Probable topical permethrin-induced neck dystonia.

A 28-year-old Caucasian man who applied topical 5% permethrin cream once for the treatment of scabies infestation experienced a severe dystonic reaction on the left side of his neck. Assessment of causality using the Naranjo algorithm revealed a probable relationship between this adverse drug event and application of topical permethrin. Dystonic or musculoskeletal adverse reactions, including muscle spasms, have been reported with pyrethroid insecticides only after inappropriate occupational dermal exposure. These reported reactions occurred 4-48 hours after exposure and lasted from a few days to a few weeks. The mechanism behind these adverse effects may be related to permethrin's ability to delay sodium channel closure within nerve cells. This may result in a lowered threshold of nerve fibers to activation of further action potentials, leading to repetitive firing and hyperexcitation of the nervous system. Clinicians should be aware of this rare but potential adverse effect of permethrin.