Pediatric Retinoblastoma & Leukocoria

Presentation: Leukocoria

Retinoblastoma classically presents with leukocoria: a white pupillary reflex in place of the normal red reflex, not a painful red eye, acute angle-closure glaucoma, or flashes and floaters. The white reflex comes from light bouncing off the pale, calcified tumor mass filling the vitreous cavity or lining the retina rather than off the normally vascular, red-appearing fundus. The red reflex test is a routine part of the newborn and well-child exam for exactly this reason. A parent who notices it, classically in a flash photograph where the normal red-eye effect is absent or replaced by white in one pupil, or a pediatrician who catches it on exam, should prompt urgent referral for a dilated fundus examination.

Strabismus is the second most common presenting sign. A tumor involving the macula can disrupt central fixation and produce a secondary deviation, which is why any new strabismus in an infant or young child warrants a dilated fundus exam rather than just a cover test and a glasses prescription.

Genetics: The RB1 Gene

RB1, a tumor suppressor gene on chromosome 13, normally regulates the cell cycle. Losing both functional copies in a retinal precursor cell removes that brake on proliferation and lets a tumor form.

The heritable and non-heritable forms are distinguished by whether the first RB1 mutation is present in every cell of the body (germline) or arose only in a single retinal cell (somatic):

• Heritable (germline) retinoblastoma: the child is born with one mutated RB1 allele in every cell, so a single further somatic "second hit" in any retinal cell is enough to cause a tumor. Because the first hit is already present everywhere, heritable disease is typically bilateral and multifocal, tends to present earlier, and carries an increased lifetime risk of second primary malignancies outside the eye (e.g. osteosarcoma). Bilateral or multifocal retinoblastoma is, for practical purposes, always a marker of a germline RB1 mutation.
• Sporadic (non-heritable) retinoblastoma: both RB1 hits occur somatically in the same retinal cell. This is a much rarer coincidence, and it essentially always produces unilateral, unifocal disease.

This two-hit pattern is the same "two-hit hypothesis" model Alfred Knudson originally proposed to explain hereditary cancer predisposition in general, with retinoblastoma as its founding example. Confirming or strongly suspecting a germline mutation (bilateral disease, or a family history of retinoblastoma) leads to genetic counseling and testing for the family, plus closer surveillance of the contralateral eye and of the patient's future offspring.

Diagnostic Imaging

B-scan ultrasound is central to the workup. The single most reliable finding on echography is calcification within the mass, producing high-amplitude internal echoes with posterior acoustic shadowing, which is essentially pathognomonic for retinoblastoma and is reproduced on CT as well. Low internal reflectivity, a mushroom-shaped configuration, and focal choroidal excavation can all turn up with various intraocular masses, but none is as specific for retinoblastoma as demonstrating calcification.

MRI is preferred over CT for further characterization and staging when available. It avoids ionizing radiation in a population, heritable retinoblastoma carriers, already at elevated lifetime cancer risk, and it can also assess for optic nerve invasion and any associated intracranial disease: trilateral retinoblastoma, a pinealoblastoma occurring alongside bilateral heritable disease.

Leukocoria Mimickers

Because leukocoria is a nonspecific sign rather than a diagnosis in itself, every child presenting with it needs a workup broad enough to separate retinoblastoma from its mimickers, since several benign or infectious entities can produce a similar white pupillary reflex:

• Persistent fetal vasculature (PFV): caused by failure of the fetal hyaloid vasculature to regress, producing leukocoria and often microphthalmos in the affected eye.
• Ocular toxocariasis: a granuloma from Toxocara canis larval infestation at the macula or periphery can mimic retinoblastoma closely enough that it's considered an important differential in any child presenting with unilateral leukocoria.
• Congenital cataract: a dense lens opacity can itself produce a white-appearing pupil, though the light typically reflects at the level of the lens rather than the retina.
• Coats disease is a separate, non-malignant condition of idiopathic unilateral retinal telangiectasia with lipid-rich subretinal exudation, predominantly affecting young males. It is a classic "retinoblastoma masquerader" because extensive exudative detachment can produce a white pupillary reflex indistinguishable from retinoblastoma on external exam alone. It is a differential diagnosis to exclude, not a form of retinoblastoma, and the two are distinguished by fundus exam and imaging: Coats disease characteristically lacks the calcification seen on B-scan in retinoblastoma.

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