Infantile Nystagmus Syndrome (Congenital Nystagmus)
Infantile nystagmus syndrome (INS), or congenital nystagmus (CN), is often recognized in the first few months of life but may not become evident until several years of age. There may be a family history of the disorder.
INS is almost always conjugate and horizontal, even in upgaze and downgaze. The nystagmus may be continuous or intermittent and can appear as jerk or pendular movements in different positions of gaze. Eye movement recordings show that these movements are often punctuated by foveation periods (brief periods in which the eyes are still and aimed at the object of regard). There is frequently a null point, the field of gaze in which nystagmus intensity is minimal and vision during the foveation period is maximized. If the null point is not in primary position, patients often adopt a head turn or posture that places the eyes in the null position in order to improve vision. Visual attention and fixation usually amplify INS (in contrast to peripheral vestibular nystagmus, discussed later in this chapter), whereas convergence on a near target damps the amplitude of the nystagmus. INS is abolished during sleep.
There are 2 characteristic signs of INS:
Reversal of the normal pattern of optokinetic nystagmus (OKN). Patients with INS respond to OKN drum rotation with slow-phase eye movements in the direction opposite to that of the rotating OKN drum.
A unique pattern in which the velocity of the slow-phase movement increases exponentially with distance from fixation. This increasing-velocity waveform is punctuated by foveation periods. Determining this pattern requires analysis of eye movement recordings (see Fig 9-1).
As mentioned previously, children with INS usually do not have oscillopsia. Visual acuity is determined by the length of the foveation period as well as the accompanying afferent pathway disease, if present. Visual acuity is often reduced, although some patients achieve normal visual acuity. Strabismus occurs in approximately 30% of patients.
INS is associated with conditions that cause bilateral, pregeniculate vision loss prior to 2 years of age. Therefore, the workup of a patient with early-onset nystagmus should include a search for clinical manifestations of pregeniculate vision loss. In some patients, the etiology may be obvious, such as in children with bilateral, dense, congenital cataracts. However, if the patient’s eyes do not have an obvious structural abnormality, the patient should be evaluated for the following conditions, which may be subtle and difficult to detect in uncooperative children:
Optic neuropathy. INS occurs with bilateral optic neuropathies that occur prior to 2 years of age, most commonly optic nerve hypoplasia. The clinician should consider obtaining neuroimaging to detect hydrocephalus or a compressive lesion. The detection of optic nerve hypoplasia requires an endocrinologic evaluation for associated hypopituitarism.
Foveal hypoplasia. INS occurs with foveal hypoplasia, often in patients with albinism or aniridia. Patients should be assessed for other signs of albinism, such as iris transillumination defects and hypopigmentation of the retinal pigment epithelium, choroid, skin, and hair. Albinism and aniridia may be familial.
Retinal dystrophy. INS may occur with early-onset retinal dystrophies such as achromatopsia and Leber congenital amaurosis. Patients should be assessed for other signs of retinal dystrophy, such as photophobia, eye pressing, paradoxical pupils, and high refractive error. Retinal examination may show mild pallor of the optic nerve head and vessel attenuation, or may be normal. The clinician should determine whether the patient has a familial history of retinal dystrophies. Ultimately, electrophysiologic testing (eg, electroretinogram) is required for diagnosis. Children with regression of developmental milestones require further workup for neurometabolic disease.
Approximately 40% of children with typical INS will have no detectable afferent pathway abnormality despite an extensive workup including electrophysiologic testing.
Memantine or gabapentin may reduce the severity of nystagmus and improve visual function in some patients with INS. However, these medications have adverse effects and are not commonly used for childhood nystagmus. The use of base-out prisms to induce convergence may help those patients whose nystagmus damps with convergence. Contact lenses may improve visual function in patients with INS. Some patients with INS can be treated with extraocular muscle surgery (eg, the Anderson-Kestenbaum procedure) to mechanically shift the null point closer to primary position (Fig 9-2; see also Fig 13-3 in BCSC Section 6, Pediatric Ophthalmology and Strabismus). A similar procedure may be performed on the vertical recti of a patient with a vertical null point, although diplopia may occur post-operatively because of limited vertical fusional amplitudes. In certain patients, horizontal rectus muscle tenotomy and reattachment of these muscles to their original insertion site may induce a variable reduction in nystagmus amplitude and improve visual function.
Abadi RV, Bjerre A. Motor and sensory characteristics of infantile nystagmus. Br J Ophthalmol. 2002;86(10):1152–1160.
Hertle RW, Dell’Osso LF. Clinical and ocular motor analysis of congenital nystagmus in infancy. J AAPOS. 1999;3(2):70–79.
Hertle RW; National Eye Institute Sponsored Classification of Eye Movement Abnormalities and Strabismus (CEMAS) Working Group. A next step in naming and classification of eye movement disorders and strabismus. J AAPOS. 2002;6(4):201–202.
Figure 9-2 Surgical treatment of infantile nystagmus syndrome. A, The nystagmus null point is in right gaze (patient maintains left face turn). B, The Anderson-Kestenbaum procedure shifts the null point centrally and reduces the head turn. The right lateral rectus and left medial rectus muscles are recessed. The right medial rectus and left lateral rectus muscles are resected.
(Illustration by Mark Miller.)
Hertle RW, Dell’Osso LF, FitzGibbon EJ, Thompson D, Yang D, Mellow SD. Horizontal rectus tenotomy in patients with congenital nystagmus: results in 10 adults. Ophthalmology. 2003; 110(11):2097–2105.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.