Rabbit syndrome in anti-NMDAR antibody encephalitis: a unique association

A Systematic Review of Oral Vertical Dyskinesia ("Rabbit" Syndrome)

Background and Objectives: Vertical rhythmic dyskinetic movements that are primarily drug-induced and affect solely the jaw, mouth, and lips without involving the tongue have been historically described as "rabbit" syndrome (RS). Evidence on the unique features and implications of this disorder remains limited. This literature review aims to evaluate the clinical-epidemiological profile, pathological mechanisms, and management of this movement disorder. Materials and Methods: Two reviewers identified and assessed relevant reports in six databases without language restriction published between 1972 and 2024. Results: A total of 85 articles containing 146 cases of RS were found. The mean frequency of RS among adults in psychiatric hospitals was 1.2% (range 0-4.4%). The mean age of affected patients was 49.2 (SD: 17.5), and 63.6% were females. Schizophrenia was the most frequent comorbidity found in 47.6%, followed by bipolar disorder (17.8%), major depressive disorder (10.3%), and obsessive-compulsive disorder (3.7%). Five cases were idiopathic. The most common medications associated with RS were haloperidol (17%), risperidone (14%), aripiprazole (7%), trifluoperazine (5%), and sulpiride (5%). The mean duration of pharmacotherapy before RS was 21.4 weeks (SD: 20.6). RS occurred in association with drug-induced parkinsonism (DIP) in 27.4% and with tardive dyskinesia (TD) in 8.2% of cases. Antipsychotic modification and/or anticholinergic drugs resulted in full or partial recovery in nearly all reported cases in which they were prescribed. Conclusions: RS occurs as a distinct drug-induced syndrome associated primarily but not exclusively with antipsychotics. Distinguishing RS from TD is important because the treatment options for the two disorders are quite different. By contrast, RS may be part of a spectrum of symptoms of DIP with similar course, treatment outcomes, and pathophysiology.

A video-based discussion of movement disorders in paediatric anti NMDAR encephalitis: A case series from Eastern India

PURPOSE

The spectrum of movement disorders associated with anti N-Methyl-d-Aspartate-Receptor (NMDAR) encephalitis is myriad, particularly in children, possibilities of which were investigated from two tertiary care centres.

METHODS

A retrospective study was conducted in two tertiary referral centres in Eastern India, analysing data of 8 paediatric patients diagnosed as anti NMDAR encephalitis, presenting with one or more movement disorders (MDs).

RESULTS

All the patients were of Bengali ethnicity with a median age of 9 years (3-16 years) and with female predilection (62.5%). CSF pleocytosis was a common feature in all. Seizures were described in 62.5%% of patients with a solitary patient exhibiting abnormalities on brain imaging. 3 out of 8 (37.5%) of patients presented with a single MD while the remaining had more than one type. Oro-linguo-facial dyskinesias and dystonia (37.5% each) were the most common movement type followed by chorea (12.5%). Complex stereotypies, myoclonus and facial tics were noted in one patient each. All patients received pulse methyl prednisolone. Escalation to second line therapy in form of rituximab was done for 5 patients (62.5%). Following immunotherapy, hyperkinetic movements resolved in 50% of patients, with persistence of movements in one (12.5%). A mortality of 37.5% was noted. Median duration of follow up was 26 months, during which none of the patients had evidence of systemic neoplasm.

CONCLUSION

MDs are a core feature of anti NMDAR encephalitis, particularly in the paediatric age group, understanding and characterization of which, is the key to early diagnosis and effective therapy.

Functional Rabbit Syndrome: A Case Report

Rabbit Syndrome is a rare involuntary movement occurring in 1.5–4.4% of patients receiving antipsychotics and characterized by rapid, regular movements (4–6 Hz) of the oral and masticatory musculature resembling the chewing motions of a rabbit. Herein we describe a middle-aged woman who presented with a rabbit syndrome characterized by several clues of psychogenicity such as sudden onset, distractibility, variability and complete “miracolous” remission.