Atypical neuroleptic malignant syndrome: diagnostic controversies and considerations.

Neuroleptic malignant syndrome (NMS) is a serious and potentially fatal adverse effect of antipsychotic drugs. The diagnosis of NMS commonly requires core symptoms of hyperthermia and muscle rigidity. Although diagnostic criteria for NMS have been established and are widely accepted and used, it should be recognized that atypical presentations pose a diagnostic dilemma, as hyperthermia and/or muscle rigidity may be absent or develop slowly over several days, leading to impairment or a significant delay in diagnosis and treatment. Evidence from case reports and retrospective evaluations supports a concept of atypical NMS, particularly with regard to treatment with atypical antipsychotics. However, it remains unclear whether these atypical presentations represent early or impending NMS. Furthermore, it is unclear whether dysfunction in other neurotransmitter systems, in addition to dopamine, may be involved in the pathogenesis of NMS induced by atypical antipsychotics. In patients receiving any antipsychotic, clinicians should carefully evaluate any features of NMS and should not prematurely exclude a diagnosis of NMS in cases where severe rigidity or hyperthermia is not initially apparent.