Objective: Describe the clinical/laboratory challenges in diagnosis of myoclonus-dystonia at a young age

Background: SGCE Myoclonus-Dystonia (SGCE-M-D) is a genetic movement disorder characterized by multifocal myoclonus and dystonia. Although symptoms may begin at a young age, the diagnosis might be challenging, and other differential diagnoses, with different treatments, should be considered.

Method: Case report

Results: A 3-year-old girl was observed in the neuropediatric clinic due to sudden and transient involuntary movements since 12-months of age, involving the neck and upper limbs. They occurred during action, causing objects to fall. There was no loss of consciousness, triggers, psychomotor regression, nor delay. A third-degree cousin also had a history of involuntary movements since 8-years-old.

At the examination, positive spontaneous and postural multifocal myoclonus was seen mainly in the upper limbs and trunk. There were no motor deficits, abnormal postures, hyperplexia, or fasciculations. Motor reflexes were normal. There was no ataxia or gait abnormalities. Video-EEG registered dozens of torso, cephalic and upper-limb episodes, consistent with positive multifocal myoclonus, without electroencephalographic correlation. Brain-MRI findings were unspecific.Thyroid function, thyroid antibodies, ammonia, muscle enzyme, ceruloplasmin, porphyrins, were normal. CSF had 5 cells, with no protein increase nor glucose consumption. An antibody panel identified contactin-associated protein-like 2 (CASPR 2) antibodies, weak title (1/32) in serum only, confirmed in a sequential analysis (1/10). We considered an auto-immune etiology and offered a trial with iv immunoglobulins 13g/2 days. However, myoclonus didn’t improve. Electromyography and peripheral nerve conduction studies were normal, with no anomalous rest activity. A thoracic-abdominal-pelvic MRI was unrevealing. Neoplastic markers were negative.

Given the persistence of the symptoms, with positive family history, an NGS test was requested, identifying a mutation in SGCE gene c.158C>G, compatible with SGCE-M-D. Cousin is awaiting genetic test results. Treatment with zonisamide until 60mg daily was initiated with no benefit, with a switch to carbamazepine 40mg.

Conclusion: SGCE-M-D misdiagnosis with inflammatory disorders may occur. Absence of response to immunosuppressants and positive family history may point to a genetic cause.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/pediatric-myoclonus-dystonia-misdiagnosed-as-caspr2-encephalitis-a-diagnostic-challenge/