Infants with severe bilateral optic nerve hypoplasia (ONH) usually present with nystagmus and poor vision. Although these patients have small pale optic discs, the optic nerve abnormality may easily be missed when examining a small infant with nystagmus. Electrophysiological testing is extremely useful in detecting visual pathway abnormalities and may prompt review of the optic disc appearance. It is important to make a specific diagnosis, as ONH may be associated with endocrine abnormalities, particularly growth hormone deficiency, which need treatment. Pattern and Flash VEPs show varying degrees of attenuation and delay [2,49] and may be undetectable in severe cases. ERGs are normal and may be of high amplitude [13,48].
• Patients with cone or cone-rod dystrophy typically present with progressive loss in visual acuity, abnormal colour vision, photophobia and often nystagmus
• The fundus may initially appear normal, but abnormalities can include peripheral hypopigmentation and/or pigment clumping, disc pallor, bull's eye maculopathy or macular atrophy
• Cone system photopic ERG's are reduced and usually delayed in cone dystrophy with additional involvement of rod-dominated scotopic ERGs in cone-rod dystrophy
• Pattern ERGs are usually markedly reduced or undetectable
Leber congenital amaurosis
• Fundi are typically normal at presentation, but disc pallor, vessel attenuation and pigmentary changes may follow
• The ERG is severely reduced or undetectable from early infancy, indicating severe photoreceptor dysfunction
Rod or S-cone monochromatism
• Poor vision, nystagmus, photophobia and abnormal colour vision from birth
• Fundi are usually normal but maculae may appear granular
• Full-field ERGs reveal an absent or severely reduced photopic 30 Hz flicker response, but good rod ERGs following even a limited period of dark adaptation
• Classical signs include foveal hypoplasia, iris transillumination and fundal hypopig-mentation in addition to nystagmus
• VEPs are of higher amplitude and/or shorter latency over the contralateral hemisphere in keeping with an abnormally high percentage of decussating temporal fibres from each eye that project to the contralateral hemisphere
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