot care, podiatry referral.

Stem 10 — Recurrence prevention: Patient cured of onychomycosis last year; recurrence noted. Best preventive measure → weekly topical antifungal + treat tinea pedis + footwear hygiene.

Stem 11 — Pediatric: 8-year-old with tinea capitis and dystrophic fingernails. Treatment → oral terbinafine weight-based × 6 weeks.

Stem 12 — Quality/safety: Hospital identifies several patients on itraconazole + simvastatin developing myopathy. Best system intervention → EHR drug-interaction alerts at order entry.

Board pearl: When the stem mentions proximal white nail, the answer pathway is almost always HIV screen, not "start terbinafine."

One-Line Recap

— Confirm before you treat: PAS-stained nail clipping is the most sensitive single test; empiric oral therapy without confirmation is wrong on Step 3.

— First-line therapy: Terbinafine 250 mg PO daily × 12 weeks (toenails) / 6 weeks (fingernails) for dermatophyte disease, with baseline LFTs and medication reconciliation (especially statins, warfarin, SSRIs).

— Know the contraindications: Itraconazole is contraindicated in CHF, pregnancy, and with simvastatin/lovastatin; both terbinafine and itraconazole are deferred in pregnancy and significant hepatic disease — topical efinaconazole is the safe alternative.

— Red-flag mimics: Proximal subungual pattern → test for HIV; single dark longitudinal band with Hutchinson sign → melanoma biopsy; nail pitting + DIP arthritis → psoriasis; green discoloration → Pseudomonas (acetic acid soaks, not antifungals).

— Prevent recurrence: Treat concurrent tinea pedis, practice footwear hygiene, use weekly topical antifungal prophylaxis, and refer diabetics for podiatric nail care to reduce risk of cellulitis, ulceration, and amputation.

The point: Onychomycosis is a common, often cosmetic but sometimes medically important fungal nail infection that must be confirmed mycologically (KOH/PAS/culture) before systemic therapy, treated first-line with oral terbinafine 250 mg daily × 12 weeks for toenails in healthy adults, and managed with attention to drug interactions, comorbidities, and long-term prevention of recurrence.

High-yield recap bullets:

CCS pearl: In a diabetic with onychomycosis, the model answer sequence is: KOH/PAS, baseline LFTs, terbinafine 250 mg daily × 12 weeks, treat tinea pedis with topical terbinafine × 4 weeks, podiatry referral for nail debridement, follow-up at 6 weeks and 6 months — embodying confirmation, treatment, comorbidity care, and longitudinal follow-up that defines Step 3 thinking.