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Background Kearns-Sayre syndrome (KSS) typically presents with chronic progressive external ophthalmoplegia, a pigmentary retinopathy, cardiac conduction abnormalities, and may cause cerebellar ataxia (1). It is associated with single large mitochondrial DNA deletions and neuro-imaging can show atrophy with high T2 signal in the subcortical white matter, basal ganglia and brainstem (2, 3).Clinical Case A 16yr old male presented with progressive unsteadiness over six months. Examination showed classical features of KSS and further history highlighted long-term poor co-ordination and a restricted diet.MRI imaging showed very extensive symmetrical subcortical white matter T2 hyperintensity effecting brain and spinal cord, with sparing of the periventricular white matter. There was also involvement of the deep grey matter structures. The DARS2 gene was tested for Leukoencephalopathy with brainstem and spinal cord involvement & lactate elevation (LBSL), but was normal.Investigations confirmed a single large Mitochondrial DNA deletion. CSF showed a raised protein of 1.6g/l and lactate of 4.3mmol/l and a very low CSF 5-MTHF of 6nmol/l.Conclusion This patient showed significantly more extensive white matter changes than is typical for KSS, and this could be missed on routine genetic testing. Cerebral folate deficiency in KSS is treatable with Folinic acid supplementation.jacob.john.roelofs@gmail.com