The level of visual function can help determine the cause of the nystagmus. Patients with nystagmus and nearly normal visual function usually have CMN, which is a benign entity. Markedly decreased visual acuity usually suggests either retinal or optic nerve abnormalities.
Because monocular occlusion can increase nystagmus intensity, particularly in FMNS (latent nystagmus), monocular acuity should be tested with at least one of the following: a fogging lens (+5.00 diopters [D] greater than the refractive error) or translucent occluder placed over the nontested eye, polarizing lenses with a polarized chart, or an occluder positioned several inches in front of the nontested eye. Binocular visual acuity is often better than monocular acuity and should be measured at distance and near, with any desired head position permitted, to assess the child’s true functional vision. Near visual acuity is usually better than distance. Children with a distance acuity below 20/400 can sometimes read as well as the 20/40 to 20/60 level at near.
In preverbal children, the optokinetic drum can be used to estimate visual acuity. If vertical rotation of an optokinetic drum elicits a vertical nystagmus superimposed on the child’s underlying nystagmus, the visual acuity is usually 20/400 or better. Preferential looking tests such as Teller Acuity Cards II (described in Chapter 1) can also be used; in patients with horizontal nystagmus, the responses can be more easily assessed with the cards held vertically.
Pupil responses are normal in CMN. Sluggish or absent responses to light, or a relative afferent defect in asymmetric cases, indicates a bilateral anterior visual pathway abnormality such as optic nerve or retinal dysfunction. However, normal responses can be seen with some sensory abnormalities such as foveal hypoplasia and achromatopsia. The normal response to darkness is the immediate dilation of the pupil. If, instead of dilating, the pupils paradoxically constrict, optic nerve or retinal disease may be present.
Examination of the anterior segment may reveal a direct cause of decreased vision (eg, congenital cataracts, corneal opacities) or clues to the cause of decreased vision (such as aniridia, or iris transillumination in albinism, both of which are associated with foveal hypoplasia).
Nystagmus may be associated with strabismus for a variety of reasons. Early-onset strabismus may cause FMNS (latent nystagmus); convergence may be used to damp nystagmus; or poor vision may be the underlying cause of both nystagmus and strabismus.
Optic nerve hypoplasia and foveal hypoplasia are common causes of congenital sensory nystagmus that may be diagnosed on fundus examination. A child with congenital sensory nystagmus from a retinal dystrophy may have vascular attenuation or optic disc pallor but may have a normal fundus; electroretinography may be required for diagnosis in patients with nystagmus and decreased vision but normal-appearing fundi (see Table 13-2, which lists primary retinal abnormalities that cause nystagmus).
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.