Evaluating a Patient for Night Blindness (Nyctalopia)
Initial Clinical Assessment
Begin by obtaining a detailed history focusing on the onset, duration, and progression of night vision difficulties, along with any associated systemic conditions that may cause vitamin A deficiency or retinal dysfunction. 1
Key History Elements
- Timing and progression: Determine when symptoms began and whether they are worsening 2
- Gastrointestinal history: Specifically ask about Crohn's disease, bowel resections, malabsorption syndromes, liver disease, or any gastrointestinal surgery (particularly Whipple procedures) 3, 4
- Dietary history: Assess intake of vitamin A-rich foods (liver, fish oils, egg yolk, butter) 5
- Medications and systemic conditions: Document current medications, history of bariatric surgery, chronic diarrhea, or conditions affecting fat absorption 6, 3
- Family history: Ask about inherited retinal dystrophies or congenital stationary night blindness 2
Comprehensive Ophthalmic Examination
Perform a complete eye examination with particular attention to fundus findings, visual acuity, and signs of vitamin A deficiency. 1
Essential Examination Components
- Visual acuity testing at distance and near with current correction 1
- Pupillary examination to assess for afferent pupillary defects 1
- Slit-lamp biomicroscopy: Look specifically for xerophthalmic changes (conjunctival dryness, Bitot spots, corneal changes) 4
- Dilated fundus examination: Evaluate for pigmentary retinal changes, retinitis pigmentosa patterns, or other retinal dystrophies 2, 4
- Intraocular pressure measurement 1
Specialized Diagnostic Testing
Electrophysiological testing is the cornerstone of objective evaluation when fundus examination appears normal or shows only nonspecific changes. 2, 4
Full-Field Electroretinogram (ERG)
- ERG is the most important diagnostic test for evaluating rod photoreceptor function in patients with nyctalopia and normal-appearing fundi 2, 4
- Scotopic (dark-adapted) responses specifically assess rod function and will be reduced or extinguished in true night blindness 2, 6, 4
- Photopic responses evaluate cone function and help differentiate between isolated rod dysfunction versus combined rod-cone dystrophies 2, 6
- 30 Hz flicker testing provides additional cone function assessment 3
Laboratory Testing
- Serum vitamin A (retinol) levels should be measured in all patients with acquired nyctalopia, particularly those with risk factors for malabsorption 6, 3, 4
- Important caveat: Normal serum vitamin A levels do not exclude vitamin A deficiency as the cause—ERG findings may be more sensitive than serum levels 4
Additional Testing When Indicated
- Subjective scotopic threshold sensitivity (STS) testing can complement ERG findings 2
- Visual field testing by automated perimetry if retinal dystrophy is suspected 1
- Optical coherence tomography of the retina and optic nerve when structural abnormalities are present 1
Diagnostic Interpretation
ERG Findings Guide Diagnosis
- Normal scotopic ERG (74% of cases): Consider optical causes (night myopia, uncorrected refractive error) or psychological factors 2
- Extinguished scotopic responses with normal photopic responses: Suggests vitamin A deficiency or isolated rod dysfunction 6, 3, 4
- Reduced scotopic and photopic responses: Indicates rod-cone dystrophy 2
- Specific ERG patterns: May suggest congenital stationary night blindness 2
Critical Clinical Pitfalls
- Do not dismiss complaints of nyctalopia even when fundus appears normal—26% of patients with normal-appearing fundi have abnormal ERG findings 2
- Do not rely solely on serum vitamin A levels—ERG responses are more sensitive markers of functional vitamin A deficiency 4
- Always inquire about gastrointestinal disease in patients presenting with acquired night blindness, as malabsorption is a common treatable cause 3, 4
- Perform ERG testing before concluding the problem is functional or psychological—objective testing prevents missed diagnoses of treatable conditions 2
Monitoring Response to Treatment
When vitamin A deficiency is identified and supplementation initiated, serial ERG testing documents functional recovery. 6