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Nystagmus in childhood


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Introduction

Nystagmus consists of rhythmic ocular oscillations. Pathological nystagmus is involuntary, although it may be modulated when performing certain tasks such as reading. Infantile nystagmus is defined as nystagmus developing in the first 3–6 months of life. Nystagmus can be acquired later in life, usually due to neurological diseases. Patients with acquired nystagmus have oscillopsia, the illusion that the environment is moving. Patients with infantile nystagmus, however, usually have a stable view of the environment, probably due to neuronal plasticity and adaptation during visual development.

Nystagmus in childhood can be idiopathic or associated with retinal diseases, low vision in infancy, and a variety of syndromes and neurological diseases. Nystagmus associated with neurological disorders in childhood may be similar in appearance and pathophysiology to acquired nystagmus. Onset may also be after 6 months of age. The estimated prevalence of nystagmus (including both infantile and acquired nystagmus) is 24 in 10,000. The prevalence of infantile nystagmus is 14 per 10,000. Idiopathic nystagmus and nystagmus associated with ocular disease are the most common forms of nystagmus in childhood. The clinical diagnoses associated with infantile nystagmus are shown in Fig. 90.1 .

Fig. 90.1, Prevalence of the main types of nystagmus in childhood. A breakdown of the various disorders in individuals aged 18 years and under with infantile nystagmus from the Leicestershire Nystagmus Survey. 1 The majority of patients with chiasmal disorders had albinism.

Causes of Infantile Nystagmus

The causes of most forms of infantile nystagmus are as yet unknown, although this is currently an active field of research. In general, infantile nystagmus is considered to be a disorder of gaze-holding and slow eye movement systems, leading to sinusoidal oscillations and/or drifts of the eye away from the intended point of fixation. These involuntary slow eye movements constitute nystagmus slow phases. The slow phases are interrupted and shaped by the interposition of nystagmus quick phases, which serve to re-align the eyes.

Many types of infantile nystagmus are associated with the presence of sensory abnormalities during early visual development. With afferent diseases such as achromatopsia and congenital cataract, the nystagmus is clearly the result of changes in the otherwise healthy ocular motor systems in response to afferent deficits present during visual development. For other conditions such as albinism and various syndromes, it is uncertain whether the nystagmus is also due to afferent deficits or directly caused by abnormalities in ocular motor neural circuitry.

Quality of Life and Infantile Nystagmus

Investigations into the quality of life of adults and children with infantile nystagmus show that the effects on visual function are considerable and are comparable to the effects of diseases such as age-related macular degeneration. Semiquantitative interviews of people with infantile nystagmus revealed universally negative experiences of living with nystagmus and allowed the development of a 29-item, nystagmus-specific QOL questionnaire. Infantile nystagmus has a much wider impact than simply reducing vision. It affects social interaction, due to lack of confidence caused by the cosmetic appearance of nystagmus, and restriction in mobility of many patients, as they are not able to drive. Consequently, treatment of nystagmus should not only aim at improving visual acuity but also at improving cosmesis. This might include, for example, the correction of abnormal head postures and the reduction of nystagmus intensity in patients with poor visual potential. Patients may also benefit from counseling services and support groups such as Nystagmus Network, UK ( http://www.nystagmusnet.org ) or the American Nystagmus Network ( http://www.nystagmus.org ).

Classification of Infantile Nystagmus

There has been significant controversy over the classification and terminology used in nystagmus. This is because some researchers have been mainly interested in the morphology of nystagmus waveforms and others in clinical etiology.

The advantages of using a clinical classification of infantile nystagmus based on the associated diseases are that the clinical implications such as prognosis, possible genetic counseling, or treatment options are immediately highlighted. Figure 90.2 lists examples of disorders using this type of classification. Idiopathic nystagmus has historically been a diagnosis of exclusion where all other eye examinations are negative. The visual acuity is 0.3 LogMAR or better in most patients, suggesting relatively good retinal function. However, retinal abnormalities have been recently identified in idiopathic nystagmus using optical coherence tomography (OCT). Mutations in the FRMD7 gene have been identified as a major cause of X-linked idiopathic nystagmus. Several genetic mutations are known for other disorders such as albinism and achromatopsia. It is likely that the nystagmus genotype will be the principal method of classification in the future.

Fig. 90.2, Classification of nystagmus based on associated diseases.

The Committee for the Classification of Eye Movement Abnormalities and Strabismus Workshop (CEMAS; http://nei.nih.gov/sites/default/files/nei-pdfs/cemas.pdf ) have grouped together idiopathic nystagmus, nystagmus associated with ocular diseases, and nystagmus associated with chiasmal misrouting into one category called “infantile nystagmus syndrome.” In a clinical setting, this classification is less helpful and makes unconfirmed assumptions about a common mechanism leading to nystagmus in all of the underlying pathologies.

Terminology Used in Nystagmus

Nystagmus can be characterized by clinical examination of the patient. Eye movement recordings can help by providing greater precision in detecting and describing nystagmus waveforms, which can assist in the diagnosis ( Figs. 90.3 and 90.4 ). The following parameters can be used to describe nystagmus:

  • Plane: The plane of oscillation can be horizontal, vertical, torsional, or a combination of more than one plane (e.g. Fig. 90.3C2 ).

  • Intensity: The intensity of the nystagmus is a measure of the speed of the eye movements and is obtained by multiplying the nystagmus amplitude (in degrees, °) and frequency (oscillations per second in Hertz, Hz).

  • Waveform: Nystagmus can be broadly classified into jerk and pendular waveforms. Jerk nystagmus consists of alternating slow and quick phase eye movements. The nystagmus direction is defined using the quick phase. Slow phases can have an increasing velocity profile where the eyes start slowly and accumulate speed (e.g. Fig. 90.4A in right gaze). Alternatively, slow phases may have decreasing velocity (e.g. Fig. 90.4C ) or linear velocity profiles. In contrast, pendular nystagmus consists of approximately sinusoidal oscillations without quick phases (e.g. Fig. 90.4A at null region). Dual jerk nystagmus is a combination of large jerk nystagmus waveforms with small pendular nystagmus waveforms superimposed along the same plane.

  • Conjugacy: If both eyes move together, i.e. with the same amplitude, frequency and plane, the nystagmus is conjugate. Dysconjugacy or dissociated nystagmus occurs if the eyes move with different amplitude (e.g. torsional eye movements in Fig. 90.3A3 ), frequency, phase (e.g. vertical eye movements in Fig. 90.3C2 ) or along different planes.

  • Foveation: Most types of infantile nystagmus show periods in the nystagmus cycle when the eyes move at a slower velocity. These slow periods are often used to align the fovea with visual targets to improve visual acuity, and hence are called foveation periods.

  • Null region: Many patients with infantile nystagmus prefer a particular gaze direction where the nystagmus is reduced in intensity and visual acuity is optimal. This is called the null region. If the null region is not in the primary position, patients may adopt an abnormal head posture (AHP) using the null region to improve vision when looking straight ahead. An example is shown in Fig. 90.4A2 where the null region is in right gaze.

Fig. 90.3, The use of eye movement recordings in the diagnosis of nystagmus. Examples of eye movement recordings from patients with infantile nystagmus associated with (A) ocular diseases, (B) neurological disorders and syndromes, (C) chiasmal misrouting disorders, and (D) spasmus nutans. (A) Ocular diseases: In achromatopsia, a fine pendular nystagmus with a vertical component is often present. In the achromat shown (A1), a fine mainly horizontal pendular nystagmus of 1–2° amplitude and 8 Hz frequency coexists with a vertical upbeat jerk nystagmus of approximately 5° and 1.5 Hz. A patient with Bardet–Biedl syndrome (A2) has horizontal pendular oscillations that are much larger than in the achromat (8–10° amplitude and 3 Hz frequency in the example shown), but the vertical component to the nystagmus is smaller. PAX6 mutations (A3) lead to a variety of waveforms, with the two eyes sometimes showing dysconjugate eye movements. The example here shows unusual horizontal waveforms with both increasing and decreasing velocity components. A significant torsional component is apparent, which is larger in the right eye. (B) Neurological disorders and syndromes: These can be associated with vertical eye movements. The example shown is from a patient with a Chiari malformation leading to a downbeat jerk nystagmus of 2–3° amplitude and 3–4 Hz frequency, with little horizontal nystagmus. (C) Chiasmal misrouting disorders: Similar to idiopathic infantile nystagmus (see Fig. 90.4A ), nystagmus associated with albinism (C1) can have pendular or jerk waveforms but always along the horizontal plane and usually with a null region. Jerk waveforms can be left beating, right beating, or bidirectional as shown in the example here. Achiasmatic disorders (C2) lead to seesaw nystagmus, so called because the eyes give the appearance of rotating around an invisible pivot positioned somewhere between the two eyes. Consequently, as one eye moves up, the other eye moves down, leading to a dysconjugate vertical waveform. The eye moving up intorts and the eye moving down extorts, leading to a large-amplitude torsional nystagmus. There is also a horizontal component to seesaw nystagmus. (D) Spasmus nutans: Nystagmus in spasmus nutans is altered by head movements. When the head is fixed, a rapid pendular oscillation develops, which is dysconjugate between the two eyes. When the head is free, head bobbing occurs with the eyes moving in the opposite direction due to the vestibulo-ocular reflex. This leads to suppression of the rapid oscillation.

Fig. 90.4, Change in nystagmus with (A) gaze, (B) time, and (C) occlusion. (A) Shows an example of the null region in a patient with idiopathic infantile nystagmus using original eye movements from a patient. The patient follows a target moving from the left to right in steps of 3° every 8 seconds (lower traces). The horizontal nystagmus changes both in waveform (A1) and intensity (A2) as the direction of gaze changes. The changing waveforms can be seen from the expanded views in the upper traces. At the null region, which is 9° to the right of central fixation, the nystagmus waveform is predominantly pendular, with occasional foveating saccades. Jerk waveforms are present to the left and right of the null region, with the nystagmus beating to the left in left gaze and right in right gaze. A plot of the nystagmus intensity in relation to the horizontal gaze angle is shown in A2. (B) Shows a patient with idiopathic infantile nystagmus and periodic alternating nystagmus as an example of a nystagmus waveform changing with time. The nystagmus completes a full cycle of left beating and right beating nystagmus every 200 seconds, with short quiet phases of several seconds between each change in beating direction. Expanded views are shown in the lower traces. (C) Shows a patient with manifest latent nystagmus illustrating the change in beating direction on covering either eye. The eyes always beat towards the open or fixing eye. The waveforms all show decreasing velocity slow phases. Note the step change in eye position on covering either eye, which is due to a large esotropia. The left eye is the dominant eye. The nystagmus is less intense when both eyes are open.

Infantile nystagmus is not always uniform but can change depending on several factors:

  • Change with gaze: Patients with nystagmus associated with albinism and idiopathic nystagmus usually have a null region. The nystagmus becomes more jerk-like (see Fig. 90.4A1 ) and intense (see Fig. 90.4A2 ) away from the null region.

  • Change with time: Most patients have a consistent oscillation when attempting to maintain a fixed gaze position. However, certain types of nystagmus vary with time. In periodic alternating nystagmus (PAN) the fast phase beats periodically to the right and to the left with quiet periods at the changeover periods (see Fig. 90.4B ).

  • Change upon covering one eye: In manifest latent nystagmus (MLN), the fast phase of nystagmus changes direction beating towards the open or fixing eye (see Fig. 90.4C ).

Clinical Assessment

History

It is important to establish the time of onset for the nystagmus since infantile nystagmus occurs usually in the first 3, or sometimes 6, months of life.

As several forms of infantile nystagmus are hereditary, establishing whether other family members have nystagmus or associated ocular diseases can help with the diagnosis. If there is a positive family history, determining the mode of inheritance is very important. Idiopathic nystagmus often occurs in an X-linked pattern in which heterozygous females are fully affected in approximately 50% of cases (i.e. 50% penetrance). In contrast, only males are fully affected in X-linked congenital stationary night blindness (CSNB), blue cone monochromatism, or ocular albinism. Oculocutaneous albinism and achromatopsia are usually autosomal recessive. Recently, a new recessively inherited syndrome consisting of foveal hypoplasia, optic nerve decussation defects, and anterior segment dysgenesis without oculocutaneous hypopigmentation (FHONDA syndrome) has been described. The most common form of autosomal dominant nystagmus is caused by mutation in PAX6 genes.

Where neurological deficits such as cerebral palsy, metabolic diseases, or other causes of developmental delay are present, they are likely to play a key role in the etiology of the nystagmus.

It is important to establish whether parents think their child has poor vision. Although nystagmus can be of very large amplitude at onset and parents can have the impression that the child is visually unresponsive, usually the nystagmus amplitude is considerably smaller by 6–9 months of age. Explaining to parents that nystagmus generally changes and becomes less evident as their child becomes older is important, and therefore caution should be taken about predicting poor vision later in life. Optical coherence tomography (OCT)-based grading of the structure of the fovea is a good predictor of later visual acuity in infantile nystagmus.

Often children with nystagmus have head nodding or bobbing. This seems to be an independent abnormal head movement, which can decrease or disappear with age. Only in spasmus nutans has head nodding been shown to reduce or abolish nystagmus (see Fig. 90.3D ).

A history of photophobia should be specifically asked for. This would be indicative of a retinal disease, particularly achromatopsia or blue cone monochromatism. Similarly, a history of night blindness should be specifically asked for. This suggests a rod–cone dystrophy and is common in congenital stationary night blindness.

Symptoms of oscillopsia are a feature of acquired nystagmus and seldom occur in children. Some patients with infantile nystagmus, however, perceive oscillopsia if they look away from the null region or if the nystagmus changes, for example in manifest latent nystagmus (MLN), which can change with the degree of strabismus. The oscillopsia in acquired nystagmus is usually sudden in onset and severe. In contrast, the time of onset of oscillopsia in infantile nystagmus is generally not as well defined and the symptoms are milder.

Clinical examination

Visual acuity

Visual acuity (VA) needs to be examined with the best optical correction and tested with both eyes open and either eye covered with a free head position. This is important, as manifest latent nystagmus, alone or superimposed on other infantile nystagmus waveforms, can increase the nystagmus and decrease VA when one eye is covered. VA should also be measured at distance and near. In infants, VA tests can be performed using preferential looking cards. In patients with horizontal nystagmus, measurement of VA can be assisted by vertically aligning the cards, making it easier to identify changes in fixation when the child looks up or down at the visual targets on the card. This can be masked by the horizontal nystagmus if the card is aligned horizontally. The presence of vertical optokinetic nystagmus can also be used to predict the likelihood of better VA in horizontal nystagmus. It has been shown that grading of the structure of the fovea on OCT is a good predictor of later visual acuity in infantile nystagmus.

Abnormal head posture

Abnormal head posture (AHP) (or torticollis) occurs commonly in nystagmus because many patients can reduce their nystagmus by looking in a certain direction of gaze, for example by directing gaze towards the null region. In most patients, the full extent of torticollis is only observed during visual effort. So to identify the full amount of AHP, ask the patient to read or look at pictures ( Fig. 90.5A ). Glasses can prevent the patient adopting the full head turn due to the spectacle frame and optical decentration. VA measurements should be repeated, therefore, without spectacles. Figure 90.5B shows a child with idiopathic infantile nystagmus (IIN) and a right head turn increasing as he reads smaller letters. With a greater visual demand, a large head turn is adopted and he looks over his glasses or prefers to read without glasses since the full head turn is prevented by the glasses.

Fig. 90.5, Abnormal head postures. (A) Patient with IIN and a mild head tilt to the left when reading large letters on VA chart (A1), which significantly increases (A2) when reading smaller letters. (B) Moderate head turn to the right due to IIN when reading large letters (B1). As the patient reads smaller letters, the head increasingly turns to the right and the patient looks over his glasses as the frames do not allow a larger head turn to the right (B2). He prefers to read without glasses in order to adopt a large head turn (B3). (C) Child with a blind right eye due to optic nerve hypoplasia adopting an extreme head turn and tilt to the left to dampen manifest latent nystagmus (C1). When the left eye is covered and there is no vision, the head turn is absent (C2), which shows that there is no muscular torticollis. (D) Manifest latent nystagmus and alternating exotropia. The patient turns the head alternately to the right to fix in adduction with the right eye (D1), or the left to fix in adduction with the left eye (D2) to dampen the manifest latent nystagmus. When one eye is patched, he turns the head continuously to the right when the right eye is open (D3), and to the left when the left eye is open (D4). (E) Girl with manifest latent nystagmus adopting chin elevation while reading (E1). She has V pattern exotropia with bilateral inferior oblique overaction (E2–E5). On downgaze she has some amount of binocularity (Bagolini positive), which causes the nystagmus to dampen. Hence she adopts a chin-up position when reading the visual chart. (F) Head turn to the left due to IIN (nystagmus null region to the right), which does not allow the patient to wear glasses as the frame is obstructing her vision (F1). After Kestenbaum surgery (F2), her head is straight and she is able to wear her 4 diopters of astigmatic correction, significantly improving her vision. (G) A patient with albinism and pronounced head tilt to the left before surgery when reading the visual acuity chart (G1). After surgery, he reads the same line on the acuity chart without a head tilt (G2).

Manifest latent nystagmus dampens on adduction. As a result, the patient adopts a head turn to keep the fixing eye adducted ( Fig. 90.5C ). In manifest latent nystagmus, the head position also needs to be examined with each eye covered, as the head position can change depending on the fixing eye ( Fig. 90.5D ). Patients with A or V pattern strabismus may adopt a chin-up or chin-down head position in order to maintain binocular function and thus reduce the amplitude of the nystagmus ( Fig. 90.5E ). Eye movement recordings are helpful in understanding abnormal head postures, for example in establishing whether the head posture is due to idiopathic nystagmus (see Fig. 90.4A ) or manifest latent waveforms (see Fig. 90.4C ). They can also be used to determine whether the adopted head posture leads to a reduction or change in the nystagmus.

Orthoptic examination

Orthoptic examination should include an assessment of strabismus at distance and near, the range of motility of each eye, binocularity, stereopsis, and fusion ranges if binocular vision is present.

Color vision testing

This is important to detect achromatopsia and other retinal or optic nerve diseases.

Examination of light sensitivity and nyctalopia

Photophobia is easily noted on slit-lamp or fundus examination and points to achromatopsia or other retinal dystrophies. Nyctalopia can be objectively measured using dark adaptometry and may indicate congenital stationary night blindness or other retinal dystrophies.

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