Night blindness
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I now have comprehensive information from multiple textbooks. Here is the full answer:
Night Blindness (Nyctalopia)
Night blindness (nyctalopia) is the inability to see adequately in dim light or darkness, while daytime vision remains relatively intact. It reflects dysfunction of the rod photoreceptors, which are responsible for scotopic (low-light) vision.
Pathophysiology
Rods contain the visual pigment rhodopsin, formed by the combination of 11-cis retinal (derived from vitamin A) and the protein scotopsin. When light strikes rhodopsin, it isomerizes to all-trans retinal and triggers rod hyperpolarization via cGMP-gated sodium channel closure. In darkness, rhodopsin is regenerated from 11-cis retinal.
Vitamin A is indispensable in this cycle: all-trans retinal is converted back to 11-cis retinal via all-trans retinol (vitamin A), using the enzyme isomerase. Without adequate vitamin A, rhodopsin cannot be sufficiently regenerated, impairing dim-light vision.
"Night blindness occurs in persons with severe vitamin A deficiency because, without vitamin A, the amounts of retinal and rhodopsin that can be formed are severely depressed. This condition is called night blindness because the amount of light available at night is too little to permit adequate vision in vitamin A-deficient persons."
- Guyton and Hall Textbook of Medical Physiology
Causes
More Common
- Refractive error (especially undercorrected myopia) - the most common overall cause of poor night vision
- Retinitis pigmentosa (RP) - rod photoreceptor degeneration
- Congenital stationary night blindness (CSNB) - non-progressive, from birth
- Advanced glaucoma or optic atrophy
- Miosis (especially pharmacologic - constricted pupil limits light entry)
- Drugs - phenothiazines, chloroquine, quinine
- After panretinal photocoagulation (PRP)
Less Common
- Vitamin A deficiency
- Gyrate atrophy
- Choroideremia
- Zinc deficiency (zinc is required for vitamin A metabolism)
(Wills Eye Manual)
Key Conditions in Detail
1. Vitamin A Deficiency
The classic nutritional cause. Large vitamin A stores in the liver mean deficiency takes months to develop. Once night blindness occurs, it can be reversed within less than 1 hour by intravenous vitamin A injection.
Symptoms: Night blindness is the earliest and most common manifestation, followed by dry eyes, ocular pain, and severe vision loss.
Ocular signs:
- Bitot spots - triangular, gray, foamy plaques of keratinized conjunctival debris (perilimbal)
- Decreased tear break-up time
- Bilateral conjunctival/corneal dryness
- Corneal epithelial defects, ulceration, perforation (keratomalacia in severe cases)
- Yellow/white peripheral retinal dots (focal RPE defects)
Systemic signs: Growth retardation (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, abetalipoproteinemia (Bassen-Kornzweig syndrome), chronic liver disease
Workup:
- History: malnutrition, GI/liver disease, previous GI surgery, measles?
- Serum vitamin A level (before treatment)
- Dark adaptation studies and ERG (may be more sensitive than serum level)
- Impression cytology (decreased goblet cell density)
Treatment (WHO-recommended dosages):
| Population | Dose |
|---|---|
| Children <12 months | 100,000 IU/day x 2 days, repeat in 2 weeks |
| Adults and children >12 months | 200,000 IU/day x 2 days, repeat in 2 weeks |
| Women of childbearing age (night blindness/Bitot spots only) | 10,000 IU/day x 2 weeks OR 25,000 IU/week x 4 weeks |
Also: ocular lubrication, treat underlying malnutrition, zinc supplementation.
(Wills Eye Manual, 13.7)
2. Retinitis Pigmentosa (RP)
Progressive rod-cone dystrophy. Night blindness and peripheral visual field loss are the cardinal early symptoms.
Classic fundus signs:
- "Bone spicule" pigment deposits in the peripheral retina (perivascular pattern)
- Arteriolar narrowing
- Waxy optic disc pallor
- Vitreous cells (most consistent sign)
- Progressive ring scotoma on visual fields
- ERG: moderately to markedly reduced
(Wills Eye Manual, 11.28)
3. Congenital Stationary Night Blindness (CSNB)
A group of non-progressive hereditary disorders with nyctalopia from birth. Key features:
- Non-progressive retinal dysfunction
- Normal visual fields
- Paradoxical pupillary response (pupil dilates in light)
- Mutations affect phototransduction or post-phototransduction signaling
- Inheritance: X-linked, autosomal dominant, or autosomal recessive
Two subtypes by fundus appearance:
Normal fundus:
- Type 1 (complete): absent rod pathway function, essentially normal cones
- Type 2 (incomplete): impairment of both rod and cone function
- X-linked and AR forms often have poor vision, nystagmus, and significant myopia
Abnormal fundus:
- Oguchi disease (AR): Fundus has a golden-yellow color in the light-adapted state, normalizing after prolonged dark adaptation (Mizuo-Nakamura phenomenon). Rod function absent at 30 minutes but recovers after prolonged dark adaptation.
- Fundus albipunctatus (AR/AD): Multitude of subtle yellow-white spots at the posterior pole, sparing the fovea; not progressive.
- Retinitis punctata albescens (AR/AD): Scattered whitish-yellow spots, most numerous at the equator; associated with arteriolar attenuation; tends to be more progressive than fundus albipunctatus.
(Kanski's Clinical Ophthalmology, 10th ed.)
4. Gyrate Atrophy
- Autosomal recessive; caused by ornithine aminotransferase deficiency
- Plasma ornithine is 10-20 times normal; lysine is decreased
- Night blindness and decreased peripheral vision in the first decade
- Scalloped RPE and choriocapillaris atrophy in midperiphery, progressing to involve the entire fundus
- Associated with posterior subcapsular cataract and high myopia
Treatment: Arginine-restricted diet + supplemental pyridoxine (vitamin B6), though only a small percentage of patients are B6 responders.
5. Choroideremia
- X-linked recessive; males present in the first to second decade
- Night blindness followed by progressive peripheral vision loss; central vision lost late
- No bone spicules; RPE and choriocapillaris progressively absent
- Female carriers have small, scattered intraretinal pigment granules
- No effective treatment currently available
Evaluation Summary
| Test | Use |
|---|---|
| Dark adaptation testing | Quantifies rod sensitivity threshold |
| Electroretinogram (ERG) | Rod vs. cone function; may be more sensitive than serum vitamin A |
| Serum vitamin A level | Confirm deficiency before treatment |
| Visual fields | Ring scotoma in RP; constriction in gyrate atrophy/choroideremia |
| Fundus examination | RP bone spicules, Oguchi golden sheen, albipunctatus white dots |
| Plasma ornithine | Gyrate atrophy |
Recent evidence: A 2025 systematic review (PMID 39694460) examined the prevalence of ocular manifestations of vitamin A deficiency in children, confirming night blindness as the most prominent and early sign - consistent with the textbook data above.
Sources: Goldman-Cecil Medicine | Guyton and Hall Textbook of Medical Physiology | Wills Eye Manual | Kanski's Clinical Ophthalmology 10th ed.
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