Nutritional deficiencies and metabolic toxicities cause a surprisingly large fraction of “unexplained” neurologic syndromes — peripheral neuropathy, myelopathy, encephalopathy, ataxia, and movement disorders. Many are reversible if recognized early but become permanent if untreated. This page covers the major nutritional and metabolic laboratory tests beyond B12 and folate: copper deficiency, vitamin E deficiency, thiamine, niacin, vitamin D, zinc, B6 (both deficiency and toxicity), heavy metals (lead, mercury, arsenic), and the role of methylmalonic acid and homocysteine. The unifying theme is that these tests should be considered in any patient with a neuropathy, myelopathy, or encephalopathy that lacks an obvious cause — especially in patients with bariatric surgery history, malabsorption, alcohol use disorder, restrictive diets, or industrial/environmental exposures.

Copper Deficiency

Clinical Syndrome

• Subacute combined degeneration-like syndrome: dorsal column + corticospinal tract dysfunction.
• Sensory ataxia, spasticity, hyperreflexia, Babinski.
• Indistinguishable from B12 deficiency myelopathy clinically.
• Sensorimotor peripheral neuropathy often coexisting.
• Hematologic clue: anemia, neutropenia, ringed sideroblasts on bone marrow biopsy.

Causes

• Bariatric surgery (Roux-en-Y gastric bypass) — copper absorbed in proximal small bowel.
• Zinc toxicity (excessive zinc supplementation, denture cream containing zinc).
• Malabsorption: celiac disease, IBD.
• Chronic enteral feeding with copper-deficient formula.
• Excessive iron supplementation.

Testing

• Serum copper level (low).
• Ceruloplasmin level (low — copper is bound to ceruloplasmin in serum).
• 24-hour urinary copper.
• Zinc level (often elevated when copper deficient).

Treatment

• Copper supplementation (oral or IV).
• Stop zinc supplementation if present.
• Hematologic abnormalities resolve; neurologic symptoms often only partially.

Vitamin E Deficiency

Clinical Syndrome

• Spinocerebellar syndrome: ataxia, dysarthria, areflexia, proprioceptive loss.
• Peripheral neuropathy (sensory-predominant).
• Pigmentary retinopathy.
• Mimics Friedreich ataxia.

Causes

• Fat malabsorption: cystic fibrosis, cholestatic liver disease, short bowel syndrome.
• Abetalipoproteinemia (rare genetic disorder).
• Ataxia with isolated vitamin E deficiency (AVED — TTPA gene mutation).

Testing

• Serum vitamin E (alpha-tocopherol) level.
• Include cholesterol level for ratio interpretation.

Treatment

• Oral or parenteral vitamin E supplementation.
• Treatment of underlying malabsorption.

Thiamine (Vitamin B1) Deficiency

Clinical Syndromes

Wernicke Encephalopathy

• Classical triad: ophthalmoplegia (especially abducens palsy, nystagmus), ataxia, confusion.
• Triad complete in only ~30%; one or two features common.
• Often misdiagnosed — give thiamine empirically if any concern.

Korsakoff Syndrome

• Anterograde and retrograde amnesia.
• Confabulation.
• Develops in many untreated Wernicke patients.

Beriberi

• Dry beriberi: peripheral neuropathy.
• Wet beriberi: cardiac failure.

At-Risk Populations

• Alcohol use disorder (classical).
• Bariatric surgery patients.
• Hyperemesis gravidarum.
• Cancer with anorexia / cachexia.
• Severe malnutrition.
• Refeeding syndrome.
• Glucose administration without thiamine in at-risk patients can precipitate Wernicke encephalopathy.

Testing

• Whole-blood thiamine (or red cell thiamine) — turnaround often slow.
• Treat empirically if clinical concern; do not wait for results.

Treatment

• Thiamine 500 mg IV TID × 3 days, then 250 mg IV/IM daily × 5 days, then oral maintenance.
• Give thiamine BEFORE glucose in at-risk patients.
• Delayed treatment may leave permanent Korsakoff syndrome.

Niacin (Vitamin B3) Deficiency — Pellagra

Classical Triad (the 3 D’s)

• Dermatitis (sun-exposed areas).
• Diarrhea.
• Dementia / encephalopathy.
• Plus death if untreated (the 4th D).

Causes

• Malnutrition (especially corn-based diets without nixtamalization).
• Alcohol use disorder.
• Isoniazid use (pyridoxine and niacin antagonism).
• Hartnup disease (tryptophan absorption defect).
• Carcinoid syndrome (tryptophan diverted to serotonin).

Treatment

• Niacin or nicotinamide supplementation.

Vitamin B6 (Pyridoxine)

Deficiency

• Peripheral neuropathy.
• Seizures, especially neonatal (pyridoxine-dependent epilepsy).
• Causes: isoniazid (without pyridoxine prophylaxis), penicillamine, alcohol use.

Toxicity

• High-dose B6 (typically >100 mg/day chronically) can cause sensory neuropathy and sensory ataxia.
• Patients self-medicating with high-dose B6 for various reasons.
• Reversible after discontinuation but recovery slow.

Testing

• Serum pyridoxal-5-phosphate (PLP) — active form.

Vitamin D

Neurologic Associations

• Proximal myopathy (low vitamin D).
• Increased fall risk in elderly.
• Associated with MS risk and disease activity (correlational, not causal proven).
• Headache, depression (associations).

Testing

• 25-hydroxyvitamin D (25(OH)D).
• Deficiency: <20 ng/mL; insufficient: 20-30; sufficient: >30.

Treatment

• Oral vitamin D3 supplementation.

Zinc

• Deficiency: rarely causes neurologic syndromes alone.
• Excess zinc → copper deficiency myelopathy (see copper section).
• Common cause: denture cream containing zinc; chronic excessive zinc supplementation.

Heavy Metal Toxicity

Lead

Adult Lead Toxicity

• Peripheral motor neuropathy (classically wrist drop, foot drop from preferential motor axon involvement).
• Encephalopathy (high levels).
• Abdominal pain, constipation.
• Hypertension, renal dysfunction.
• Hematologic: basophilic stippling, anemia.

Pediatric Lead Toxicity

• Cognitive impairment, behavioral problems.
• Lead encephalopathy at high levels.

Exposure Sources

• Old paint (pre-1978 in US), water from old pipes.
• Industrial: smelting, battery work, painting.
• Traditional remedies, cosmetics from some regions.

Testing

• Blood lead level.
• Urine lead (24-hour) — if exposure-mediated chelation suspected.

Treatment

• Source removal.
• Chelation: succimer (DMSA), EDTA, or dimercaprol depending on severity.

Mercury

Methylmercury (Organic — Most Important)

• Sensory ataxia, paresthesias, visual field constriction.
• Cognitive impairment, tremor.
• Sources: large predatory fish (tuna, swordfish, shark, king mackerel); traditional / artisanal gold mining.

Elemental Mercury Vapor

• Tremor, behavioral changes, psychiatric symptoms (“mad hatter”).
• Sources: industrial, thermometer breakage, dental amalgam (low concern).

Testing

• Blood mercury (recent exposure).
• Hair mercury (longer-term).
• 24-hour urine mercury.

Arsenic

Clinical Syndrome

• Sensorimotor peripheral neuropathy (painful, often Guillain-Barré-like presentation).
• Encephalopathy with high acute exposure.
• Skin: hyperpigmentation, hyperkeratosis (palms/soles), Mees’ lines on nails.
• Gastrointestinal symptoms.

Exposure Sources

• Contaminated groundwater (Bangladesh, parts of US Southwest).
• Pesticides (older formulations).
• Industrial: smelting, semiconductor manufacture.
• Traditional remedies.

Testing

• 24-hour urinary arsenic (speciation distinguishes organic seafood-related from toxic inorganic).
• Hair, nails for chronic exposure.

Thallium

• Rare, often criminal/intentional.
• Painful sensorimotor neuropathy.
• Alopecia: hallmark (occurs 2-3 weeks after exposure).
• Mees’ lines.

Manganese

• Parkinsonism (often with dystonia, postural instability, atypical features for idiopathic PD).
• Exposure: welding, mining, ferromanganese smelting, hyperalimentation (TPN-related), chronic liver disease.
• MRI: T1 hyperintensity in basal ganglia (pallidum).

Methylmalonic Acid (MMA) and Homocysteine

Methylmalonic Acid

• Elevated in B12 deficiency (B12 cofactor needed for methylmalonyl-CoA → succinyl-CoA conversion).
• Most sensitive marker of B12 deficiency at tissue level.
• Useful when B12 level is borderline (200-400 pg/mL) and clinical suspicion exists.

Homocysteine

• Elevated in B12 OR folate deficiency.
• Also elevated in: MTHFR variants, chronic kidney disease, hypothyroidism, certain medications.
• Severe elevation (homocystinuria) causes mental retardation, lens dislocation, marfanoid habitus, vascular events.
• Moderate elevation associated with vascular disease risk.

Vitamin A

• Deficiency: night blindness; xerophthalmia.
• Toxicity: idiopathic intracranial hypertension (pseudotumor cerebri) — headache, papilledema, visual disturbance.
• High-dose supplementation, isotretinoin therapy.

Bariatric Surgery Patients — Special Considerations

Bariatric surgery patients require lifelong monitoring for multiple nutritional deficiencies:

• B12 (intrinsic factor / parietal cell loss).
• Folate.
• Iron.
• Copper.
• Thiamine.
• Vitamin D.
• Calcium.
• Protein (severe restriction).

Any neurologic symptom in a bariatric surgery patient warrants comprehensive nutritional workup.

Pitfalls and Pearls

• Copper deficiency myelopathy: indistinguishable from B12 deficiency clinically; check in any “B12-like” syndrome with normal B12.
• Bariatric surgery: lifelong risk for B12, copper, thiamine, folate, vitamin D deficiencies.
• Excessive zinc → copper deficiency: denture cream, supplements.
• Vitamin E deficiency: spinocerebellar syndrome mimicking Friedreich ataxia.
• Wernicke encephalopathy: ophthalmoplegia + ataxia + confusion (triad in 30%); treat empirically.
• Give thiamine BEFORE glucose in at-risk patients.
• Pellagra: dermatitis + diarrhea + dementia (3 D’s).
• B6 toxicity: sensory neuropathy from high-dose supplements.
• Lead: motor neuropathy (wrist drop, foot drop); encephalopathy; basophilic stippling.
• Mercury (methylmercury): sensory ataxia + visual field constriction; large predatory fish.
• Arsenic: GBS-like neuropathy + Mees’ lines + palmar/plantar hyperkeratosis.
• Thallium: painful neuropathy + alopecia.
• Manganese: parkinsonism + T1 hyperintensity in pallidum; welders, TPN, chronic liver disease.
• MMA: most sensitive for B12 deficiency at tissue level.
• Homocysteine: B12 or folate deficiency; MTHFR variants; vascular risk.
• Vitamin A toxicity: pseudotumor cerebri.
• Niacin deficiency: pellagra (3 D’s).
• Any unexplained neuropathy / myelopathy / encephalopathy: consider nutritional workup beyond just B12.

References

1. Kumar N. Copper deficiency myelopathy (human swayback). Mayo Clin Proc. 2006;81(10):1371-1384.
2. Sechi G, Serra A. Wernicke’s encephalopathy: new clinical settings and recent advances in diagnosis and management. Lancet Neurol. 2007;6(5):442-455.
3. Galvin R, Bråthen G, Ivashynka A, et al. EFNS guidelines for diagnosis, therapy and prevention of Wernicke encephalopathy. Eur J Neurol. 2010;17(12):1408-1418.
4. Schaumburg H, Kaplan J, Windebank A, et al. Sensory neuropathy from pyridoxine abuse. A new megavitamin syndrome. N Engl J Med. 1983;309(8):445-448.
5. Patrick L. Mercury toxicity and antioxidants: Part 1: role of glutathione and alpha-lipoic acid in the treatment of mercury toxicity. Altern Med Rev. 2002;7(6):456-471.
6. Kumar N. Neurologic presentations of nutritional deficiencies. Neurol Clin. 2010;28(1):107-170.