Fungal nail infections, also called onychomycosis, are among the most common nail disorders. With rising prevalence of conditions like diabetes and increasing number of the aged population the numbers of individuals with these infections are also on the rise.
Types of fungal nail infection
Fungal nail infections may be classified according to the part of the nail affected. (1-5) They can be categorised into Distal and lateral subungal onychomycosis (DLSO), superficial white onychomycosis (SWO) and so forth.
Distal and lateral subungual onychomycosis
Distal and lateral subungual onychomycosis is also called DLSO. This is the most common form of onychomycosis. It is caused commonly be dermatophytes and may affect a healthy nail or one already diseased, e.g. by psoriasis.
The most common feature is affliction of the lateral edges or sides of the nail bed initially. The spread occurs over the nail bed leading to yellowish or creamy discoloration of the nails. The white and creamy patches indicate places where the nail has been detached from the nail beds.
The nail also becomes thickened. The nail becomes soft or brittle and crumbles or breaks away. The spread may be limited to one side of the nail or spread sideways to involve the whole nail bed.
The progress of the nail infection usually takes weeks or more slowly over months or years. As time progresses the nail becomes opaque, thickened and cracked, friable and raised from the nail bed.
Skin around the nail becomes inflamed, scaly and shows signs of fungal infections. Toe nails are more commonly affected than finger nails with 80% of cases affecting the feet.
Superficial white onychomycosis
Superficial white onychomycosis is also known as SWO. This is less common form of fungal nail infection. It is caused by T. mentagrophytes.
There is usually a white chalky plaque on the nail and occurs almost exclusively on the toenails. The top of the nails or nail plate is more commonly affected than the nail bed unlike DLSO.
Nail plate may become eroded and even lost. There is a whitish yellow discoloration of the nails and flakes over the nail plate. The nail may become brittle and friable and break off.
Fungal infection of the skin around the nails or elsewhere is less common than seen with DLSO.
Proximal subungual onychomycosis
This is a rarer form of fungal skin infection. It is seen in those with a suppressed immunity like those with HIV infection and AIDS. It is also seen in those with diabetes and those with poor blood supply to fingers and toes.
This is usually caused by dermatophyte infection. The infection begins with a white spot beneath the nail bed that fills up the nail bed completely.
There is concomitant athlete’s foot or fungal foot infection. The nail becomes white especially towards the base of the nail and remains normal at the tips.
This may be of three types. Candida paronychia occurs as swelling of the nail bed with pain. There are usually patchy opaque spots that may be discoloured appearing white, yellow, green or black. Sometimes furrows may be seen over the nails.
Most cases are on fingernails usually the middle finger. There may also be a Candida granuloma where there is direct invasion and thickening of the nail plate. It is uncommon and seen with Candida paronychia.
Subungual abscess with DLSO is another form where there is collection of pus in the nail beds.
The third type of infection is Total nail dystrophy that leads to thickening of the entire nail. Those dealing with water exposure and psoriasis are more at risk of this condition. There is complete destruction of the nail.
Secondary bacterial infections
Fungal nail infections may be complicated by superimposed bacterial nail bed or nail infections. This is extremely painful and there may be formation of pus.
In rare cases (especially those with diabetes or the elderly) there may be a risk of cellulitis or bacterial infection under the skin or osteomyelitis or infection of the bone.
Diagnosis of fungal nail infection
Diagnosis of fungal nail infection is usually made clinically. However, only about 20-50% of discoloured nails have a fungal infection confirmed with dermatophyte.
The rest may be due to:
- damage to nails
- other conditions like Lichen planus
- psoriatic nail disease
- poor foot care
- bacterial nail infections
- discoloration of nails due to health conditions like thyroid disease, diabetes etc.
- drug reactions or side effects e.g. with tetracyclines or quinolone antibiotics (1, 5)
For appropriate diagnosis the affected nail is clipped and sent to the laboratory for microscopic examination. The nail bits are stained with special dyes and under the microscope the organism may be seen.
The specimen can be immersed in a solution of 10 to 30% KOH or NaOH mixed with 5% glycerol before examination.
Results are usually obtained in around 3 to 4 days.
For DLSO cases a scraper like instrument is used to take bits of tissues from under the nails or the nail beds.
For SWO the top of the nail is scraped and sent to the laboratory. Sometimes diagnosis may be confirmed by allowing the fungi on the nail to grow in culture media in the laboratory and examine once the growth is obtained. This usually may take weeks for a confirmed diagnosis.
The nail anatomy under the microscope helps determine other causes of nail discoloration and deformity like psoriasis. (1, 5)
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)