Likely Diagnosis: Vitamin A Deficiency (Night Blindness/Nyctalopia)
The most likely diagnosis for a child experiencing unilateral nocturnal vision loss with preserved daytime vision is vitamin A deficiency causing night blindness (nyctalopia), though the unilateral presentation is atypical and warrants urgent comprehensive ophthalmologic evaluation to rule out structural pathology.
Critical Immediate Actions
Urgent comprehensive ophthalmic examination is mandatory, including binocular red reflex test, visual acuity assessment, pupillary examination (specifically swinging-light test for relative afferent pupillary defect), cycloplegic refraction, binocular alignment testing, and dilated funduscopic examination to identify structural abnormalities 1.
A relative afferent pupillary defect on the affected right eye would indicate unilateral optic nerve or anterior visual pathway pathology requiring immediate neuroimaging 2.
Detailed history must include dietary intake (vitamin A sources), birth history, developmental milestones, trauma history, family history of retinal diseases (retinitis pigmentosa), and progression timeline 1.
Differential Diagnosis Framework
Primary Consideration: Nutritional Deficiency
- Vitamin A deficiency causes impaired rhodopsin regeneration in rod photoreceptors, leading to night blindness while cone function (daytime vision) remains intact initially.
- Classic presentation is bilateral, making unilateral symptoms concerning for additional pathology 3, 4.
Structural Causes Requiring Exclusion
- Retinal pathology: Early retinitis pigmentosa, congenital stationary night blindness, or chorioretinal scarring can present with night vision complaints 2, 1.
- Optic nerve abnormalities: Optic nerve hypoplasia or other structural optic nerve pathology may manifest as reduced vision in dim lighting 2.
- Amblyopia with refractive error: Anisometropic amblyopia could theoretically cause differential light sensitivity, though this would affect daytime vision more prominently 2, 1.
Red Flag Diagnoses
- Retinoblastoma: Must be ruled out with dilated funduscopic examination and red reflex testing, as vision loss can be an early sign 3.
- Congenital cataract or media opacity: Would cause visual deprivation amblyopia and abnormal red reflex 2, 3.
Diagnostic Algorithm
Step 1: Ophthalmologic Examination
- Perform red reflex examination bilaterally—asymmetry or absence suggests media opacity, retinal pathology, or retinoblastoma 2, 3.
- Conduct swinging-light pupillary test—afferent pupillary defect indicates optic nerve or retinal disease requiring neuroimaging 2.
- Complete dilated funduscopic examination to visualize retina, optic nerve, and macula for structural abnormalities 1.
Step 2: Specialized Testing
- Dark adaptation testing or electroretinography (ERG) if available to assess rod photoreceptor function.
- Cycloplegic refraction to identify significant anisometropia or high refractive error contributing to symptoms 1.
Step 3: Nutritional Assessment
- Serum retinol and retinol-binding protein levels if vitamin A deficiency suspected.
- Dietary history focusing on vitamin A intake (dairy, eggs, orange/yellow vegetables, liver).
Step 4: Neuroimaging Indications
- Order MRI brain and orbits if: afferent pupillary defect present, optic nerve abnormality on funduscopy, progressive vision loss, or neurological signs 2, 1.
Treatment Algorithm
If Vitamin A Deficiency Confirmed
- Immediate vitamin A supplementation per WHO guidelines (age-appropriate dosing).
- Dietary counseling to ensure adequate vitamin A intake.
- Monitor for improvement in night vision over 2-4 weeks.
If Structural Pathology Identified
- Amblyopia treatment: Correct refractive error with spectacles, followed by occlusion therapy or pharmacologic penalization of the fellow eye if amblyopia confirmed 2, 1.
- Retinal disease: Refer to pediatric retina specialist for management of inherited retinal dystrophies.
- Optic nerve pathology: Neurologic workup and management based on underlying etiology 1.
If Cataract or Media Opacity
- Surgical removal within 2 months of age for dense congenital cataracts to prevent irreversible amblyopia 2.
- Immediate optical correction post-surgery with aggressive amblyopia treatment 2.
Critical Prognostic Considerations
- Untreated amblyopia results in permanent visual handicap, decreased educational achievement, and doubled lifetime risk of bilateral vision impairment 2, 1.
- Early vitamin A deficiency can progress to corneal xerosis, keratomalacia, and permanent blindness if untreated.
- The unilateral nature of symptoms is highly atypical for nutritional deficiency and raises concern for structural pathology requiring urgent evaluation 1.
- Accidental trauma to the fellow eye poses significant risk in children with unilateral vision loss, emphasizing importance of protecting remaining vision 2.
Common Pitfalls to Avoid
- Dismissing unilateral night blindness as benign: Unilateral presentation demands thorough structural evaluation as nutritional deficiencies typically affect both eyes 1, 3.
- Delaying referral: Any child with unexplained vision loss requires prompt ophthalmologic evaluation, as treatment efficacy decreases with age for amblyogenic conditions 2, 1.
- Missing retinoblastoma: Always perform red reflex testing and dilated funduscopy in children with vision complaints 3.
- Overlooking dietary history: In resource-limited settings or restrictive diets, vitamin A deficiency remains a preventable cause of childhood blindness.