Night blindness
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Night Blindness (Nyctalopia)
Night blindness (nyctalopia) is the inability to see adequately in dim or low light conditions. It reflects impaired rod photoreceptor function, since rods mediate scotopic (dark-adapted) vision.
Pathophysiology
Rods depend on rhodopsin, a photosensitive pigment formed by the combination of 11-cis retinal (derived from vitamin A) and scotopsin. When light hits rhodopsin, it decomposes into all-trans retinal + scotopsin, generating a nerve signal. Regeneration requires reconversion back to 11-cis retinal via vitamin A.
In darkness, rod outer segments maintain high cGMP levels, keeping sodium channels open (dark current). Light triggers rhodopsin decomposition → cGMP falls → Na⁺ channels close → rod membrane hyperpolarizes → signal transmitted. This cycle is critically vitamin A-dependent.
- Guyton and Hall Textbook of Medical Physiology, p. 635
Causes / Differential Diagnosis
More Common
| Cause | Key Feature |
|---|---|
| Refractive error (undercorrected myopia) | Corrects with glasses |
| Advanced glaucoma / optic atrophy | Associated field loss |
| Miosis (especially pharmacologic) | Drug history |
| Retinitis pigmentosa (RP) | Bone-spicule pigmentation, tunnel vision, abnormal ERG |
| Congenital stationary night blindness (CSNB) | Present from birth, non-progressive |
| After panretinal photocoagulation | History of laser treatment |
| Drugs (phenothiazines, chloroquine, quinine) | Medication history |
Less Common
| Cause | Key Feature |
|---|---|
| Vitamin A deficiency | Bitot spots, xerosis, malnutrition history |
| Gyrate atrophy | Elevated plasma ornithine (10-20x normal), AR |
| Choroideremia | X-linked recessive, males predominantly affected |
| Zinc deficiency | Zinc needed for vitamin A metabolism |
- The Wills Eye Manual, p. 40-41
True night blindness (inability to see any stars on a clear night, inability to ambulate without help in darkness) is seen in retinitis pigmentosa, vitamin A deficiency, and phenothiazine toxicity.
- Goldman-Cecil Medicine, p. 2500
Vitamin A Deficiency - Details
Vitamin A is essential for photoreceptor protein synthesis and epithelial maintenance. Night blindness is the earliest and most common manifestation of vitamin A deficiency.
Ocular signs:
- Bitot spots - triangular, foamy, keratinized gray plaques at the perilimbus (due to Corynebacterium xerosis)
- Conjunctival and corneal xerosis (dryness)
- Corneal epithelial erosions → ulceration → keratomalacia → perforation
- Yellow/white peripheral retinal dots (RPE defects)
Systemic signs:
- Growth retardation in children
- Dry, hyperkeratotic skin
- Increased susceptibility to infections
Etiology:
- Primary: dietary deficiency, chronic alcoholism
- Secondary: lipid malabsorption - cystic fibrosis, chronic pancreatitis, IBD, celiac disease, postgastrectomy, chronic liver disease, abetalipoproteinemia (Bassen-Kornzweig syndrome)
Note: Vitamin A is stored in large quantities in the liver; deficiency takes months to develop. Once night blindness occurs, it can be reversed within 1 hour by IV vitamin A.
- Guyton and Hall, p. 635; Kanski's Clinical Ophthalmology, p. 7.38; The Wills Eye Manual, pp. 1044-1046
WHO Classification (Xerophthalmia)
| Grade | Feature |
|---|---|
| XN | Night blindness |
| X1A | Conjunctival xerosis |
| X1B | Bitot spots |
| X2 | Corneal xerosis |
| X3A | Corneal ulceration (<1/3 cornea) |
| X3B | Corneal ulceration (≥1/3 cornea - keratomalacia) |
| XS | Corneal scar |
| XF | Xerophthalmia fundus |
Congenital Stationary Night Blindness (CSNB)
A group of non-progressive retinal disorders with nyctalopia from birth. Caused by mutations affecting phototransduction and post-phototransduction transmission.
With normal fundus:
- Type 1 (complete): Complete absence of rod pathway function, normal cone ERG
- Type 2 (incomplete): Impaired rod + cone function
- X-linked, AD, and AR forms exist; X-linked/AR often have poor vision with nystagmus and high myopia
With abnormal fundus:
-
Oguchi disease (AR): Fundus has golden-yellow appearance in light-adapted state, reverts to normal after prolonged dark adaptation (Mizuo-Nakamura phenomenon). Rod function absent at 30 min dark adaptation but recovers after extended dark adaptation.
-
Fundus albipunctatus: Multiple subtle yellow-white spots at the posterior pole, sparing the fovea; both cones and rods may be affected on ERG; AR or AD
-
Retinitis punctata albescens: Whitish-yellow spots most numerous at the equator, arteriolar attenuation; may be a variant of fundus albipunctatus; RLBP1 mutations identified
-
Kanski's Clinical Ophthalmology, pp. 658-659; The Wills Eye Manual, p. 889
Paradoxic pupillary response (pupil dilates in light instead of constricting) is seen in CSNB, congenital achromatopsia, Leber congenital amaurosis, RP, and albinism.
Hereditary Causes with Progressive Night Blindness
- Gyrate atrophy: AR; elevated plasma ornithine (10-20x normal) due to ornithine aminotransferase deficiency; scalloped RPE/choriocapillaris atrophy progressing centrally; posterior subcapsular cataract; high myopia. Treatment: arginine-restricted diet + vitamin B6 (pyridoxine)
- Choroideremia: X-linked recessive; males present in 1st-2nd decade with nyctalopia then progressive field loss; total absence of RPE and choriocapillaris late; female carriers show scattered pigment granules
Workup
- Detailed history: diet, alcohol use, GI surgery, liver disease, medications, family history
- Full ophthalmic exam including slit lamp and fundus
- Serum vitamin A level
- Dark adaptation testing (may be more sensitive than serum vitamin A)
- Electroretinogram (ERG) - scotopic (rod) ERG confirms rod dysfunction
- Impression cytology of conjunctiva (decreased goblet cell density in Vit A deficiency)
- Plasma ornithine level if gyrate atrophy suspected
- Genetic testing for inherited dystrophies
Treatment by Cause
| Cause | Treatment |
|---|---|
| Vitamin A deficiency | WHO protocol: 200,000 IU/day x 2 days, repeat in 2 weeks (adults/children >12 mo); reduce dose in women of childbearing age |
| CSNB | No treatment; supportive; genetic counseling |
| Gyrate atrophy | Arginine-restricted diet; vitamin B6 up to 500 mg/day |
| Choroideremia | No effective treatment; tinted sunglasses; genetic counseling |
| RP | Gene therapy (AAV-RPE65) for RPE65-associated RP; supportive otherwise |
| Drug-induced | Discontinue offending agent if possible |
- The Wills Eye Manual, pp. 888-889, 1046
Recent evidence note: A 2025 systematic review (PMID 39694460) on ocular manifestations of vitamin A deficiency in children confirms night blindness remains the most prevalent early manifestation globally, consistent with textbook teaching.
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