Question:

A 4-year-old boy is brought to the clinic by his mother due to concerns about his vision. While wearing a pirate costume at home last week, the patient refused to use the eye patch, stating that he could not see. This occurred only when the patch was placed over the right eye. The patient was born at 34 weeks gestation and takes no daily medications. Vital signs are normal. Eye examination shows symmetric corneal reflexes. The red reflex is equal bilaterally. Cover test shows no eye deviation, and extraocular movements are intact. Visual acuity is 20/30 in the right eye. In the left eye, the patient is unable to see the visual acuity chart and is able to count fingers only at a close distance. Funduscopic examination shows sharp disc margins with normal vessel caliber and course. Which of the following is the most likely cause of this patient's clinical findings?

Detachment of the retina

Increased intracranial pressure

Malalignment of the eyes

Opacity in the lens

Retinopathy of prematurity

Uncorrected refractive error

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Amblyopia
Definition	Functional reduction in visual acuity due to visual disturbances in early childhood Unilateral (most common): ≥2-line difference in vision between eyes (eg, 20/30 & 20/50) Bilateral: vision worse than 20/40 at age ≥4
Etiology	Strabismus Asymmetric refractive error Vision deprivation (eg, cataracts, ptosis)
Management	Corrective lenses Encourage use of amblyopic eye Patching eye with better vision Cycloplegic drops to blur normal eye Surgery (eg, cataract removal)

This patient has a functional reduction in visual acuity of the left eye, a condition known as amblyopia. Amblyopia, which is usually unilateral, occurs when binocular vision is disturbed while the visual system is developing (age <5). During early childhood, decreased visual stimuli from one eye results in preferentially enhanced development of the visual cortex associated with the other, normally functioning eye and therefore further decreased visual acuity of the affected eye.

Causes of amblyopia include strabismus (ocular malalignment), uncorrected refractive error, and less commonly, vision deprivation due to conditions such as cataracts, ptosis, or corneal opacities. This patient does not have an eye deviation that would suggest strabismus or a dull red reflex due to a lens opacity (Choices C and D). Therefore, the most likely etiology of his amblyopia is an asymmetric refractive error. Because of compensation by the normal eye, the presentation of refractive amblyopia is often delayed until the normal eye is occluded (eg, pirate patch).

Unilateral amblyopia is diagnosed by a ≥2-line difference in vision between eyes using the Snellen chart. Bilateral amblyopia should be suspected in a patient age ≥4 with a visual acuity worse than 20/40. Management includes strengthening the amblyopic eye by applying a patch or cycloplegic vision-blurring drops (eg, atropine) to the eye with better vision. Patients also typically require prescription lenses to correct the refractive error. Without treatment, vision loss can worsen and may become permanent.

(Choice A) Retinal detachment causes acute vision loss and is seen on funduscopic examination as a retinal tear or an elevated retina. This patient's funduscopic examination is normal.

(Choice B) Increased intracranial pressure due to edema, hemorrhage, or a mass typically causes headache, vomiting, and papilledema, none of which are seen in this patient.

(Choice E) Retinopathy of prematurity is a cause of decreased visual acuity that typically affects infants born at <30 weeks gestational age. Funduscopic examination shows retinal neovascularization.

Educational objective:
Amblyopia is a functional reduction in the visual acuity of one or both eyes and is caused by disturbances in binocular vision during early childhood. Because of compensation from the normal eye, amblyopia from an uncorrected refractive error may not present until the normal eye is occluded.