20.3 Ophthalmology (Strabismus, Red Eye, Leukocoria)

Ophthalmology: Strabismus, Red Eye, and Leukocoria

Why This Matters for the Exam

Pediatric vision screening findings are classic single-best-answer material because the underlying question is almost always whether a finding is benign or a must-not-miss diagnosis. The exam rewards recognizing red-flag findings — particularly an abnormal red reflex — over common benign mimics.

Strabismus

Strabismus is ocular misalignment, meaning the eyes do not point in the same direction simultaneously.

Types (by Direction of Deviation)

Strabismus is also described by pattern: constant (present at all times) versus intermittent, and unilateral versus alternating, where the deviating eye switches from one side to the other.

Exam nuance — pseudostrabismus: a flat, broad nasal bridge with prominent epicanthal folds in an infant can make the eyes appear crossed, called pseudoesotropia, when alignment is actually normal. This is distinguished from true strabismus using the corneal light reflex and cover test described below, and it does not require treatment or referral by itself.

Amblyopia Risk

Amblyopia, sometimes called lazy eye, is reduced visual acuity in one eye caused by abnormal visual development during the critical period of childhood, roughly up to age 7–9, most often because the brain suppresses input from a misaligned or blurred eye to avoid double vision. Strabismus is one of the three major causes of amblyopia, alongside significant refractive error (anisometropia) and visual deprivation, such as from a dense congenital cataract.

Exam-critical point: amblyopia is a diagnosis of visual development, not just an eye exam finding, and it is treatable only within the critical period. Delayed recognition of strabismus, or of any condition obstructing a clear image, risks permanent, irreversible vision loss in the affected eye even after the primary problem is later corrected. This is why prompt referral matters more than the specific type of strabismus.

Screening Tools

• Red reflex test: shining a direct ophthalmoscope light into both pupils simultaneously from about 18 inches should produce a symmetric, bright red or orange reflex. Asymmetry, dullness, or whiteness is abnormal and requires urgent ophthalmology referral (see Leukocoria below).
• Corneal light reflex (Hirschberg test): a light shone at both corneas should reflect symmetrically in both pupils; asymmetric reflection suggests true strabismus.
• Cover test: covering the fixating, presumed normal eye and watching the other eye for a refixation movement is the definitive test for manifest strabismus, since a shift means that eye was misaligned. The cover-uncover test and alternate cover test variants distinguish manifest deviations (tropia) from latent deviations (phoria).

Red Eye in Children: Conjunctivitis and Serious Mimics

Most pediatric red eye is conjunctivitis, but the exam expects recognition of features suggesting a more serious process requiring urgent referral.

Red Flags That Argue Against Simple Conjunctivitis

The following features should prompt urgent evaluation rather than a diagnosis of routine conjunctivitis:

• Photophobia, significant pain, or a foreign-body sensation — consider corneal abrasion, keratitis, or uveitis
• Decreased visual acuity
• Perilimbal (ciliary) flush, meaning redness concentrated around the cornea rather than diffusely across the conjunctiva — suggests keratitis, uveitis, or angle disease rather than simple conjunctivitis
• Proptosis, limited or painful eye movement, or eyelid swelling with fever and systemic illness — raises concern for orbital cellulitis, a sight- and life-threatening emergency, as opposed to preseptal (periorbital) cellulitis, which spares eye movement and vision
• An unequal or poorly reactive pupil
• Neonatal conjunctivitis in the first month of life always warrants urgent evaluation for Neisseria gonorrhoeae and Chlamydia trachomatis, given the risk of corneal perforation and systemic disease.

Leukocoria: The Must-Not-Miss Red Flag

Leukocoria is an abnormal white pupillary reflex, a white reflex instead of the normal red reflex, often first noticed by parents in a flash photograph where one pupil appears white instead of red. It is never a normal finding and always requires urgent ophthalmology referral.

The exam's single most important association is that retinoblastoma is the diagnosis that must never be missed when leukocoria is present.

• Retinoblastoma is the most common primary intraocular malignancy of childhood, usually presenting before age 2, and can be unilateral, which is usually sporadic with a somatic RB1 mutation, or bilateral and multifocal, which is usually hereditary with a germline RB1 mutation and a higher risk of second malignancies later in life.
• Leukocoria is the most common presenting sign; strabismus can also be an early presenting sign, because a tumor obstructing the visual axis leads to poor fixation.
• Prompt diagnosis, including a dilated fundoscopic exam under anesthesia and imaging, and prompt treatment are both vision- and life-saving; delayed diagnosis allows local and metastatic spread.

Other causes of leukocoria to keep in the differential, since the exam may ask you to distinguish retinoblastoma from these, include congenital cataract, persistent fetal vasculature (persistent hyperplastic primary vitreous), Coats disease (a retinal vascular malformation with exudation), retinopathy of prematurity, and toxocariasis.

Exam trap: because leukocoria can have several causes, some answer options may describe more common, more benign explanations, such as congenital cataract. The exam is testing whether you know that any white pupillary reflex mandates urgent, prompt ophthalmology referral to exclude retinoblastoma, regardless of what the most likely alternative diagnosis might be.