Unexplained Becomes Explained

7 years old male presented to the ophthalmology clinic for the evaluation of low vision. He was born with caesarean section after an uneventful pregnancy at 38 weeks, 4.05kg. The mother denied any perinatal hypoxia, ischemia or head injury during delivery. He first visited pediatric neurology clinic for hypotonia at the age of 6 months. He started to walk when he was 18 months. He showed delayed development and have mild intellectual disability with intelligence quotient (IQ) of about 77-80. He is taking methylphenidate for attention deficit hyperactivity disorders. He can communicate well with others and, he can walk and run without any support. Cycloplegic refraction showed -cyl0.50 axis 180 in the right eye and +sph0.50 -cyl0.50 axis180 in the left eye. His best corrected visual acuity was 20/100 in the both eyes. Extraocular motility examination showed full duction and version. Alternate cover test showed orthophoria at distance and near. When covering one eye, jerk nystagmus beating toward; uncovered eye was noted, which is consistent with latent nystagmus. Slit lamp examination showed normal, and dilated fundus examination revealed diffuse optic atrophy in both eyes. Spectral-domain optical coherence tomography showed thinning of retinal nerve fiber layer in both eyes. Electroretinography with skin electrode was performed and the results was inconclusive. Brain magnetic resonance imaging was not remarkable. Biochemistry test showed normal lactate to pyruvic acid ratio. Neurological examination showed normal cerebellar function and gait. Other cranial nerve examination was normal. Further diagnostic testing was then performed.