Nutritional Optic Neuropathy

Bilateral symmetric optic neuropathy caused by deficiency of vitamins and minerals essential to mitochondrial metabolism and myelin: vitamin B12 (cobalamin), folate, thiamine (vitamin B1), riboflavin, copper, and sometimes protein malnutrition. Tobacco-alcohol amblyopia has a similar picture with nutritional deficiency.

• Causes: pernicious anaemia (autoimmune atrophic gastritis), strict vegan diet, malabsorption (coeliac disease, Crohn disease, bariatric surgery, gastrectomy, small intestinal bacterial overgrowth), metformin and proton pump inhibitor use (low B12), nitrous oxide abuse, chronic alcoholism, famine or prisoner-of-war malnutrition, epidemic outbreaks (nutritional epidemics), excess zinc intake (copper deficiency), and parenteral nutrition without supplements.
• Mechanism: demyelination and axonal loss of the papillomacular bundle, with increased methylmalonic acid and homocysteine in B12 deficiency.
• Risk is higher in the elderly, in patients after bariatric surgery (copper, B12, thiamine), and in alcohol dependence.

• Subacute, painless, progressive bilateral blurred vision with dyschromatopsia (colours look washed out, particularly red), and central or centrocaecal scotomas; vision 6/12 to 6/60 in most.
• Discs normal at onset; later temporal pallor; peripheral fields preserved; pupils briskly reactive (no relative afferent pupillary defect if symmetric).
• Systemic: fatigue, glossitis, anaemia (macrocytic), paraesthesia, sensory ataxia, myelopathy (subacute combined degeneration of the cord in B12 deficiency), cognitive decline, weight loss, diarrhoea, burning feet, and neuropathy.
• Thiamine deficiency (Wernicke): confusion, ataxia, ophthalmoplegia, nystagmus, and rarely optic neuropathy.
• Copper deficiency: myeloneuropathy and anaemia, neutropenia.

• History: diet, alcohol, smoking, surgery, gastrointestinal disease, medications (metformin, proton pump inhibitors, nitrous oxide, anti-tuberculous drugs), and weight loss.
• Visual acuity, colour vision, central fields (cecocentral scotomas), pupils, dilated fundus examination, optical coherence tomography (temporal retinal nerve fibre layer thinning), and visual evoked potentials.
• Blood tests: full count and film (macrocytosis, hypersegmented neutrophils), vitamin B12 (low below 200 pg/mL; borderline 200 to 300 with raised methylmalonic acid and homocysteine), folate, thiamine (or erythrocyte transketolase), copper, caeruloplasmin, zinc, vitamin A and E, intrinsic factor and parietal cell antibodies, tissue transglutaminase antibodies, liver function, and HIV and syphilis serology.
• MRI brain and orbits with contrast and spine MRI (B12 and copper myelopathy) to exclude compressive and demyelinating causes.
• Endoscopy and small bowel evaluation when malabsorption is suspected.

• Vitamin B12 deficiency with neurological or ocular involvement: hydroxocobalamin 1000 micrograms IM on alternate days for 2 weeks (or 6 loading doses over 2 weeks), then 1000 micrograms every 2 to 3 months for life in pernicious anaemia or malabsorption (or oral cyanocobalamin 1000 to 2000 micrograms daily for dietary causes); never give folic acid alone before B12 is replaced.
• Folate deficiency: folic acid 5 mg daily for 4 months, after B12 has been replaced.
• Thiamine: thiamine 100 mg IV or orally three times daily for suspected deficiency, and in Wernicke's disease 500 mg IV three times daily for 3 days, then 250 mg daily for 5 days, then 100 mg orally daily; give thiamine before glucose in alcoholics.
• Copper deficiency: copper sulphate or gluconate 2 to 8 mg elemental copper daily orally (or 2 to 4 mg IV daily for 6 days if absorption is poor), stop zinc supplements, and monitor levels.
• General: balanced diet with high-protein foods, multivitamin, alcohol and tobacco cessation (reduces risk of recurrence), treatment of malabsorption (gluten-free diet in coeliac disease), and nutritional support after bariatric surgery (lifelong vitamin B12, iron, folate, calcium, copper).
• Monitor visual acuity, colour vision and fields every 1 to 3 months; recovery occurs within weeks to months if treated early; delayed treatment leaves permanent atrophy. Patients with irreversible loss need low-vision rehabilitation.
• Public health measures: screen at-risk groups (vegans, elderly, bariatric patients, people on metformin) and fortify food where deficiencies are common.