Arrhythmia, Astrocytosis, Ataxia, Atrophy/Degeneration involving the caudate nucleus, Babinski sign, Basal ganglia cysts, Basal ganglia gliosis, Bilateral tonic-clonic seizure, Blindness, Blurred vision, Bulbar signs, CNS demyelination, Cardiac conduction abnormality, Cardiomyopathy, Central retinal vessel vascular tortuosity, Central scotoma, Centrocecal scotoma, Cerebral cortical atrophy, Chorea, Choreoathetosis, Cognitive impairment, Cogwheel rigidity, Constriction of peripheral visual field, Corticospinal tract atrophy, Decreased activity of mitochondrial respiratory chain, Delayed speech and language development, Dementia, Demyelinating peripheral neuropathy, Developmental regression, Developmental stagnation, Diabetes mellitus, Difficulty walking, Dilated cardiomyopathy, Distal lower limb muscle weakness, Dysarthria, Dyskinesia, Dysphagia, Dyspnea, Dystonia, Emotional lability, Episodic respiratory distress, Episodic vomiting, Failure to thrive, Fever, Focal T2 hyperintense basal ganglia lesion, Frequent falls, Gait ataxia, Gait disturbance, Gastroesophageal reflux, Generalized hypotonia, Generalized myoclonic seizure, Gliosis, Global developmental delay, Headache, Hearing impairment, Hemiplegia/hemiparesis, Hepatic failure, Hepatocellular necrosis, Hepatomegaly, Horizontal pendular nystagmus, Hyperalaninemia, Hyperreflexia, Hypertrichosis, Hypertrophic cardiomyopathy, Hyperventilation, Hyporeflexia, Hypothermia, Hypotonia, Impaired vibration sensation in the lower limbs, Increased CSF lactate, Increased circulating lactate concentration, Infantile muscular hypotonia, Infantile spasms, Intellectual disability, Intellectual disability, mild, Intellectual disability, severe, Irritability, Lactic acidosis, Lacticaciduria, Leber optic atrophy, Leg muscle stiffness, Loss of ability to walk, Low plasma citrulline, Lower limb muscle weakness, Lower limb pain, Lower limb spasticity, Mitochondrial myopathy, Mitochondrial respiratory chain defects, Multiple glomerular cysts, Muscle spasm, Muscle weakness, Myoclonic spasms, Myoclonus, Myopathy, Neurodevelopmental delay, Neurological speech impairment, Nystagmus, Ophthalmoparesis, Ophthalmoplegia, Optic atrophy, Optic disc pallor, Optic neuropathy, Pendular nystagmus, Peripheral axonal neuropathy, Peripheral neuropathy, Pigmentary retinopathy, Polyneuropathy, Postural tremor, Progressive gait ataxia, Progressive ophthalmoplegia, Progressive spastic paraplegia, Proximal muscle weakness, Psychomotor retardation, Ptosis, Ragged-red muscle fibers, Respiratory insufficiency, Retinal arteriolar tortuosity, Retinal pigment epithelial mottling, Retinal telangiectasia, Retinal vascular tortuosity, Retinopathy, Rigidity, Rod-cone dystrophy, Segmental peripheral demyelination/remyelination, Seizure, Sensorimotor neuropathy, Sensorineural hearing impairment, Sensory neuropathy, Severe global developmental delay, Short stature, Slow decrease in visual acuity, Small basal ganglia, Spastic tetraparesis, Spasticity, Strabismus, Supraventricular arrhythmia, Tetraparesis, Undetectable light- and dark-adapted electroretinogram, Upper limb muscle weakness, Ventricular preexcitation, Ventriculomegaly, Visual loss