Waveforms seen on eye movement recordings in children
FMNS is common in childhood cataract especially where there are significant interocular differences in visual acuity (such as typically seen in unilateral cataract). In some cases, it can be associated with a head posture or even cross-fixation, in order to hold the fixing (better) eye in adduction. This is thought to reduce the nystagmus intensity by holding gaze in the opposite direction to that of the nystagmus fast phases, a phenomenon seen in many forms of nystagmus (Alexanders law) .
20.5 Infantile Nystagmus Syndrome (Previously Including Congenital Idiopathic Nystagmus, CIN and Some Forms of Sensory Deficit Nystagmus, SDN)
The criteria for diagnosis of Infantile Nystagmus Syndrome (INS) include; infantile onset (usually identified between 3 and 6 months) and diagnostic accelerating slow phases on eye movement recordings (see Fig. 20.1) . Typically, it is predominantly horizontal, increases with fixation attempt, stays horizontal in up and down-gaze, decreases with convergence and is associated with null zones. OKN can be either absent, or in some cases described as ‘reverse’ and squints and refractive errors are common. Some diagnostic confusion can occur here as the presence of INS does not point to a specific etiology and patients with sensory deficits such as albinism, achromatopsia and even congenital cataract may fall into this category if they meet the other diagnostic criteria (particularly accelerating waveforms on eye movement recordings).
20.6 Saccadic Intrusions
Saccadic intrusions are fast eye movements taking the fovea away from the object of regard. According to the CEMAS classification , the group of eye movements collectively named ‘saccadic intrusions and oscillations’ comprises 12 different subtypes including: square wave jerks, opsoclonus and psychogenic (voluntary) flutter. However, the term saccadic intrusion has been used in much of the paediatric cataract literature to describe any involuntary, conjugate, horizontal fast eye movement of less than 1° in amplitude . The term will therefore be used in this context in this chapter (see Fig. 20.1).
20.7 What Type of Oculomotor Instabilities Do Children with Cataract Have?
Studies have found a variety of oculomotor instabilities in children with cataract. The most commonly reported is FMNS but INS, undefined multiplanar nystagmus and saccadic intrusions have also been reported in a number of studies [16–18].
Interestingly, saccadic intrusions can be recorded in normal infants, whenever they are in a reduced state of arousal and bursts of monophasic square-wave intrusions have also been recorded in healthy infants aged between 2 and 7 weeks of age [16, 19, 20]. They are also known to occur more commonly in amblyopia.
It has been observed that saccadic intrusions are a common physiological finding in younger children dependent upon their state of arousal and perhaps reflecting a failure to attend to, or actively engage in, fixation . Therefore, the significance of their presence in children with cataract is difficult to interpret although features separating them from those seen in healthy infants appears to be their increased amplitude and occurrence in short bursts .
Early publications describing nystagmus associated with childhood cataract were typically limited to noting the presence or absence of nystagmus and did not differentiate between FMNS and INS or any other type of nystagmus [21, 22].
In the small number of studies that have used eye movement recordings to differentiate between FMNS and INS in children with cataract, the patient cohorts have differed substantially as have the criteria used for diagnosis of FMNS or INS. Unsurprisingly the reported relative frequency of FMNS to INS [16, 18] has varied between series. Interestingly, it has in some cases, also varied between the two eyes of individual patients and between pre and postoperative measurements in individual patients . There is some evidence that the type and risk of nystagmus does not appear to vary with laterality of the cataract . It is also evident that the characteristics of the nystagmus seen in children with cataract can evolve over time as they do in ‘pure’ FMNS or INS seen in other contexts.
In 2012, Birch et al studied 41 children with dense cataracts from a variety of aetiologies. They found that by 5 years of age, 29 (71 %) had nystagmus. Of these, 18 had a predominantly INS phenotype and 11 had a predominantly FMNS phenotype. They found that risk factors for nystagmus included: infantile onset of cataract (<12 months of age) and duration of cataract of longer than 6 weeks but interestingly NOT congenital onset (cataract observed in the 1st week of life) or laterality (unilateral vs bilateral). Furthermore, they found that congenital onset and unilaterality WERE associated with a significant risk of inter-ocular asymmetry of nystagmus . Strabismus was also strongly associated with the presence of nystagmus.
Much debate still exists over the anatomical substrates for FMNS and INS and the observations of their coexistence in many patients with congenital cataract suggests that each is a consequence of a unique combination of timing, duration, laterality and the nature of sensory deprivation .
In summary, it seems that there are overlapping oculomotor disturbances at play in children with cataract, each perhaps with different but overlapping sensitive periods. However, early interruption of binocular vision appears to be a common causative insult. There are quite significant differences in the reported prevalence of nystagmus and in the characteristics of the nystagmus noted, between studies using eye movement recordings and others using cruder clinical assessment. This may explain why series describing children assessed using eye movement recordings report no difference in the incidence of fixation instabilities between bilateral and unilateral cataract cases [16, 18] whereas clinical reviews have described nystagmus as ‘rare’ in unilateral cataract .
20.8 How Common Is Nystagmus in Children with Cataract and Is It a Useful Prognostic Indicator for Cataract Surgery?
The reported prevalence of nystagmus and ocular oscillations in children with cataract varies between 38 and 100 % [16, 17, 24–27]. This makes judging the prognostic value of the presence of nystagmus difficult. However, the reported series and the types of patients included within them, varied widely in parameters including laterality, morphology of cataract, age of intervention, age of examination, stage of examination (pre-op or post-op), eye movement examination technique (observation or recordings) and discrepancy between types of ocular oscillations (i.e. saccadic intrusions vs nystagmus) [16, 17, 24, 25]. However, it is generally agreed that untreated dense bilateral cataracts in early infancy are associated with nystagmus or roving eye movements which develop by the age of 13 weeks [26, 27].
Robb and Peterson reported the visual results of 51 patients with congenital cataract following surgery and included a clinical observation of the presence of ‘nystagmus’. A precise description of the abnormal eye movements observed is not provided but clinically defined ‘nystagmus’ was associated with the following: density of the cataract, signs of other ocular developmental anomalies (such as poor pupillary dilation and reduced corneal diameter) as well as poor acuity outcome . Bradford et al studied 33 patients with dense bilateral congenital cataract and found that ‘clinically observed nystagmus’ was not a prognostically significant determinant of visual outcome either as a pre or post op finding  (although interestingly they similarly also found that in this series age at surgery was not a significant factor in determining visual outcome).
However, in a later study Lambert et al found that in 43 children undergoing surgery for dense bilateral congenital cataracts, the presence of preoperative nystagmus was a more powerful predictor of visual outcome than age at surgery .
Interestingly, other clinical studies have reported good visual outcomes in children with nystagmus but this appears to be largely confined to when the nystagmus is described as ‘mild’ [26, 29].
As described above, early studies provided qualitative judgement of eye movements and did not include eye movement recordings or assessment of the waveforms or beat direction of the oscillations . In 2006, Abadi et al addressed some of these issues by studying 33 neonates, infants and children with infantile cataract without other associated ocular or systemic abnormalities . The cataracts were graded according to morphology, density and position. Eye movement recordings were taken before and after surgery where possible. The authors found that those children with denser cataract, representing more severe form deprivation had a high probability of developing nystagmus.
In summary, most published work supports the premise that pre and post-operative nystagmus is associated with poorer final visual acuity outcomes. However, it remains unclear whether nystagmus is separate from other types of oculomotor instability such as saccadic intrusions or part of a spectrum of fixation instability. Similarly it is unclear whether the nystagmus subtype is important or whether the association with poorer vision is because nystagmus per se reduces acuity. Furthermore, given that the critical periods for visual acuity and oculomotor stability are generally considered to be distinct (but perhaps overlapping), it is possible that risk factors such as the timing, depth and duration of form deprivation operate independently on the development of acuity and ocular stability.
We would thus recommend that clinicians include careful assessment of the presence or absence of nystagmus and some details of its type (see section below) in their pre-operative assessment of children with cataract but advise that its presence does not necessarily preclude a good outcome from surgery.
20.9 Can Cataract Surgery Affect or Alter Nystagmus/Fixation Instability?
Felius et al described the oculomotor recordings, taken at 4.5 years old from 83 children who had undergone surgery at less than 7 months of age for unilateral cataract as part of the IATS study . Preoperative measurements were not reported but nystagmus was identified in 38 % of subjects and ‘saccadic oscillations’ in 31 %. Although it is not possible to comment on the effect of surgery here it is notable that there was no difference between children who had contact lens or an intraocular lens for correction of aphakia.