Objective: To describe a pediatric case with a pathogenic SCN8A variant who presented with neurodevelopmental delay, seizure and prominent cerebellum signs, along with providing a literature review of the movement disorder phenomenology described in SCN8A gene.

Background: SCN8A is a gene that encodes the Nav1.6 voltage-gated sodium channel, which is essential in propagating the electrical signal in neurons. Multiple pathogenic variants in SCN8A have been recognized, and these have been linked to a broad spectrum of neurological manifestations which mostly related to epilepsy and neurodevelopmental disorders. Various published studies have described the different epilepsy phenotypes in detail. Although movement disorders have been linked to this mutation, this aspect remains sparsely described in literature.

Method: Case report.

Results: A 5-year-old girl with neurodevelopmental delay who developed seizures at the age of 4 months, she later began exhibiting cerebellar symptoms including ataxia, dysmetria and dysarthria at 9 months. Then at the age of 3 years, she experienced dystonic posturing of the lower limbs. Genetic testing performed revealed a de novo pathogenic heterozygous variant in SCN8A c.4850G>A (p.Arg1617Gln). Her movement disorder followed a fluctuating course over time, while her seizures remained well-controlled with antiseizure medications.

A literature review was conducted identifying 83 patients across 24 published studies which described the movement disorders associated with SCN8A variants. The reported phenomenology was heterogeneous, with ataxia being the most frequent manifestation (33) followed by dystonia (20), dyskinesia (16), and chorea (14). Less frequently reported features included paroxysmal dyskinesia (11), tremor (8), myoclonus (5), stereotypies (4) and athetosis (3), hyperkplexia (3), tics (1), and paroxysmal tonic upgaze (1). Parkinsonism has not been reported and 4 individuals had unclassified movements. Co-occurrence of multiple movement disorders was common (36/65).

Conclusion: This report highlights the expanding movement disorder spectrum associated with SCN8A-related disorders, underscoring the importance of recognizing these features in clinical practice.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/scn8a-related-disorders-expanding-the-movement-disorder-phenotype-through-a-case-report-and-literature-review/