Thiamethoxam-Induced Subclinical Onychomadesis

Irritant contact dermatitis affecting the nail unit may lead to nail matrix damage and onychomadesis, which may initially be subclinical, becoming overt later. We describe a patient who developed these changes after using a chemical pesticide.

A 52-year-old woman presented with discoloration of the nails of both hands of two days' duration, associated with mild digital pain. She had used an over-the-counter insecticide product containing thiamethoxam (a broad-spectrum systemic insecticide) for her houseplants, preceding the nail changes. Examination revealed onycholysis and subclinical onychomadesis involving multiple fingernails as well as toenails. Onychoscopic examination showed proximal nail plate separation with an erythematous regular border. Histopathology showed an essentially normal nail plate with spongiosis, epidermal cell necrosis, and hypergranulosis. However, there was no evidence of bacterial or fungal infection, and dermal inflammation was mild. The patient was diagnosed with thiamethoxam-induced irritant dermatitis with subclinical onychomadesis and was advised to take general precautions with avoidance of any further contact with the insecticide. She was managed with topical steroids and emollient; however, on follow-up, she developed green nails, with progression to overt onychomadesis in some nails. Additionally, onychomycosis was observed in few nails in the long-term, which needed to be treated.

Thiamethoxam is an over-the-counter broad-spectrum insecticide used for houseplants, but its safety data does not mention acute adverse effects on nails, which was a novel finding in our patient. The safety data does mention the use of gloves for preparing and administering the product. Secondary bacterial and fungal infections, which can occur after the initial insult, further worsen the prognosis.