A severe disease with onset in infancy primarily associated with brain dysfunction combined with muscle weakness. Symptoms include hypotonia, failure to thrive, delayed development of mental and motor skills, severely impaired speech development, seizures, movement abnormalities, microcephaly and cerebral atrophy. All individuals with the disease have lactic acidosis. Also associated with congenital heart defects or arrhythmias, vision problems, hearing loss, hepatopathy and immune deficiency. Caused by mutation in the FBXL4 gene responsible for producing a protein found within mitochondria. Inherited in an autosomal recessive pattern.