Segmental Myoclonus and Epilepsy in a Child with GAD 65 Antibodies

Keywords: GAD 65 antibodies, Child, Myoclonus, Epilepsy Glutamic acid decarboxylase (GAD) is a rate-limiting enzyme for the synthesis of γ-aminobutyric acid (GABA), an inhibitory neurotransmitter.It is widely distributed in the central nervous system but is also present in other organs, especially the pancreas.Elevated GAD 65 antibodies have diverse clinical associations, including stiff person syndrome (SPS), cerebellar ataxia, limbic encephalitis, epilepsy, ocular motor disorders, type 1 diabetes mellitus, 1,2 and rarely myoclonus.3-6 GAD 65 antibody-associated neurological syndromes, mostly SPS or epilepsy, are rarely reported in pediatrics.2,7 Many patients improve after immune therapy and some, whose presentation includes seizures, become seizure free. 1 Elevated GAD 65 antibodies may be associated with tumors.However, such an association is rare in children. 1 A 7.5-year-old-girl, previously healthy and intellectually normal, presented with a few days history of self-limited right-sided throbbing headache and a new-onset focal tonicclonic seizure with bilateral spread lasting 5 min.She concurrently developed right sided then bilateral continuous, high frequency, semi-rhythmic facial muscle twitching, consistent with myoclonus, most prominent on the right face and upper lip (Video).The myoclonus was present during wakefulness and sleep.Six weeks later, the myoclonus spread to her right thumb and tongue.She did not develop myoclonus anywhere else.She did not feel the myoclonus.The myoclonus responded partially to treatment with clonazepam.Levetiracetam was started following a second, 2.5 min focal unaware seizure, 10 d after the first seizure.There were no signs of encephalopathy, cognitive decline, or regression.She could eat, drink, and speak normally.Pregnancy, delivery, development, and past medical history were unremarkable.Parents are non-consanguineous.There was no family history of seizures, autoimmune, or movement disorders.The patient's full neurological exam was normal apart from the myoclonus.Blood tests were normal including CBC, extended electrolytes, glucose, liver and thyroid function, uric acid, copper, ceruloplasmin, ESR, CRP, C3, C4, immunoglobulins, rheumatoid factor, ANCAs, anti-TPO, and phospholipid (cardiolipin and β2-glycoprotein) antibodies.ANA screen was positive (low titers at 1:40 for speckled and 1:80 for nucleolar antinuclear antibodies detected by immunofluorescence), but dsDNA and extractable nuclear antigens were negative.Urinary vanillylmandelic acid and homovanillic acid were negative.CSF investigations revealed normal glucose and lactate, with a slightly elevated protein (0.29 g/l, normal range: 0.12-0.24)and cell count (8 cells, 97% lymphocytes).Oligoclonal bands were present in the CSF with no corresponding bands in the serum.GAD 65 antibodies were 'highly positive' in CSF and serum as tested by a cell-based immunoassay (CBA: Euroimmun AG, Luebeck, Germany), performed by MitogenDx, Calgary, Canada.Serum level of anti-GAD 65 as tested by an enzyme-linked

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