The evidence to date, both published and unpublished, which addresses the validity of the proposed unique subgroup of children with early and abrupt onset of obsessive–compulsive disorder (OCD) and/or tic disorders subsequent to streptococcal infections was reviewed. The aetiology of OCD and tic disorders is unknown, although it appears that both disorders may arise from a variety of genetic and environmental factors. Post-streptococcal autoimmunity has been postulated as one possible mechanism for some. The acronym PANDAS (for paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections) has been given to a subgroup of paediatric patients who meet five inclusionary criteria: presence of OCD and/or tic disorder, pre-pubertal symptom onset, sudden onset or episodic course of symptoms, temporal association between streptococcal infections and neuropsychiatric symptom exacerbations, and associated neurological abnormalities. The proposed model of pathophysiology provides for several unique treatment strategies, including the use of antibiotic prophylaxis to prevent streptococcal-triggered exacerbations, and the use of immunomodulatory interventions (such as intravenous immunoglobulin or therapeutic plasma exchange) in the treatment severe neuropsychiatric symptoms. For the latter study group, long-term (2–5 yr) follow-up revealed continued symptom improvement for the majority of patients, particularly when antibiotic prophylaxis had been effective in preventing recurrent streptococcal infections. In addition, the episodic nature of the subgroup’s illness provides for opportunities to study brain structure and function during health and
disease, as well as allowing for investigations of the aetiologic role of anti-neuronal antibodies and neuroimmune dysfunction in both OCD and tic disorders. Although much research remains to be done, an increasing body of evidence provides support for the postulate that OCD and tic disorders may arise from post-streptococcal autoimmunity. The unique clinical characteristics of the PANDAS subgroup, the presence of volumetric changes in the basal ganglia, and the dramatic response to immunomodulatory treatments, suggest that symptoms arise from a combination of local, regional and systemic dysfunction. Ongoing research is directed at understanding the nature of the abnormal immune response, as well as identifying at-risk children, in order to provide for novel strategies of prevention and treatment.