Amblyopia is often the principal barrier to a successful visual outcome in the management of pediatric cataracts. Leinfelder¹ described a condition of “congenital amblyopia” in children with congenital cataracts and poor visual acuity (VA) results following lensectomy. Indeed, it was once debatable if surgical management of unilateral congenital cataracts was advisable.² Advances in the understanding of the pathophysiology of amblyopia³ supported more aggressive therapy. Frey et al.⁴ suggested, in the 1970s, that treatment of monocular cataracts in children may be worthwhile. Over the past several decades, the understanding of deprivation amblyopia physiology and treatment has continued to evolve. This, coupled with ongoing refinement of surgical timing and technique, has resulted in even better VA outcomes in pediatric cataracts. Treatment of childhood cataracts is now commonplace worldwide, and surgical correction of even unilateral congenital cataracts is standard of care.⁵ This chapter reviews the visual outcomes in children with unilateral and bilateral cataracts, discusses the mechanisms and treatment of deprivation amblyopia caused by cataracts, considers the optimal timing of surgical treatment, suggests postoperative amblyopia therapy regimens, and explores the relationship between cataract-related amblyopia and binocular vision.

A review of VA outcomes will frame the approaches and expectations of amblyopia therapy. These data represent the full range of pediatric cataracts—unilateral and bilateral, congenital and acquired—and varied surgical and amblyopia management approaches. As expected, unilateral cataracts result in worse VA compared to bilateral cataracts, and age of surgery is strongly correlated with final vision.

In one of the largest reviews of childhood cataracts, Zwann et al.⁶ reported that 44% of 306 pseudophakic eyes achieved vision of 20/40 or better, with amblyopia as the major factor in acuity outcome. This result was similar to the mean acuity outcome of 20/55 in 100 pseudophakic eyes described by McClatchey et al.⁷ Another large study of 139 mixed childhood cataracts treated with intraocular lens (IOL) implantation had a median final VA of 20/30. Forty-five eyes in this study had a final VA of worse than 20/40; amblyopia was the cause of poor vision in 34.⁸

Within these large studies, we find poorer outcomes among cases of unilateral cataracts. Ledoux et al.⁸ reported a median final VA of 20/40 in these patients, compared to 20/25 in bilateral cases. Other series of unilateral cases confirm this finding and often find much worse final acuity. Hosal et al.⁹ reviewed 74 unilateral cataracts and found that only 37% had a final acuity of 20/40 or better. A series of unilateral traumatic cataracts in children had a mean final VA of 20/50, with amblyopia affecting 75% of children under 6 years of age.¹⁰ When only unilateral congenital cataracts are studied, the outcomes may be even worse.¹¹ In a review of 62 such patients, with a mean age of lensectomy of 5 weeks, only 32% had a final VA of 20/80 or better.¹² Early surgery was correlated with better final acuity.

It has been suggested that the method of optical correction (IOL implantation versus contact lens or spectacle correction) is an important factor in final VA. Some studies found no significant overall difference in VA between pseudophakia and aphakia,¹³,¹⁴ and others found that the difference depended on whether the child had unilateral or bilateral cataracts.¹⁵ Overall, though, these early studies did not study IOL implantation in children under 2 years of age. The Infant Aphakia Treatment Study (IATS) randomized infants 1 to 7 months of age with unilateral cataracts to IOL implantation versus aphakia and found no significant difference in grating acuity at 1 year.¹⁶ Follow-up is ongoing for infants in this study and
is planned until 5 years of age. Observational studies have suggested that pseudophakia may result in better final VA in congenital cataracts depending on the timing of surgery.¹⁴,¹⁷ This was also shown with traumatic cataracts.¹⁰,¹⁸ The literature on final VA outcomes in pediatric cataracts presents a wide spectrum of pathology and results.

The heterogeneous nature of these data makes conclusions difficult, but general trends are evident. Certainly, early surgery is preferred. The use of IOLs may not significantly improve final VA; however, pseudophakia may have other advantages to visual function, such as improved binocularity. Unilateral cataracts, particularly when congenital, present the greatest challenge for visual rehabilitation. The reasons behind these findings, and their practical implications, are the focus of the remainder of this chapter.

The relatively worse outcomes in unilateral cataracts, and those treated later in life, stem directly from the pathophysiology of deprivation amblyopia. Early visual experience is required for normal development of the visual cortex.³ Following a latent period during which vision is mediated via subcortical pathways,¹⁹,²⁰ the visual system is highly dependent on robust visual input. Inducing monocular deprivation in animals (usually by suturing of eyelids) during this “sensitive period” causes a loss of normal cortical cell response to the deprived eye.³ Cells in the ocular dominance columns of the brain’s striate visual cortex that are normally attributed to the deprived eye switch their response to the functioning eye, resulting in corresponding shrinkage and growth of the columns.²¹,²²,²³ This cortical brain correlate of amblyopia can be partially reversed by preventing continued deprivation.²² Within this sensitive period, there is a “critical period” when early visual exposure is essential; during this time, absence of equal quality input to each eye can lead to irreversibly poor vision.

Management of amblyopia has been well described and is generally quite successful for strabismic and anisometropic amblyopia.²⁴,²⁵,²⁶,²⁷ Deprivation amblyopia, representing approximately 3% of all cases of amblyopia, is the most refractory to treatment.²⁸ The most common cause of deprivation amblyopia is unilateral congenital cataract, and bilateral congenital cataracts are a common cause as well.²⁹ Failure to promptly remove congenital cataracts, especially unilateral ones, can result in dense amblyopia refractory to management. The literature on VA outcomes consistently documents a universal age-related effect. Early surgery has the best chance of a good visual outcome, but what is early enough? Gregg and Parks³⁰ described a child with a unilateral congenital cataract who underwent lensectomy at 1 day of life and had a final acuity of 20/25 at age 8.

Further studies have attempted to determine the critical period during which cataract surgery should be conducted for congenital cataracts. Birch et al.³¹ found that very early surgery for unilateral congenital cataracts resulted in better VA. Subsequent analysis led to the conclusion that acuity outcomes drastically declined if surgery was conducted after 6 weeks.³² There was not a significant benefit of earlier surgery within these first 6 weeks (see Chapter 6).

Similarly, a critical period may exist for bilateral congenital cataracts. Lambert et al.³³ reviewed VA outcomes in 43 children and found that poor vision outcomes increased if surgery was conducted after 10 weeks of age. No eyes operated before 10 weeks of age had worse than 20/80 VA, though both groups (before and after 10 weeks) had an equal amount of children with 20/40 or better VA. Birch et al.³⁴ did not find a specific break point in 37 infants with bilateral congenital cataracts but did provide evidence that surgery before 14 weeks of age resulted in a better outcome. Surgery prior to 14 weeks did result in improved final outcome in a linear fashion. A smaller series of 13 children suggested that the decline was exponential, and no specific breakpoint was identified.³⁵ Given that bilateral deprivation causes less specific effects on the visual cortex,³⁶,³⁷ it is not surprising that a reproducible critical period was not identified in these infants. Overall, we can conclude that early surgery is also better in bilateral cataracts, though it may be delayed relative to unilateral cases.

Surgical correction of bilateral cataracts should occur in close succession, if the cataracts are symmetrical. A prolonged interval between cases could result in dense deprivation amblyopia in the untreated eye. Some have advocated for full-time occlusion of the treated eye until the second eye is operated on, though most surgeons attempt to remove the fellow cataract within 2 weeks.³³,³⁴,³⁵ Limiting the delay between surgical treatment of bilateral cataracts is essential; indeed, immediate sequential same-day surgery has been advocated in select cases.

Asymmetrical bilateral cataracts that require surgical removal present a unique challenge, as deprivation amblyopia may be present in the worse eye. A novel approach to these cases has been suggested by Yu and Dahan³⁸: both eyes were operated simultaneously on the same day and only the worse eye received an IOL. The better eye was left aphakic; a secondary IOL was implanted once the amblyopic eye has improved. A similar approach has been described in bilaterally aphakic eyes: the contact lens is periodically removed from the better eye. Another approach is to operate on the worse eye and delay surgery on the fellow eye until VA is approximately equal.

Overall, a critical period likely exists for congenital cataracts. Unilateral cases should be operated on within 6 weeks to maximize VA; even earlier surgery may be beneficial for motor and sensory outcomes.³⁹,⁴⁰ Bilateral cases should be treated early in life as well, although the critical period likely extends to a few months of age.

Because early visual experience is essential for a good visual outcome, congenital cataracts must be removed quickly. Acquired cataracts have a better outcome as patient age increases, since the visual experience is usually normal before acquiring the cataract.⁸,¹⁰ Similarly, better outcomes are correlated with a shorter duration from cataract onset to removal.¹⁰

The development of nystagmus predicts worse VA outcomes. This sensory-type nystagmus tends to develop by 13 weeks of age when congenital cataracts are left untreated,⁴¹ suggesting that a critical period of fixation stability exists.⁴⁰ Nystagmus therefore likely represents a surrogate for insufficient early visual exposure and has a direct effect on VA. Indeed, the presence of nystagmus was more strongly correlated with poor visual outcome than was late surgery in a series of 43 children with bilateral congenital cataracts.³³ Nonetheless, other series⁴¹,⁴² have reported good outcomes in these children. Rabiah et al.⁴¹ reviewed 95 cases of bilateral congenital cataracts presenting with sensory nystagmus at the time of their cataract surgery. They found 46% with 20/60 or better vision in the better eye. Interestingly, they reported that the nystagmus was eliminated or reduced in 40% of these children following treatment.