lished by placing a four-prism diopter base-out prism sequentially in front of one eye, then the other eye, while the child fixates a distant target binocularly. A rapid horizontal refixation movement is observed in one eye but not in the opposite eye with the central scotoma. The absence of central fusion with preservation of peripheral fusion is also confirmed by performing the Worth 4-Four Dot test using the handheld flashlight at near and at a distance. In the monofixation syndrome, this test reveals the presence of fusion for near targets (which subtend a large angle and thereby stimulate peripheral fusion) and suppression of the involved eye for small, distant targets (for which images fall within the scotoma).

Refractive Abnormalities

Children with bilateral hyperopia of 6 diopters or more can present with bilaterally decreased vision in the range of 20/100. When given their full cycloplegic refraction, their vision initially improves to 20/40–20/70 and, in some cases, may approach normal. The subnormal vision is presumed to represent a bilateral form of ametropic (form deprivation) amblyopia. Because the optic discs appear small in high hyperopes, the diagnosis of optic nerve hypoplasia may be entertained, but close examination reveals a normal peripapillary nerve fiber layer. Children with corrected bilateral meridional amblyopia may present with unexplained visual loss when the increased visual demands of school work brings attention to their visual difficulty.

One should inquire about recent ingestion of medications with anticholinergic side effects in any child who complains of blurred vision that is worse for near tasks. If accommodative amplitudes are found to be decreased, other systemic disorders associated with hypoaccommodation, such as botulism, dorsal midbrain syndrome, diabetes, head and neck trauma, diphtheria, and familial forms of hypoaccommodation, must be considered in the differential diagnosis.380,389

Cornea

Keratoconus in children can reduce vision in the absence of any biomicroscopic findings. Keratoconus is a progressive, noninflammatory ectasia, in which the cornea assumes a progressively conical shape secondary to central thinning and protrusion. Its incidence has been estimated at 50–230 per 100,000 people. It is usually bilateral, but may be unilateral or highly asymmetrical. Most patients have no family history of keratoconus, but a few autosomal dominant and recessive pedigrees have been described. Keratoconus is generally an isolated finding but is occasionally associated with systemic disease, most notably atopic disease and Down syndrome.17

In children with unexplained visual loss, retinoscopy through an undilated pupil is a sensitive office screening test, because the earliest changes may be confined to the central cornea, and the bright reflex obtained from dilated retinoscopy may obscure these changes. The diagnosis of early keratoconus can be confirmed by keratoscopy.17 Slit lamp biomicroscopic signs may be absent early in the disorder. Corneal topography is a sensitive means of diagnosing keratoconus, although expense and availability limits its general application.

Mucolipidosis IV is a rare autosomal recessive disorder that is more common in Ashkenazi Jews than in other ethnic groups. Mothers may note decreased intrauterine motility. There is delayed psychomotor development from birth, corneal clouding, and some coarseness of facial features (not as striking as that seen in the mucolipidoses). The corneal clouding is epithelial in origin and can easily be mistaken for congenital cataracts, leading to unnecessary cataract surgery.329a These patients develop a retinal dystrophy, go blind, and die in their teens and twenties. Although the enzymatic defect has been identified, known, no treatment is currently available. Diagnosis is especially important for future genetic counseling, because the risk of this disease in subsequent children is 25%.

Retina

Stargardt macular dystrophy should be a major diagnostic consideration in the child who presents with unexplained or psychogenic visual loss in both eyes. Stargardt macular dystrophy is a hereditary condition (usually autosomal recessive but, rarely, autosomal dominant) in which central vision decreases in childhood. Occasionally, patients become symptomatic in adulthood.118 Over time, atrophic macular degeneration develops, surrounded by yellow pisciform flecks that increase in size and number and may subsequently disappear.39 A bull’s-eye maculopathy may be seen as an intermediate stage. Peripheral pigmentary clumping is also occasionally seen.

Although children with Stargardt disease eventually develop distinct retinal abnormalities, the retina may appear normal until visual acuity approaches 20/200. Some children with Stargardt disease experience significant visual loss over the course of weeks to months.109 Once vision decreases to 20/40, it usually deteriorates rapidly to 20/200. The final visual acuity usually stabilizes in the range of 20/200–20/400.109,418 Despite their diffuse retinal involvement, children with Stargardt disease have mild dyschromatopsia, mildly constricted visual fields, and no symptoms of night blindness.

The diagnosis of Stargardt disease should be suspected in a child whose “psychogenic” visual loss fails to improve with reassurance and whose color vision is relatively preserved despite poor acuity.236 ERGs and electro-oculograms are generally unhelpful in establishing the diagnosis, because they are