Occasional Nystagmus
Chapter 89 Nystagmus in childhood
- Nystagmus And Vertigo
- Occasional Nystagmus Definition
- Occasional Nystagmus In One Eye
- Nystagmus To The Right
Nystagmus is a vision condition in which the eyes make repetitive, uncontrolled movements. These movements often result in reduced vision and depth perception and can affect balance and coordination. These involuntary eye movements can occur from side to side, up and down, or in a circular pattern. Nystagmus is an eye condition characterized by rapid, jerking eye movements. It does not always cause any noticeable symptoms, but it can be associated with issues such as dizziness and vision problems. A number of different neurological illnesses can cause nystagmus.
Nystagmus in childhood can be idiopathic or associated with retinal diseases, low vision in infancy, and a variety of syndromes and neurologic diseases. Nystagmus associated with neurologic disorders 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 10 000.1 Among the infantile forms of nystagmus, idiopathic nystagmus is the most common, followed by nystagmus associated with ocular disease. The diagnoses associated with infantile nystagmus are shown in Figure 89.1.
The causes of most forms of infantile nystagmus are unknown. Infantile nystagmus is considered to be a disorder of gaze-holding and slow eye movement systems,2 leading to sinusoidal oscillations and/or drifts of the eye away from 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 realign the eyes.
Many types of infantile nystagmus are associated with sensory abnormalities during early visual development.3 With “afferent” diseases such as achromatopsia and congenital cataract, the nystagmus is 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 abnormalities in ocular motor neural circuitry.
Nystagmus And Vertigo
Quality of life studies of adults and children with infantile nystagmus show that the effects on visual function are considerable and are comparable to diseases such as age-related macular degeneration.4 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 causes restriction in mobility of many patients, as they are not able to drive.5 Treatment of nystagmus should not only aim at improving visual acuity (VA) 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 (www.nystagmusnet.org).
• American Nystagmus Network (www.nystagmus.org).
The advantages of using a classification of infantile nystagmus based on the associated diseases are that the clinical implications such as prognosis, possible genetic counseling, or treatment options are highlighted.6Figure 89.2 lists examples of disorders using this type of classification. Idiopathic nystagmus is a diagnosis of exclusion where all other eye examinations are normal. VA is logMAR 0.3 (6/12, 20/40, 0.5) or better in most patients.7,8 Mutations in the FRMD7 gene have been identified as a major cause of X-linked idiopathic nystagmus.9,10 Several genetic mutations are known for other disorders such as albinism11,12 and achromatopsia.13 It is likely that the nystagmus genotype will be the principal method of classification in the future.
The Committee for the Classification of Eye Movement Abnormalities and Strabismus Workshop (CEMAS, www.nei.nih.gov/news/statements/cemas/pdf) have grouped idiopathic nystagmus, nystagmus associated with ocular diseases, and nystagmus associated with chiasmal misrouting into one category, “infantile nystagmus syndrome.” This classification makes unconfirmed assumptions about a common mechanism leading to nystagmus in all of the underlying pathologies.
Nystagmus can be characterized by clinical examination of the patient. Eye movement recordings can provide greater precision in detecting and describing nystagmus waveforms which can assist in the diagnosis (Figs 89.3 and 89.4). The following parameters can be used to describe nystagmus:
Waveform Nystagmus can be 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 increasing velocity profiles where the eyes start slowly and accumulate speed (e.g. Fig. 89.4A in right gaze). Alternatively, slow phases may have decreasing velocity (e.g. Fig. 89.4C) or linear velocity profiles. In contrast, pendular nystagmus consists of sinusoidal oscillations with small or no quick phases (e.g. Fig. 89.4A 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. Disconjugacy (dissociated nystagmus) occurs if the eyes move with different amplitude (e.g. torsional eye movements in Figure 89.3A,3), frequency, phase (e.g. vertical eye movements in Figure 89.3C,2), or along different planes.
Null region Many patients with infantile nystagmus prefer to use a particular gaze direction where the nystagmus is reduced in intensity and the VA 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 (see Chapter 81). An example is shown in Figure 89.4A,2 where the null region is in right gaze.
Change with gaze Patients with albinism and idiopathic nystagmus usually have a null region. The nystagmus becomes more jerk-like (Fig. 89.4A,1) and intense (Fig. 89.4A,2) away from the null region.
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 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).9,10 In contrast, only males are fully affected in X-linked congenital stationary night-blindness,15 blue cone monochromatism,13 or ocular albinism.12 Oculocutaneous albinism11 and achromatopsia13 are usually autosomal recessive. The most common form of autosomal dominant nystagmus is caused by mutations in PAX6 genes.16
Establish whether the parents think the child has poor vision. Nystagmus can be of very large amplitude at onset and parents can have the impression that the child is visually unresponsive. Usually the amplitude is considerably smaller by 6 to 9 months of age17 Video 89.1). Explaining to parents that nystagmus changes and becomes less evident with age is important. Caution should be taken about predicting poor vision later in life.
Oscillopsia seldom occurs in infantile nystagmus. Some children, however, perceive oscillopsia if they look away from the null region18 or if the nystagmus changes, for example in MLN which can change with the degree of strabismus (Video 89.18). Oscillopsia in acquired nystagmus is usually sudden in onset and severe. When it occurs in infantile nystagmus, the time of onset is generally not well defined and the symptoms are milder.
Occasional Nystagmus Definition
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. MLN, alone or superimposed on infantile nystagmus, can increase the nystagmus and decrease VA when one eye is covered. VA should 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 cards19 making it easier to identify changes in fixation when the child looks up or down. This can be masked by the horizontal nystagmus if the card is aligned horizontally. The presence of vertical (optokinetic nystagmus) OKN suggests the likelihood of better VA in horizontal nystagmus.
Occasional Nystagmus In One Eye
AHP occurs commonly in nystagmus because patients can reduce their nystagmus by looking in a certain direction of gaze. In most patients, the full extent of torticollis is only observed during visual effort. To identify the full amount of AHP, ask the patient to read or look at pictures (Fig. 89.5A, Video 89.5). 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 89.5B and Video 89.6 show 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.