ABSTRACT

Several under-recognized or newly described rare nail diseases may mimic other well-known and common nail diseases. The objective of this chapter is to highlight these conditions to help physicians to diagnose and manage the disease at an early stage. Asymmetric gait nail unit syndrome presents as structural changes in nail plate closely resembling onychomycosis, and early diagnosis may avoid unnecessary investigations and treatments. Hang nail is triangular-shaped extensions of cuticle that may cause swelling and redness of nail fold, which need to be differentiated from paronychia. Brittle nail syndrome is characterized by increased fragility of nail plate with or without other underlying dermatological or non-dermatological disease. Retronychia is another recently established entity that is characterized by ingrowing of proximal nail plate in proximal nail fold with old nail plate underneath the uppermost nail. Disappearing nail bed is loss of nail bed due to prolonged distal onycholysis. Parakeratosis pustulosa is an under-recognized condition that starts with a pustule or vesicle that later develop scaling, fissures, and erythema and may be misdiagnosed as eczema or psoriasis. Worn-down nails usually develop on the dominant hand due to repetitive trauma to the nail unit and present as erythematous triangular areas of thinning of the distal nail plate. Ectopic nail (EN) or onychoheterotopia may be congenital or a post-trauma-acquired rare condition where an additional nail unit is present at an unusual location (ungula or periungual region). It presents as small spikes of nail tissue and needs to be differentiates from cutaneous horn and rudimentary digit.