Onychomycosis (Nail Fungus) ICD-10: B35.1

Fungal / Infectious nail condition

Often searched as: yellow toenails, thick toenails, nails turning yellow and crumbling, nail fungus treatment, toenail lifting off, toenail falling off, brown nails, nail fungus photos…

Clinical urgency level

lowmediumhigh

Systems Affected

nailsperiungual skin

Severity Levels

mild

Discoloration or mild thickening of one or two toenails, no pain, limited to the distal nail — may respond to topical antifungal treatments.

moderate

Multiple nails affected, significant thickening or crumbling, some tenderness — requires oral antifungal therapy (terbinafine) for adequate cure.

severe

Total nail dystrophy, complete destruction of nail plate, secondary bacterial infection, pain limiting walking, or extensive infection in a diabetic or immunocompromised patient — requires urgent medical management.

Red Flags

Nail infection in a person with diabetes — risk of secondary cellulitis or deeper infection
Pain or difficulty walking due to severely thickened toenails
Signs of secondary bacterial infection: redness, warmth, pus around the nail
Fingernail fungus spreading rapidly — can indicate immune suppression
Nail changes in a child (nail fungus is uncommon in children — consider other causes)
Nail that completely separates from the nail bed

Clinical Presentation

Onychomycosis — yellow, thickened, crumbling toenails

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Onychomycosis image gallery — DermNet NZ

View gallery on DermNet NZ (image gallery reference)

When to See a Doctor

See a doctor if you have thick, discolored, or crumbling nails that don't improve with over-the-counter treatments, if more than one nail is affected, or if you have diabetes or a weakened immune system. A nail sample can confirm the diagnosis before starting oral antifungal treatment, which has potential side effects worth discussing with a doctor.

Differential Diagnosis

Psoriatic nail disease (pitting, oil drop sign, salmon patches — no fungus on culture)
Traumatic nail dystrophy (from repeated pressure or injury)
Yellow nail syndrome (lymphatic association — all nails yellow, slow growing)
Lichen planus of the nails
Pachyonychia congenita (genetic — present since childhood)
Alopecia areata nail changes (pitting, trachyonychia)

Comorbidities

Diabetes mellitus (impaired circulation and immunity — higher risk and severity)
Peripheral vascular disease
HIV / immunosuppression
Tinea pedis (athlete's foot — frequent source of nail infection)
Hyperhidrosis
Psoriasis (can coexist and complicate diagnosis)

Prognosis

Onychomycosis is curable but challenging to treat due to slow nail growth. Oral terbinafine (12 weeks for toenails) achieves mycological cure in 70–80% of patients and remains the most effective option. Topical treatments (amorolfine, ciclopirox) work for mild or superficial infections but have significantly lower cure rates (~30–50%). Recurrence is common — up to 25% within 3 years — especially if predisposing factors (sweaty feet, communal showers) are not addressed. Nails grow slowly, so visible improvement may take 9–12 months even after the fungus is eliminated.

Detailed Overview

Causes, Symptoms, and When to Take Action Onychomycosis is a fungal infection of the nail unit. It most often affects toenails but can also involve fingernails. While not life-threatening, it is common and can cause discomfort and cosmetic concerns. How It Looks Texture: Thickened, brittle, or crumbly nails Color: Yellow, white, brown, or gray discoloration Shape: Irregular nail surface, lifting from the nail bed, or distortion of the nail plate Location: More frequent in toenails than fingernails Why Does It Appear? Onychomycosis develops when fungi invade the nail unit, usually in warm and moist conditions. Factors that may contribute include: Aging Repeated trauma to the nails Underlying skin conditions Weakened immune defenses Should You Be Concerned? Onychomycosis is generally benign, but it may lead to nail thickening, pain, and functional problems. In some cases, it can predispose to secondary bacterial infections. Medical evaluation is recommended if nails become painful, severely distorted, or if multiple nails are affected. Can You Prevent It? While not always preventable, risk may be reduced by: Keeping nails trimmed, dry, and clean Wearing protective footwear in communal areas Avoiding sharing personal items such as shoes or nail clippers 👉 How Piel AI Can Help Piel AI allows you to track changes in your nails over time using regular imaging. While not a diagnostic tool, it helps you stay aware of progression or changes that may need professional attention. Summary Onychomycosis is a fungal nail infection. It causes thickened, discolored, and sometimes painful nails. Risk factors include age, nail trauma, and reduced immunity. Prevention focuses on hygiene and protective practices.

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