*Equal contribution from B. Sweeney and DA. Olszewska

To report the first Irish family with Salla disease; to expand the phenotype by the first description of myoclonus in this condition; to report new radiological features.

Salla disease is an autosomal recessive, lysosomal storage disorder caused by pathogenic variants in a lysosomal sialic acid transporter protein gene (SLC17A5). It was first described in 1979 and is named after a small municipality in Finland. The phenotypic spectrum ranges from mild cognitive impairment and spasticity to intermediate severe Salla disease involving global developmental delay/seizures/hypotonia, and hypomyelination on brain magnetic resonance imaging(MRI). To date, there have only been 204 cases described, with MRI brain reported on 35/204 and no videos available (10 images published).

We conducted a prospective study of five siblings with genetically confirmed Salla disease born to nonconsanguineous parents. We examined the phenotype, genotype, and radiological features of the family including the parents. Video recordings and a detailed literature review accompany this report.

Our cohort included two females and three males who were homozygous for a SLC17A5 p.K136E (c.406A>G) pathogenic variant. Their parents were asymptomatic carriers. All five affected siblings had symptom onset in early childhood with hypotonia/psychomotor retardation/delayed developmental milestones, and dystonia. Four of the siblings had startle myoclonus. An MRI brain performed on one of our patients displayed putaminal cysts and hypomyelination.

Salla disease is a rare lysosomal storage disease with a handful of cases described worldwide. This is the first Irish report of Salla disease. We describe the previously unreported clinical feature of myoclonus. Furthermore, the presence of putaminal cysts on MRI is a newly reported radiological feature which could serve as a diagnostic clue for this condition. Although rare, Salla disease should be considered in patients with progressive developmental delay and hypomyelination on MRI.