Broadening the Spectrum of SLC22A5 Phenotype: Primary Carnitine Deficiency Presenting with Focal Myoclonus

Abstract

Primary carnitine deficiency (PCD) is caused by pathogenic variants of the SLC22A5 gene, which encodes a transmembrane protein that functions as a high affinity carnitine transporter. Carnitine is essential for the transport of acyl-CoA, produced from fatty acids, into the mitochondria where they are oxidised to produce energy. We present the case history of an 8-year-old boy who presented with fever, lethargy, focal rhythmic (3 Hz) left wrist twitching, and severe encephalopathy. MRI brain showed basal ganglia involvement. Metabolic investigations revealed low serum carnitine; whole genome sequencing confirmed compound heterozygous SLC22A5 mutations. With carnitine replacement, intensive care support, and neurorehabilitation, he made a remarkable recovery, regaining independent breathing, speech, mobility, and hand use. Seizure presentation in PCD is rare and presentation with sustained focal myoclonus has not been previously reported. This case expands the known phenotype of PCD. Prompt carnitine replacement is imperative.

Keywords: focal myoclonus; genotype phenotype; primary carnitine deficiency.

Figure 1.

MRI head. A, Day 1 of PICU admission. Subtle bilateral high signal in the lentiform and caudate nuclei (arrows), swelling of the caudate nuclei, no restricted diffusion, contrast enhancement. MRS was non-specific (not shown). B, Day 5. Persistent basal ganglia changes, mild reduction in swelling of the caudate nuclei with reduced mass effect on the frontal horns of the lateral ventricles (arrows). C, Two months later, showing resolution of basal ganglia signal changes and swelling; frontal horns of lateral ventricles no longer compressed, but mild basal ganglia atrophy.