12.1Overview

The management of congenital nystagmus presents a complex problem, which requires the accurate diagnosis 12 of the underlying causes of congenital nystagmus and an understanding of the compensatory mechanisms used.

Diagnosis can involve detailed clinical examination with ancillary testing such as the eye movement recordings and electrodiagnostics. It is important to delineate between the di erent forms of congenital nystagmus such as congenital periodic alternating nystagmus (PAN) and manifest latent nystagmus (MLN) before treatment is considered.

Treatment of congenital nystagmus is rapidly evolving, with new methods of treatment emerging which are now proving to be beneficial. The armamentarium of treatment of congenital nystagmus includes optical, medical, and surgical treatments. Currently, in most nystagmus forms there is no definite answer as to which is the best treatment option. This chapter highlights the di erent modes of treatment.

The first section of this chapter discusses in detail the clinical characteristics of patients with congenital nystagmus with and without sensory deficit, MLN, and PAN. In the second section, the compensatory mechanism involved and methods to identify them are considered. The third section discusses the treatment options available for congenital nystagmus.

12.1.1Congenital Nystagmus with and Without Sensory Deficits

Memantine

Gabapentin

Placebo

CIN

SN

CIN

SN

CIN

SN

Congenital nystagmus consists of involuntary periodic to-and-fro oscillations of the eye. It usually presents within the first 3 months of life; however, onset as late as 12 months to 10 years has been reported [1]. The incidence of congenital nystagmus is estimated to be 1 in 2,000, in a population-based survey done in UK.

The eye movements in congenital nystagmus are mainly in the horizontal plane, although they can be vertical or torsional, or in a combination of di erent planes. Congenital nystagmus is often described in the literature as being a jerk nystagmus with accelerating slow phase; however, IIN may show di erent waveforms that usually vary with eccentricity. Frequently, congenital nystagmus consists of underlying pendular oscillations interrupted by regularly occurring foveating saccades (quick phases) as shown in Fig. 12.1. Nystagmus intensity often changes with the direction of gaze. The region of lowest nystagmus intensity and longest foveation periods is known as the “null region.” This is often the preferred region of

3º

1sec

Fig. 12.1 Original horizontal eye movement recordings of right eyes of (first row) a patient with congenital idiopathic nystagmus (CIN) and (second row) a patient with secondary nystagmus (SN) associated with albinism before and during memantine treatment; (third row) a patient with CIN and (fourth row) a patient with SN associated with achromatopsia before and during gabapentin treatment; (fifth row) a patient with SN and (sixth row) a patient with SN associated with albinism before and during placebo treatment at examinations one and four. Eye movements to the right are represented by an upward deflection, and eye movements to the left by a downward deflection.The eye movement recordings show the variability in waveforms with the common occurrence of an underlying pendular waveform. They also show reduction of intensity after treatment with memantine and gabapentin but not with placebo

fixation for optimal vision with the head position being used to maintain vision in the null region. Consequently, patients often exhibit an anomalous head posture (AHP)

if the null region is eccentric. Typically, the oscillation drifts toward the null region with the drift becoming accentuated further away from the null region. This results in the quick phases usually beating away from the null region with slow phases often accelerating toward the null region.

Congenital nystagmus can be idiopathic with the most likely cause being abnormal development of the brain areas controlling eye movements and gaze stability. It can also occur in association with defects in the a erent visual system such as albinism, congenital optic atrophy, optic nerve hypoplasia, congenital retinal dystrophies or

retinal dysfunction disorders (such as achromatopsia and congenital stationary night blindness (CSNB) ), and congenital cataracts. To assess visual potential when treating a patient, it is important to carefully diagnose whether an a erent visual defect is present. Ocular albinism is frequently misdiagnosed as idiopathic nystagmus as the phenotypical characteristics might be subtle. Figure 12.2 shows clinical signs seen in a patient with oculocutaneous albinism as well as in a patient with ocular albinism. The patient with ocular albinism has dark hair and skin, very mild iris transillumination, but a hypopigmented fundus. Both patients have foveal hypoplasia to varying

Fig. 12.2 Phenotypical characteristics of patients with oculocutaneous

(a, c, e, g) and ocular (b, d, f, h) albinism. The patient with oculocutaneous albinism has light hair and more prominent iris transillumination than the patient with ocular albinism. Both patients have fundus hypopigmentation, macular hypoplasia, and small optic nerves. Optical coherence tomography (OCT) shows foveal thickening in both patients (g, h) with total absence of foveal pit in the patient with oculocutaneous albinism (g)

Appearance

Iris transillumination

Fundoscopy

Optical

Coherence

Tomography

Fig. 12.3 Examples of scotopic, photopic, and flicker electroretinograms (ERGs) of (a) a normal subject, (b) a patient with congenital stationary night blindness (CSNB) with a negative scotopic ERG, and (c) a patient with achromatopsia with extinguished photopic ERG and flicker ERG. (d) Visual evoked potential of a patient with albinism showing asymmetry between recording from the right and left hemisphere

(see placements of electrodes on scalp in upper right corner) when the right and left eye are individually stimulated. Owing to increased crossing of optic nerve fibers in the chiasm, the evoked potentials are more pronounced in the contralateral hemisphere

(O1, O2, and O3 are electrodes placed over the

back of the head (near the occipital pole of the cortex) in left, central, and right positions, respectively; FZ is the reference electrode)

c

d

Achromatopsia

500µv

20ms

Visual Evoked Potentials

Albinism

Right Eye Open

Left Eye Open