|
During a routine yearly check-up of a 3-year-old boy, an intermittent, outward deviation of his right eye was detected. His growth and development had been normal, and the problem with strabismus had not been noted previously on physical examination. The child’s mother had noticed it occasionally during recent months but had been told by a family member that the child would "grow out of it." A sibling of the mother had a "lazy eye" as a child, but the condition resolved during early childhood without treatment. On examination, the child’s pupils were reactive and equal. He was not cooperative with the funduscopic examination, but he did have a red reflex present bilaterally. Reflection of light off the cornea (i.e. Hirschberg’s cornea reflex test) appeared to be symmetrical while he was looking toward the light. Covering his eyes alternately while he focused on an object induced the outward deviation in the right eye, and a normal accommodation reflex was noted. The left eye remained normally aligned during testing. |
Children with an eye that turns in or out may have various causes of decreased vision in the affected eye or may have other forms of strabismus. Causes of unilateral loss of vision include a cataract, macular damage or scarring, retinal detachment, scarring in the cornea, defects in the optic tract, long-standing amblyopia, and neurological abnormalities causing defects in visual processing.
Bilateral deviation of the eyes can occur with congenital abnormalities as well as with bilateral causes of decreased vision. Some congenital causes of eye deviation include infantile esotropia, congenital blindness, cerebral palsy, and some rare congenital disorders such as craniofacial syndrome. Retinopathy of prematurity, intraocular tumours, and abnormalities of the vitreous should also be considered.
In most cases of young children with amblyopia, the cause is related to a decrease in vision. This decrease can be the result of refractive errors (severe myopia, severe astigmatism, severe hyperopia) leading to a preference for fixation with one eye. Muscle weakness in one of the ocular muscles or other eye abnormalities may be present. In some cases, no cause can be identified.
Further evaluation of the child's vision and the fundus is necessary to determine whether serious causes of decreased vision in the affected eye are present. On the basis of the patient's age, the correction of the amblyopia upon fixation, unilateral involvement, and an initial eye examination that does not reveal a defect in the cornea, retina, or the pupillary reflexes, the most likely diagnosis is amblyopia. In most children, this is caused by poor refraction in the affected eye.
The most readily recognized sign is the "lazy eye." The affected eye may deviate inward or outward. Some patients may fixate with one eye and then the other, allowing the non-fixating eye to drift inward or outward. Some patients with amblyopia have a significant or complete loss of vision. Loss in visual discrimination occurs, such as a loss of sensitivity to contrast. Some patients may have mild abnormalities in pupillary reflexes and tracking movement in the affected eye.
Testing for visual acuity and evaluation of the fundus are necessary and require special testing by an ophthalmologist. Full examination for refraction may require measurements with retinoscopy after treatment with a cycloplegic agent, particularly in young patients who are unable to cooperate with an ophthalmic examination. Challenging the affected eye with a diopter lens or using opticokinetic testing are other tests that can differentiate nystagmus caused by a refractive error from other types of visual loss.
Children with developmental delay and those who were born prematurely or who had a low birth weight have an increased risk for amblyopia. Evidence also exists that children with a positive family history for amblyopia are at increased risk.
Untreated amblyopia can lead to permanent vision loss. Most cases of amblyopia occur in patients younger than 5 years, yet it is a common underlying cause for unilateral vision loss in adults because it must be treated before the age of 6 years to prevent vision loss.
In a recent population study, 1.8% of children younger than 6 years were found to have amblyopia, confirming the usually quoted prevalence rate of 2%. Most (58.7%) of the children had significant hyperopia.
Because of the need to treat amblyopia early and the increased prevalence of amblyopia before age 5, screening of children when they begin school is recommended. However, evaluation should be done when signs of amblyopia are found or if reason exists to suspect a refractive error.
It is critical that treatment be started at an early age to prevent progression to blindness in the affected eye. In many patients, glasses that provide vision correction are adequate for treatment.
Patching the better eye is useful to force the weaker eye to fixate. Occlusion therapy can be difficult for young patients, and compliance may be difficult to achieve. Atropine eye drops can be used to reduce the visual acuity in the good eye by dilating the pupil and paralyzing the ciliary muscles.
For lesions such as cataracts, surgical intervention is necessary for obvious reasons. Treatment of retinal disorders, retinal tumours, and other ocular abnormalities would be tailored to the specific cause. Muscle disorders may be amenable to surgery or to ocular exercises designed to strengthen the weak muscles.