Does neonatal ocular misalignment predict later abnormality?

2016 International Orthoptic Congress Burian Lecture: Folklore or Evidence?

The theme of the 2016 Burian Lecture is how our understanding of strabismus has been changed by the research carried out in our laboratory in Reading over the years. Accommodation and convergence are fundamental to orthoptics, but actual responses have often been very different compared to what we had expected. This paper outlines how our laboratory's understanding of common issues such as normal development of accommodation and convergence, their linkage, intermittent strabismus, anisometropia, orthoptic exercises, and risk factors for strabismus have changed. A new model of thinking about convergence and accommodation may help us to better understand and predict responses in our patients.

Horizontal and vertical optokinetic eye movements in macaque monkeys with infantile strabismus: directional bias and crosstalk

PURPOSE

Optokinetic eye movements stabilize gaze by tracking motion of the visual scene during sustained movement of a creature's body. The purpose of this study was to describe vertical and horizontal optokinetic nystagmus (OKN) in nonhuman primates (NHPs) with normal binocular vision, and to compare their responses to NHPs with binocular maldevelopment induced by prism-rearing.

METHODS

Optical strabismus was created in infant macaques (n = 6) by fitting them with prism goggles. The goggles were removed after 3, 6, 9, or 12 weeks to determine the effects of increasing durations of binocular noncorrespondence. Infant NHPs (n = 2) reared wearing plano goggles served as controls. OKN was evoked by horizontal or vertical stripe motion. Eye movements were recorded by using binocular search coils.

RESULTS

NHPs reared in early infancy under conditions of binocular noncorrespondence for durations of 6 weeks or longer had horizontal OKN responses biased directionally in favor of nasalward motion. NHPs reared with prisms for any duration had vertical OKN responses more biased than normal NHPs in favor of upward motion. Diagonal "crosstalk" during horizontal or vertical OKN (vertical slow phases during horizontal stimulus motion, and vice versa) was present to some degree in all NHPs. However, crosstalk-upward during horizontal OKN and nasalward during vertical OKN-was most pronounced in NHPs reared with prism for durations long enough to induce a permanent esotropic strabismus (longer than 3 weeks).

CONCLUSIONS

With fusion maldevelopment, the OKN pathways retain a nasalward and upward bias. During forward locomotion, optic flow excites temporalward and downward visual motion in each eye. The OKN biases would act in counterbalance. The biases attenuate with emergence of fusion, but may persist and crosstalk when fusion is impeded.

Decorrelation of cerebral visual inputs as the sufficient cause of infantile esotropia

BACKGROUND AND PURPOSE

Human infants at greatest risk for esotropia are those who suffer cerebral insults that could decorrelate signals from the two eyes during an early critical period of binocular, visuomotor development. The authors reared normal infant monkeys under conditions of binocular decorrelation to determine if this alone was sufficient to cause esotropia, and associated behavioral as well as neuroanatomic deficits.

METHODS

Binocular decorrelation was imposed using prism-goggles for durations of 3-24 weeks (control monkeys wore plano goggles), emulating unrepaired strabismus of durations 3 months to 2 years in human infants. Behavioral recordings were obtained, followed by neuroanatomic analysis of ocular dominance columns and binocular, horizontal connections in the striate visual cortex (area V1).

RESULTS

Concomitant, constant esotropia developed in each monkey exposed to decorrelation for a duration of 6-24 weeks. The severity of ocular motor signs (esotropia angle; dissociated vertical deviation; latent nystagmus; pursuit / optokinetic tracking asymmetry; fusional vergence deficits), and the loss of V1 binocular connections increased as a function of decorrelation duration. Stereopsis was deficient and motion visually evoked potentials were asymmetric. Monkeys exposed to decorrelation for 3 weeks showed transient esotropia, but regained normal alignment, visuomotor behaviors, and binocular V1 connections.

CONCLUSIONS

Binocular decorrelation is a sufficient cause of infantile esotropia when imposed during a critical period of visuomotor development. The systematic relationship between severity of visuomotor signs and severity of V1 connectivity deficits provides a neuroanatomic mechanism for these signs. Restoration of binocular fusion and V1 connections after short durations of decorrelation helps explain the benefits of early strabismus repair in humans.

Gender differences in early accommodation and vergence development

A remote haploscopic photorefractor was used to assess objective binocular vergence and accommodation responses in 157 full-term healthy infants aged 1-6 months while fixating a brightly coloured target moving between fixation distances at 2, 1, 0.5 and 0.33 m. Vergence and accommodation response gain matured rapidly from 'flat' neonatal responses at an intercept of approximately 2 dioptres (D) for accommodation and 2.5 metre angles(MA) for vergence, reaching adult-like values at 4 months. Vergence gain was marginally higher in females (p = 0.064), but accommodation gain (p = 0.034) was higher and accommodative intercept closer to zero (p = 0.004) in males in the first 3 months as they relaxed accommodation more appropriately for distant targets. More females showed flat accommodation responses (p = 0.029). More males behaved hypermetropically in the first two months of life, but when these hypermetropic infants were excluded from the analysis, the gender difference remained. Gender differences disappeared after three months. Data showed variable responses and infants could behave appropriately and simultaneously on both, neither or only one measure at all ages. If accommodation was appropriate (gain between 0.7 and 1.3; r(2) > 0.7) but vergence was not, males over- and under-converged equally, while the females who accommodated appropriately were more likely to overconverge (p = 0.008). The apparent earlier maturity of the male accommodative responses may be due to refractive error differences but could also reflect gender-specific male preference for blur cues while females show earlier preference for disparity, which may underpin the earlier emerging, disparity dependent, stereopsis and full vergence found in females in other studies.

Final report of the early vs. late infantile strabismus surgery study (ELISSS), a controlled, prospective, multicenter study

BACKGROUND

The optimal age for surgery for infantile esotropia is controversial. Proponents of early surgery believe that further loss of binocular vision can be prevented by early surgery, a minority believes that binocular vision can even be restored by early surgery. The ELISSS compared early with late surgery in a prospective, controlled, non-randomized, multicenter trial.

METHODS

Fifty-eight clinics recruited children aged 6-18 months for the study. Each clinic operated all eligible children either 'early', i.e. at age 6-24 months, or 'late', i.e. at age 32-60 months. At baseline the angle of strabismus, refraction, degree of amblyopia and limitation of abduction were assessed. Intermediate examinations took place every six months. Children were evaluated at age six in the presence of independent observers. Primary endpoints were (i) level of binocular vision, (ii) manifest angle of strabismus at distance and (iii) remaining amblyopia. Secondary endpoints were number of operations, vertical strabismus, angle at near and the influence of surgical technique.

RESULTS

A total of 231 children were recruited for early and 301 for late surgery. Age at entry examination was 11.1 months (SD 3.7 months) in the early group and 10.9 (SD 3.7) months in the late group. Refraction, amblyopia and limitation of abduction were distributed equally in the early and late groups, but the angle of strabismus was slightly larger in the early group. Dropout-rates were 26.0% in the early and 22.3% in the late group. At age six, 13.5% of the early vs. 3.9% of the late group recognized the Titmus Housefly; 3.0% of the early and 3.9% of the late group had stereopsis beyond Titmus Housefly. No significant difference was found for angle of strabismus. 35.1% of the early group and 34.8% of the late group did not have an angle between 0 degrees and 10 degrees , the thresholds set for re-operation. For ratio of the visual acuities (remaining amblyopia) there was a small but significant advantage for the early group. There was hardly any correlation between the baseline parameters and the primary endpoints. Children scheduled for early surgery had first been operated at 20 (SD 8.4) months, but 8.2% had not been operated at age six. Children scheduled for late surgery had been operated at 49.1 (SD 12.7) months, but 20.1% had not been operated at age six. The number of operations per child was 1.18 (SD 0.67) in the early and 0.99 (SD 0.64) in the late group. Age at recruitment, age that strabismus reportedly had started and refraction at entry examination were similar among operated and non-operated children. Only the angle of strabismus at entry predicted, to some extent, whether a child had been operated at age six.

DISCUSSION

Children operated early had better gross stereopsis at age six as compared to children operated late. They had been operated more frequently, however, and a substantial number of children in both groups had not been operated at all.

Too much or too little: neonatal ocular misalignment frequency can predict later abnormality

BACKGROUND

214 orthoptists' infants have been followed for up to 15 years, relating neonatal misalignment (NMs) and first convergence onset to later childhood ocular abnormalities. NMs are shown in a companion paper to reflect the onset of first convergence, but if frequent or absent may predict a higher risk of refractive error and esodeviation.

METHODS

In a prospective postal survey, orthoptist mothers observed their own infants during the first months of life and regularly reported ocular behaviour and alignment, visual development, and any subsequent ocular abnormalities.

RESULTS

Later strabismus and refractive error were less common in infants who showed NMs occasionally compared with those who never or frequently did. There was a significant linear trend for fewer ocular abnormalities to be found in children with more frequent NMs (p<0.001). Hypermetropes were later to show first convergence than emmetropes or myopes (p = 0.006)

CONCLUSIONS

NMs usually reflect an emerging and normally developing vergence system. This study suggests that delayed onset of convergence (and lack of NMs) is associated with later defects, especially hyperopia. Possible causal relations are discussed.

Neonatal ocular misalignments reflect vergence development but rarely become esotropia

BACKGROUND

214 orthoptists' infants have been followed for up to 15 years, relating neonatal misalignment (NMs) behaviour to onset of convergence and 20 Delta base out prism response, and also to later childhood ocular abnormalities.

METHODS

In a prospective postal survey, orthoptist mothers observed their own infants during the first months of life and regularly reported ocular behaviour and alignment, visual development, and any subsequent ocular abnormalities.

RESULTS

Results confirm previously reported characteristics of NMs. Infants who were misaligned more frequently were misaligned for longer periods (p <0.01) and were later to achieve constant alignment (p <0.001) but were earlier to attempt first convergence (p = 0.03). Maximum NM frequency was usually found at or before the onset of first convergence (p = 0.0002).

CONCLUSIONS

NMs occur in the first 2 months of life and usually reflect a normally developing vergence system. They appear to represent early attempts at convergence to near targets. Emerging infantile esotropia is indistinguishable from frequent NMs before 2 months.