CHAPTER 13

Childhood Nystagmus

The prevalence of nystagmus in young children is approximately 0.35%. Nystagmus can be due to a motor defect that is compatible with relatively good vision, an ocular abnormality that impairs vision or fusion, or a neurologic abnormality. Distinguishing between these causes can be challenging. See BCSC Section 5, Neuro-Ophthalmology, for additional discussion of nystagmus.

General Features

Nystagmus is an involuntary, rhythmic oscillation of the eyes. In pendular nystagmus, the eyes oscillate with equal velocity in each direction. Jerk nystagmus denotes a movement of unequal speed. The fast component defines the direction of the nystagmus—for example, a right jerk nystagmus has a slow movement to the left and a fast movement (jerk) to the right.

The nystagmus movement can be further classified according to the frequency (number of oscillations per unit of time) and the amplitude (the angular distance traveled during the movement). The movements can be characterized as horizontal, vertical, or torsional or a combination of these. The characteristics of the nystagmus may change with gaze direction. For example, pendular nystagmus can become jerk nystagmus on extreme gaze.

Gaze position can affect the amplitude and frequency (intensity) of the nystagmus. This is especially true of jerk nystagmus, which can have a null point (or null zone, the gaze position in which the intensity of oscillations is diminished and the visual acuity improves), or which can decrease in intensity with gaze opposite the fast-phase component (analogous to Alexander’s law for vestibular nystagmus). The abnormal head position that these patients assume in order to reduce nystagmus can be the most prominent manifestation of their condition.

Nomenclature

The National Eye Institute (NEI) has reclassified eye movement abnormalities, including nystagmus. For the discussion of nystagmus in this chapter, we use the traditional, familiar designations. The terminology recommended by the NEI-sponsored Committee for the Classification of Eye Movement Abnormalities and Strabismus (CEMAS) is indicated in parentheses throughout this chapter. The document produced by the CEMAS committee is available on the NEI website at www.nei.nih.gov/site s/default/files/nei-pdfs/cemas.pdf.

Evaluation

History

Many forms of nystagmus are inherited, either as a direct genetic abnormality or in association with other ocular conditions (Table 13-1). Thus, a family history, including inherited ocular or systemic diseases, is an important part of the initial evaluation. Although many cases of congenital motor nystagmus are sporadic, autosomal dominant, autosomal recessive, and X-linked inheritance have been described. Examination of family members with nystagmus can provide valuable prognostic information.

Table 13-1

Perinatal events can affect the developing visual system and, if severe enough, can result in nystagmus. For this reason, the history should also include questions about the labor and delivery, maternal infections, and prematurity. For children older than 3 months, parental observations about head tilts, head movements, gaze preference, and viewing distances can aid in diagnosis.

Ocular Examination

Visual acuity

The level of visual function can be helpful for determining the cause of nystagmus. Patients with nystagmus and nearly normal visual acuity usually have congenital motor nystagmus (see the section “Congenital motor nystagmus” later in the chapter), which is a benign entity. Markedly decreased visual acuity usually implies either retinal or optic nerve abnormalities. Monocular visual acuity should be tested with fogging because monocular occlusion may increase nystagmus intensity. Binocular visual acuity 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 distance acuity of less than 20/400 can sometimes read at the 20/40–20/60 level at near.

In preverbal children, the optokinetic nystagmus (OKN) drum can be used to estimate visual acuity. If vertical rotation of an OKN 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. The examiner should keep in mind that, in patients with a horizontal nystagmus, the response can be more easily observed with the card held vertically.

Pupils

Pupils should be assessed for direct reaction to light, afferent defect, reaction to darkness, and