Night blindness

Night blindness (nyctalopia) is poor vision in dim light with normal or near normal vision in good light. It reflects impaired rod function or poor dark adaptation. Causes include vitamin A deficiency, inherited rod dystrophies, glaucoma, cataract, high myopia, and drug toxicity.

Physiology. Rods mediate scotopic vision and need rhodopsin, formed from vitamin A (retinol). Vitamin A deficiency impairs rhodopsin regeneration first, causing night blindness, then xerophthalmia (dry conjunctiva with Bitot spots, corneal xerosis, ulceration and keratomalacia).

Classification. Acquired (vitamin A deficiency from poor intake, malabsorption, liver disease and alcoholism; drugs; glaucoma and optic nerve disease; cataract; myopia; paraneoplastic and autoimmune retinopathy) and inherited (retinitis pigmentosa, congenital stationary night blindness, Oguchi disease, fundus albipunctatus, Usher syndrome, choroideremia, gyrate atrophy, Refsum disease, Bietti crystalline dystrophy).

Key distinctions. Progressive night blindness with peripheral field loss and a family history indicates retinitis pigmentosa. Night blindness with dry eyes, Bitot spots and malnutrition indicates vitamin A deficiency. Lifelong stable night blindness suggests congenital stationary night blindness.

History.

• Age of onset (childhood or adult) and progression.
• Symptoms: difficulty seeing in dim rooms, at dusk and when moving from light to dark; bumping into objects; peripheral vision loss; glare; colour vision change; dry or sore eyes.
• Family history (consanguinity, X linked pattern) and hearing loss.
• Diet (limited animal products, vegetables, fruit), malnutrition, alcohol intake and chronic diarrhoea.
• Fat malabsorption (coeliac disease, cystic fibrosis, pancreatic disease, bariatric surgery, cholestasis), liver disease and eating disorders.
• Drugs and cancer history.

Drug history. Phenothiazines (thioridazine), chloroquine and hydroxychloroquine, vigabatrin, ethambutol, isotretinoin, deferoxamine, quinine, miotic drops (pilocarpine), and chelators.

Examination.

• Visual acuity, colour vision, pupils, and visual fields (peripheral constriction).
• Slit lamp: Bitot spots, corneal xerosis, cataract, posterior subcapsular opacities, keratomalacia, dry eye.
• Intraocular pressure.
• Dilated fundus: pigmentary bone spicule changes, attenuated vessels, waxy disc pallor, retinal flecks.
• Growth and nutritional assessment, skin (xerosis, follicular hyperkeratosis), and hearing.

Red flags

• Corneal xerosis, ulceration or keratomalacia (vitamin A deficiency, emergency)
• Bitot spots with malnutrition, measles, diarrhoea or severe illness in a child
• Progressive field loss with night blindness
• Night blindness with hearing loss (Usher syndrome) or with neurological features (Refsum disease)
• Acute onset with drug exposure
• Cancer history with subacute visual loss (paraneoplastic retinopathy)
• Pregnancy with night blindness (vitamin A deficiency)

Special groups. Children: vitamin A deficiency (a leading cause of preventable blindness and mortality in low income settings), measles, and congenital stationary night blindness. Pregnancy: vitamin A deficiency; high dose retinol is teratogenic. Older adults: cataract, glaucoma and malabsorption. Region specific: vitamin A deficiency is common in South Asia and sub-Saharan Africa.

Nutritional and acquired

• Vitamin A deficiency (dietary, malabsorption: coeliac disease, cystic fibrosis, short bowel, bariatric surgery, pancreatic insufficiency, cholestatic liver disease, chronic alcohol use, chronic diarrhoea, measles, eating disorders)
• Zinc deficiency (impaired retinol mobilisation), vitamin E deficiency
• Drug toxicity (thioridazine, hydroxychloroquine, vigabatrin, ethambutol), and siderosis

Ocular

• Glaucoma, cataract, high myopia, optic atrophy, retinal detachment (history)
• Autoimmune and paraneoplastic retinopathy (cancer associated, melanoma associated), diffuse unilateral subacute neuroretinitis, posterior uveitis, birdshot choroidopathy, syphilitic retinopathy

Inherited

• Retinitis pigmentosa (rod cone dystrophy, syndromic: Usher syndrome, Bardet Biedl syndrome, Refsum disease, abetalipoproteinaemia)
• Congenital stationary night blindness (complete and incomplete), Oguchi disease, fundus albipunctatus, Sorsby dystrophy, choroideremia, gyrate atrophy, Bietti dystrophy

Reasoning. Look for vitamin A deficiency (history, signs, level) and correct it urgently. If the eye signs point to retinal dystrophy, use electrophysiology and genetic testing.

First line. Visual acuity, colour vision, fields, slit lamp examination for xerophthalmia signs, dilated fundus examination, and nutritional assessment.

Targeted tests.

• Serum retinol (under 0.70 micromol/L, or 200 micrograms/L, indicates deficiency; under 0.35 micromol/L is severe), retinol binding protein, zinc, vitamin E, vitamin B12, folate, and iron; liver function, full blood count, albumin, coeliac serology, faecal elastase, and measles status.
• Dark adaptometry and full field electroretinogram (rod responses reduced or absent in rod dystrophy and vitamin A deficiency, reversible with replacement), electro-oculogram.
• Automated perimetry, optical coherence tomography, fundus autofluorescence and fundus photography.
• Genetic testing with counselling for suspected inherited retinal disease; hearing tests for Usher syndrome; lipid profile and phytanic acid for Refsum disease; blood film and lipid profile for abetalipoproteinaemia.
• Anti-retinal antibodies and cancer screening (CT chest, abdomen and pelvis) for suspected paraneoplastic retinopathy.
• Intraocular pressure and optic nerve imaging for glaucoma.

Limitations. Serum retinol is reduced in inflammation and protein deficiency independently of stores. Electroretinogram needs specialist equipment.

Vitamin A deficiency with night blindness or xerophthalmia (treat as an emergency when corneal changes are present).

• Retinol (vitamin A) 200,000 units orally on days 1, 2 and 14 for adults and children over 12 months (100,000 units for 6 to 11 months; 50,000 units for under 6 months).
• When the child is vomiting or has malabsorption, give the equivalent intramuscular dose (water miscible).

In pregnancy and women who could become pregnant, use lower doses (10,000 units daily or 25,000 units weekly) for night blindness without corneal disease. Use the full dose only for corneal xerophthalmia under specialist direction because of teratogenic risk.

Treat the cause:

• dietary diversification (liver, eggs, dairy, orange and yellow fruits, dark green leafy vegetables, fortified foods)
• treat infection and diarrhoea
• treat malabsorption (pancreatic enzyme replacement, gluten free diet in coeliac disease, fat soluble vitamin supplements)
• correct protein energy malnutrition
• correct zinc deficiency (zinc sulphate 220 mg orally once daily with food)
• give measles immunisation and treatment

Corneal xerophthalmia and ulcer. Same day ophthalmology. Give lubricants (hypromellose 0.3 percent every 1 to 2 hours), topical antibiotic (chloramphenicol 0.5 percent drops 4 times daily), cycloplegic drops, and retinol. Protect the eye from pressure.

Prevention. Vitamin A supplementation in high risk children (200,000 units every 4 to 6 months for ages 12 months to 5 years; 100,000 units at 6 to 11 months), fortified foods, breastfeeding promotion, and immunisation.

Retinitis pigmentosa and inherited rod disease.

• Genetic counselling and family screening.
• No treatment restores vision except gene therapy in selected genotypes (voretigene neparvovec for biallelic RPE65 related disease).
• Sunglasses for glare and cataract surgery when needed.
• Treat cystoid macular oedema with topical dorzolamide 2 percent 1 drop three times daily or acetazolamide 250 to 500 mg orally daily (monitor potassium and bicarbonate).
• Low vision services, orientation and mobility training, and psychological support.

Vitamin A palmitate 15,000 units daily may slow decline in typical retinitis pigmentosa (evidence is limited). Give it under specialist supervision, with liver function and retinol monitoring, and avoid it in pregnancy and with smoking. Refsum disease: dietary phytanic acid restriction and plasmapheresis by specialists.

Congenital stationary night blindness. Stable; counselling and genetic testing; correct refractive error.

Drug toxicity. Stop or change the culprit drug after discussion with the prescriber; hydroxychloroquine: annual screening after 5 years (maximum 5 mg/kg actual body weight daily).

Glaucoma and cataract. Treat as indicated: pressure lowering drops (latanoprost 0.005 percent 1 drop at night, or timolol 0.5 percent 1 drop twice daily; avoid timolol in asthma, heart block and bradycardia) and cataract surgery.

Paraneoplastic retinopathy. Oncology care, treatment of the primary tumour, and immunosuppression by specialists.

Safety. Avoid driving at night, use bright and well positioned lighting, nightlights, contrast markings on steps, and mobility training.

Monitoring. Repeat retinol and night vision at 4 to 8 weeks after supplementation; electroretinogram and fields annually for inherited disease; growth and nutrition in children.

Referral and escalation. Same day ophthalmology for xerophthalmia with corneal involvement. Dietitian and gastroenterology for malabsorption. Genetics and inherited retinal disease clinic for suspected dystrophy. Oncology for paraneoplastic disease. Public health and nutrition programmes for deficiency in communities.

Admission criteria. Corneal ulceration or keratomalacia, severe malnutrition, or severe infection in a child with xerophthalmia.

Discharge and safety netting. Return for eye pain, redness, clouding of the cornea, reduced vision, worsening night vision despite treatment, or new symptoms in other family members.