Neurotoxicology & Nutritional Deficiencies
Neurotoxicology & Nutritional Deficiencies
What Do You Need to Know?
- Thiamine (B1) deficiency → Wernicke encephalopathy (confusion, ophthalmoplegia, ataxia) — always give thiamine before glucose; mamillary body necrosis is pathognomonic
- B12 deficiency → subacute combined degeneration (posterior columns + lateral corticospinal tracts); elevated methylmalonic acid + homocysteine; nitrous oxide inactivates B12
- B6 excess causes pure sensory neuropathy/ganglionopathy — one of the few vitamins where toxicity (not deficiency) causes neurologic disease
- Copper deficiency mimics B12 deficiency (myelopathy + neuropathy + cytopenias) — check copper in any "B12-like" presentation with normal B12; zinc excess is a common cause
- Lead toxicity: encephalopathy in children, wrist/foot drop in adults, basophilic stippling on blood smear, treat with EDTA/succimer/dimercaprol
- Arsenic: painful sensorimotor neuropathy + Mees lines (transverse white nail lines) + GI symptoms; mercury (organic): Minamata disease with visual field constriction and ataxia
- Manganese toxicity: parkinsonism affecting globus pallidus (not caudate) with T1 hyperintensity on MRI — seen in welders and miners
- Carbon monoxide: bilateral globus pallidus necrosis; delayed neuropsychiatric syndrome days to weeks after exposure
🚩 Don’t Miss — Test-Day Priorities
- Thiamine before glucose: in any suspected Wernicke (alcoholic, bariatric, hyperemesis, refeeding), give IV thiamine FIRST — glucose without thiamine precipitates/worsens Wernicke encephalopathy
- Wernicke triad: ophthalmoplegia (CN VI, conjugate gaze palsy, nystagmus) + ataxia + confusion; MRI shows mammillary body, medial thalamus & periaqueductal gray hyperintensity; Korsakoff = irreversible amnestic syndrome with confabulation
- B12 deficiency = SCD: posterior columns (vibration/proprioception loss) + lateral corticospinal tracts (spasticity, Babinski) ± sensorimotor neuropathy + megaloblastic anemia; ↑ methylmalonic acid AND homocysteine; treat with IM cobalamin
- Nitrous oxide (whip-its, dental): oxidizes cobalt → functional B12 deficiency → SCD-like myelopathy + ataxia; check B12, MMA, homocysteine in young patient with myelopathy
- B6 (pyridoxine) paradox: deficiency (INH, cycloserine) → seizures + neuropathy; TOXICITY (>200 mg/day from supplements) → pure sensory neuronopathy/ganglionopathy (DRG attack) — one of few vitamins where excess is the danger
- Copper deficiency mimics B12 SCD: after gastric bypass or zinc overload (denture cream, supplements); myelopathy + neuropathy + cytopenias with NORMAL B12 — check ceruloplasmin/copper, replace copper, stop zinc
- Lead in adults: wrist drop (radial motor neuropathy) + abdominal colic + basophilic stippling + microcytic anemia + Burton gum line; children get encephalopathy + cognitive decline; treat succimer (DMSA) or Ca-EDTA + dimercaprol (BAL) for severe
- Manganese parkinsonism: welders, miners, chronic liver disease; T1 hyperintensity in globus pallidus (NOT T2); poor levodopa response, early psychiatric & dystonia — distinguishes from idiopathic PD
- Methanol vs ethylene glycol: both cause anion-gap metabolic acidosis; METHANOL → bilateral putaminal necrosis + visual loss (blindness); ETHYLENE GLYCOL → calcium oxalate crystals + AKI; treat both with fomepizole ± dialysis
- Organophosphate poisoning: SLUDGE-M cholinergic crisis + miosis + fasciculations; treat ATROPINE + PRALIDOXIME (2-PAM); watch for intermediate syndrome (24–96 hr respiratory/proximal weakness) and OPIDN (delayed neuropathy 1–3 wk)
- Carbon monoxide: bilateral globus pallidus necrosis on MRI; delayed neuropsychiatric syndrome (parkinsonism, leukoencephalopathy) days to weeks after recovery; treat ALL suspected CO with 100% normobaric oxygen; reserve hyperbaric oxygen for selected severe/high-risk cases (LOC, neurologic deficits, cardiac ischemia, severe acidosis, very high COHb, or pregnancy per local protocol)
🔍 Buzzwords & Pathognomonic FindingsClinical / exposure · Diagnostic / imaging · Treatment / antidote
Clinical / exposure
- Ophthalmoplegia + ataxia + confusion in alcoholic / bariatric / hyperemesis → Wernicke encephalopathy (thiamine B1 deficiency)
- Amnesia + confabulation, irreversible → Korsakoff syndrome
- Posterior column + corticospinal myelopathy + megaloblastic anemia → B12 deficiency / subacute combined degeneration
- Whip-it / nitrous oxide / dental assistant with myelopathy → N₂O-induced functional B12 deficiency
- Dermatitis + diarrhea + dementia (Casal necklace) → Pellagra (niacin B3 deficiency) — also carcinoid, Hartnup, isoniazid
- Sensory ataxia + areflexia + retinitis pigmentosa in fat malabsorption → Vitamin E deficiency / AVED
- Wrist drop + abdominal colic + gum lead line in adult → Lead poisoning
- Contaminated fish, Minamata Bay, visual field constriction + ataxia → Methylmercury (organic mercury) poisoning
- “Mad Hatter” tremor + erethism + gingivitis → Inorganic mercury poisoning
- Welder / miner with parkinsonism + “cock-walk” gait → Manganese toxicity
- Painful sensorimotor neuropathy + alopecia + Mees lines → Thallium poisoning
- Glue / paint sniffing → cerebellar ataxia + cognitive decline → Toluene leukoencephalopathy
- Bootleg liquor / windshield fluid → blindness + acidosis → Methanol poisoning
- Antifreeze ingestion + anion-gap acidosis + AKI → Ethylene glycol poisoning
- Farmer / pesticide exposure with SLUDGE-M + miosis + fasciculations → Organophosphate poisoning
- “Chasing the dragon” heroin inhalation → Spongiform leukoencephalopathy
- Rave / MDMA + hyperthermia + clonus → Serotonin syndrome / MDMA toxicity
- Red wine drinker with corpus callosum demyelination → Marchiafava-Bignami disease
- Cassava (konzo) / grass pea (lathyrism) → Tropical spastic paraparesis
Diagnostic / imaging
- Mammillary body + medial thalamus + periaqueductal gray T2/FLAIR hyperintensity → Wernicke encephalopathy
- ↑ Methylmalonic acid AND ↑ homocysteine → B12 deficiency (folate gives high homocysteine with NORMAL MMA)
- Posterior + lateral column T2 hyperintensity (“inverted V” sign) → Subacute combined degeneration (B12 or copper)
- Bilateral putaminal necrosis + visual loss → Methanol toxicity
- Bilateral globus pallidus necrosis → Carbon monoxide poisoning
- T1 hyperintensity in globus pallidus (no T2 change) → Manganese deposition (also chronic liver failure)
- Basophilic stippling on peripheral smear + microcytic anemia + elevated ZPP → Lead poisoning
- Mees lines (transverse white lines on nails) → Arsenic or thallium poisoning
- Calcium oxalate crystals in urine + anion-gap acidosis → Ethylene glycol
- Red ↑ RBC transketolase activity after thiamine → Thiamine deficiency
- Splenium / corpus callosum demyelination in alcoholic → Marchiafava-Bignami
- Symmetric central pontine T2 hyperintensity after rapid Na correction → Central pontine myelinolysis (osmotic demyelination)
- Giant axonal swellings on nerve biopsy in glue sniffer → n-Hexane neuropathy
Treatment / antidote
- IV thiamine 500 mg TID BEFORE any glucose → Wernicke encephalopathy
- IM cyanocobalamin 1000 µg weekly ×4 then monthly → B12 deficiency / SCD / N₂O myelopathy
- Pyridoxine (B6) supplementation → INH-induced neuropathy / seizures, hydrazine, cycloserine
- Stop the supplement (B6 >200 mg/day) → Pyridoxine sensory neuronopathy
- Stop zinc + replace copper → Copper deficiency myeloneuropathy
- Nicotinamide → Pellagra
- Succimer (DMSA, oral) → Mild–moderate lead, arsenic, mercury
- Ca-EDTA ± dimercaprol (BAL) → Severe lead encephalopathy
- Dimercaprol (BAL) → Acute arsenic, inorganic mercury, severe lead
- Penicillamine → Inorganic mercury (also Wilson disease — not in this topic)
- Atropine + pralidoxime (2-PAM) → Organophosphate poisoning
- Fomepizole (or ethanol) + hemodialysis + folate → Methanol poisoning
- Fomepizole + hemodialysis + thiamine/pyridoxine → Ethylene glycol poisoning
- 100% normobaric oxygen for ALL suspected CO; hyperbaric oxygen (HBO) for SELECTED severe/high-risk cases (LOC, neuro deficits, cardiac ischemia, severe acidosis, very high COHb, pregnancy per protocol) → Carbon monoxide poisoning
- Folinic acid (leucovorin) rescue → Methotrexate neurotoxicity
- High-dose vitamin E replacement → AVED / abetalipoproteinemia
Heavy Metal Toxicity
Overview of Neurotoxic Heavy Metals
| Metal | Source / Exposure | Neurologic Features | Other Key Findings | Treatment |
|---|---|---|---|---|
| Lead | Old paint, contaminated water, batteries, occupational | Children: encephalopathy (irritability, seizures, cerebral edema). Adults: wrist drop (radial nerve), foot drop (peroneal nerve), peripheral neuropathy | Basophilic stippling on blood smear; lead lines on gums (Burton lines); abdominal colic; microcytic anemia; impaired heme synthesis | Succimer (oral, mild); EDTA (moderate-severe); dimercaprol + EDTA (encephalopathy) |
| Arsenic | Pesticides, contaminated groundwater, homicide attempts | Painful sensorimotor neuropathy (axonal, length-dependent); encephalopathy in acute poisoning | Mees lines (transverse white lines on nails); "rain drop" skin pigmentation; GI symptoms (watery diarrhea, garlic breath); QT prolongation | Dimercaprol (acute); succimer (chronic) |
| Mercury (organic) | Contaminated fish (methylmercury), Minamata Bay | Minamata disease: visual field constriction (calcarine cortex), cerebellar ataxia, sensory neuropathy, paresthesias, hearing loss | Constriction of visual fields is characteristic; affects calcarine cortex and granule cell layer of cerebellum; teratogenic | Succimer; selenium supplementation |
| Mercury (inorganic) | Industrial exposure, thermometers, dental amalgam (minimal) | Intention tremor; erethism (psychiatric — irritability, shyness, insomnia); peripheral neuropathy | "Mad Hatter" syndrome (hat-making industry); gingivitis, salivation | Succimer; penicillamine |
| Manganese | Miners, welders, smelters; chronic liver disease (impaired hepatic clearance) | Parkinsonism — affects globus pallidus preferentially (spares caudate/putamen); "cock-walk" gait (strutting on toes); psychiatric symptoms early | T1 hyperintensity in basal ganglia (especially globus pallidus) on MRI; poor response to levodopa (unlike idiopathic PD) | Remove exposure; chelation with EDTA (limited evidence) |
| Thallium | Rodenticides, homicidal poisoning | Painful sensorimotor neuropathy (ascending); encephalopathy | Alopecia (hallmark; 2–3 weeks after exposure); Mees lines (like arsenic); GI symptoms | Prussian blue (potassium ferric hexacyanoferrate); activated charcoal |
| Aluminum | Dialysis patients (contaminated dialysate), antacids | Dialysis dementia: progressive speech difficulty (stuttering/dysarthria), myoclonus, seizures, cognitive decline | Osteomalacia; microcytic anemia; EEG shows generalized slowing with bursts | Deferoxamine; remove aluminum source |
| Bismuth | Pepto-Bismol (bismuth subsalicylate), other bismuth compounds | Myoclonus (prominent); encephalopathy; ataxia; seizures | Reversible with discontinuation; dark discoloration of tongue/stool | Discontinue bismuth-containing products |
| Cadmium | Battery/electroplating/welding factories; cigarette smoke (major source); contaminated rice/seafood | Length-dependent sensorimotor polyneuropathy; anosmia (olfactory nerve direct injury); parkinsonism and progressive cognitive decline with chronic exposure | Long half-life (10–30 years); itai-itai disease (osteomalacia + renal tubulopathy); BBB permeability increased; no reliable chelator | Remove exposure; supportive care |
Board Pearl
Manganese parkinsonism vs. idiopathic Parkinson disease: Manganese affects the globus pallidus (T1-bright on MRI), not the substantia nigra. It causes a distinctive "cock-walk" gait and does not respond to levodopa. Idiopathic PD affects the substantia nigra pars compacta, causes resting tremor (not action tremor), and responds to levodopa. Look for exposure history (welder, miner) and T1 basal ganglia hyperintensity.
Board Pearl
Mees lines (transverse white bands on nails) are seen in both arsenic and thallium poisoning. To differentiate: thallium causes prominent alopecia (often the most striking finding), while arsenic causes characteristic "rain drop" skin pigmentation. Both cause painful neuropathy. Lead causes Burton lines (blue-black line on gums), not Mees lines.
Organic Solvents & Environmental Toxins
Key Organic and Environmental Neurotoxins
| Toxin | Source / Exposure | Neurologic Features | Key Findings | Treatment |
|---|---|---|---|---|
| Carbon monoxide | Fires, car exhaust, gas heaters, charcoal burning | Acute: headache, confusion, coma, seizures. Delayed neuropsychiatric syndrome (days–weeks later): cognitive decline, parkinsonism, personality changes | Cherry-red skin (classic but unreliable); bilateral globus pallidus necrosis on MRI; carboxyhemoglobin level elevated | 100% normobaric O2 for ALL suspected CO poisoning; hyperbaric oxygen for SELECTED severe/high-risk cases (loss of consciousness, neurologic deficits, cardiac ischemia, severe acidosis, very high COHb, or pregnancy per local protocol) |
| Organophosphates | Pesticides, nerve agents (sarin, VX) | Cholinergic crisis (DUMBELS: Diarrhea, Urination, Miosis, Bradycardia, Emesis, Lacrimation, Salivation). Intermediate syndrome: proximal weakness, respiratory failure (days 1–4). OPIDN: delayed distal neuropathy (weeks later) | Acetylcholinesterase inhibition (irreversible); depressed RBC cholinesterase; fasciculations, muscle cramps | Atropine (muscarinic blockade) + pralidoxime (reactivates AChE if given within 24–48 hours, before "aging") |
| n-Hexane | Glue sniffing, solvents, shoe manufacturing | Sensorimotor peripheral neuropathy (distal → proximal) | Giant axonal swellings on nerve biopsy (accumulation of neurofilaments); metabolite 2,5-hexanedione is the toxic agent | Remove exposure; slow recovery |
| Toluene | Paint thinners, spray paint huffing | Cerebellar ataxia, cognitive decline, dementia, anosmia, hearing loss | White matter changes on MRI; renal tubular acidosis; hypokalemia | Remove exposure; supportive care |
| Methanol | Moonshine, windshield wiper fluid, industrial solvent | Visual loss (optic nerve and retinal toxicity); blindness may be permanent; headache, confusion | Putaminal necrosis (bilateral) on MRI; high-anion-gap metabolic acidosis; formate is the toxic metabolite | Fomepizole (preferred) or ethanol (competitive alcohol dehydrogenase inhibition); dialysis for severe cases |
| Ethylene glycol | Antifreeze | Altered mental status, seizures, cranial neuropathies (especially CN VII) | Calcium oxalate crystals in urine; renal failure; high-anion-gap metabolic acidosis; osmolar gap | Fomepizole or ethanol; dialysis |
Board Pearl
Organophosphate toxicity has three phases: (1) Acute cholinergic crisis (DUMBELS) — treat with atropine + pralidoxime; (2) Intermediate syndrome (1–4 days later) — proximal and respiratory muscle weakness, may need ventilation; (3) OPIDN (organophosphate-induced delayed neuropathy, 2–4 weeks later) — distal axonal neuropathy affecting legs > arms. Pralidoxime must be given early before the AChE-organophosphate complex "ages" (becomes irreversible).
Clinical Pearl
Carbon monoxide poisoning classically causes bilateral globus pallidus necrosis on MRI. The delayed neuropsychiatric syndrome occurs days to weeks after apparent recovery and includes cognitive decline, personality changes, parkinsonism, and incontinence. This delayed syndrome occurs in up to 40% of severely poisoned patients. All suspected CO poisoning warrants 100% normobaric oxygen; hyperbaric oxygen is reserved for selected severe/high-risk cases (loss of consciousness, neurologic deficits, cardiac ischemia, severe acidosis, very high COHb, or pregnancy per local protocol). Evidence for HBO benefit is debated.
Marine Sodium-Channel Neurotoxins
| Toxin | Source | Mechanism | Clinical Features |
|---|---|---|---|
| Ciguatoxin | Large reef fish that bioaccumulate dinoflagellate toxins (grouper, snapper, barracuda, amberjack) | Opens voltage-gated Nav channels | GI prodrome (hours) → paresthesias, perioral tingling, cold allodynia / hot-cold temperature reversal, dental pain; bradycardia/hypotension; symptoms persist weeks-months |
| Tetrodotoxin (TTX) | Pufferfish (fugu), blue-ringed octopus | Blocks Nav channels | Rapid onset (15–60 min); ascending paralysis, respiratory failure, hypotension; sensory phenomena minimal; supportive care |
| Saxitoxin | "Red tide" dinoflagellates → bivalves (paralytic shellfish poisoning) | Blocks Nav channels | Same mechanism / phenotype as TTX (paralysis without sensory features) |
| Maitotoxin | Same dinoflagellate group as ciguatoxin | Activates voltage-dependent Ca2+ channels | Bradycardia, hypotension, profound weakness |
| ω-Conotoxin (cone-snail) | Cone-snail venom | Blocks N-type voltage-gated Ca2+ channels | Therapeutic analog = ziconotide (intrathecal for refractory neuropathic pain) |
💎 Board Pearl — Ciguatera
Cold allodynia / temperature reversal after eating grilled grouper or barracuda = ciguatera fish poisoning. Mechanism: ciguatoxin opens Nav channels (vs. tetrodotoxin/saxitoxin, which block them — the paralytic shellfish/pufferfish pair). Mannitol IV may shorten acute symptoms; avoid ethanol and fish for months.
Staple-Food Neurotoxins — Konzo & Lathyrism
- Konzo — abrupt-onset, permanent, symmetric spastic paraparesis in children and women of childbearing age during food shortages in sub-Saharan Africa. Mechanism: chronic dietary cyanogenic glycosides from under-processed bitter cassava + sulfur-deficient diet. Sensation, bowel/bladder, and cognition are preserved.
- Lathyrism (neurolathyrism) — gradual-onset progressive spastic paraparesis in men consuming the grass pea (Lathyrus sativus) as a famine staple in South Asia / Ethiopia. Mechanism: β-ODAP (oxalyldiaminopropionic acid), a glutamate analog → AMPA-receptor excitotoxicity of corticospinal Betz cells.
- Both spare cognition and bowel/bladder; both are public-health/global-neurology vignettes.
Drug-Induced Neurotoxicity
Chemotherapy Agents
| Drug | Neurotoxicity | Key Details |
|---|---|---|
| Vincristine | Peripheral neuropathy (dose-limiting toxicity) | Sensorimotor neuropathy (distal → proximal); loss of ankle reflexes earliest sign; autonomic neuropathy (constipation, ileus); avoid in Charcot-Marie-Tooth patients |
| Cisplatin | Sensory neuropathy; ototoxicity | Large-fiber sensory ganglionopathy; high-frequency hearing loss; neuropathy may worsen after stopping treatment ("coasting") |
| Methotrexate | Leukoencephalopathy | Acute (aseptic meningitis with intrathecal use); subacute (stroke-like episodes); chronic (progressive leukoencephalopathy, white matter changes on MRI); risk increased with radiation |
| 5-Fluorouracil (5-FU) | Cerebellar toxicity | Acute cerebellar syndrome (ataxia, dysarthria, nystagmus); usually reversible; risk higher with DPD deficiency |
| Ifosfamide | Encephalopathy | Acute confusion, somnolence, seizures, cerebellar dysfunction; treated with methylene blue |
| Oxaliplatin | Acute cold-induced dysesthesias; chronic sensory neuropathy | Acute cold-induced dysesthesias (transient channelopathy — perioral and acral paresthesias triggered by cold) + chronic cumulative sensory neuropathy (length-dependent); used in colorectal cancer regimens (FOLFOX) |
| Paclitaxel | Sensory neuropathy | Cumulative dose-dependent sensory neuropathy; microtubule stabilizer; "stocking-glove" distribution; may worsen after discontinuation ("coasting") |
| Bortezomib | Painful sensory neuropathy | Painful distal sensory neuropathy (proteasome inhibitor); used in multiple myeloma; subcutaneous administration reduces neurotoxicity vs IV |
| Brentuximab | Sensorimotor neuropathy | Sensorimotor neuropathy (anti-CD30 antibody-drug conjugate, MMAE payload disrupts microtubules); used in Hodgkin lymphoma and anaplastic large-cell lymphoma |
Antibiotics & Other Medications
| Drug | Neurotoxicity | Key Details |
|---|---|---|
| Aminoglycosides | Ototoxicity; NMJ blockade | Predominantly vestibulotoxic (streptomycin, gentamicin) vs predominantly cochleotoxic (neomycin, kanamycin, amikacin, tobramycin) — overlap exists; can unmask or worsen myasthenia gravis; avoid in NMJ disorders |
| Isoniazid (INH) | Peripheral neuropathy (B6-deficiency mediated) | Inhibits pyridoxine (B6) metabolism → neuropathy; prevent with B6 supplementation; overdose causes refractory seizures treated with IV pyridoxine |
| Metronidazole | Neuropathy; cerebellar toxicity; optic neuropathy; encephalopathy | With prolonged use (>4 weeks cumulative); T2/FLAIR hyperintensity in dentate nuclei on MRI; peripheral and optic neuropathy and encephalopathy also reported; usually reversible with discontinuation |
| Cefepime / Imipenem | Encephalopathy with myoclonus; nonconvulsive status epilepticus | Encephalopathy with myoclonus, nonconvulsive status epilepticus — especially in renal impairment; cefepime > imipenem; EEG may show generalized periodic discharges or NCSE; resolves with discontinuation |
| Linezolid | Serotonin syndrome; optic and peripheral neuropathy; lactic acidosis | Serotonin syndrome with concurrent SSRI/MAOI (linezolid is a weak MAOI); optic and peripheral neuropathy with prolonged use (>28 days); also lactic acidosis (mitochondrial inhibition) |
| Amiodarone | Peripheral neuropathy; optic neuropathy | Mixed demyelinating and axonal sensorimotor neuropathy; optic disc edema; also causes thyroid dysfunction and pulmonary fibrosis |
| Statins | Myopathy | Myalgia → myopathy → rhabdomyolysis (rare); elevated CK; risk increased with concomitant fibrates or CYP3A4 inhibitors; immune-mediated necrotizing myopathy (anti-HMGCR antibodies) can persist after discontinuation |
| Cyclosporine / Tacrolimus | PRES (posterior reversible encephalopathy syndrome) | Headache, seizures, visual changes, confusion; bilateral parieto-occipital white matter edema on MRI; treat by dose reduction or discontinuation |
| Lithium | Tremor; cerebellar toxicity | Fine postural tremor at therapeutic levels; cerebellar ataxia, nystagmus, and irreversible cerebellar damage with toxicity; narrow therapeutic window; diabetes insipidus |
Antiseizure Medications
| Drug | Neurotoxicity | Key Details |
|---|---|---|
| Phenytoin | Cerebellar atrophy; nystagmus | Chronic use → irreversible cerebellar atrophy (Purkinje cell loss); acute toxicity: nystagmus (first sign), ataxia, confusion; gingival hyperplasia, hirsutism |
| Carbamazepine | Hyponatremia (SIADH); diplopia; ataxia | Dose-dependent diplopia and dizziness; SIADH most common cause of hyponatremia with AEDs; SJS/TEN risk (screen HLA-B*1502 in Southeast Asian patients) |
| Valproate | Tremor; hepatotoxicity; teratogenicity | Postural tremor (most common neurologic side effect); hepatic failure (especially children <2 on polytherapy); neural tube defects (highest teratogenic risk of all AEDs); contraindicated in known/suspected POLG-related disease (Alpers; fatal hepatotoxicity) — use caution and individualize choice in other mitochondrial disorders rather than treating all mitochondrial disease as an absolute contraindication; hyperammonemia; weight gain; alopecia |
Board Pearl
Metronidazole cerebellar toxicity is characterized by T2/FLAIR hyperintensity in the dentate nuclei on MRI. This is a classic imaging finding on boards. The toxicity typically occurs with prolonged cumulative use and is usually reversible with drug discontinuation. Other drugs causing cerebellar toxicity include phenytoin (irreversible Purkinje cell loss), lithium, and 5-FU.
Vitamin B1 (Thiamine) Deficiency
Wernicke Encephalopathy
- Classic triad: confusion (encephalopathy), ophthalmoplegia — nystagmus is the most common ocular sign; lateral rectus (CN VI) palsy and conjugate gaze palsies also seen, and gait ataxia
- The full triad is present in only ~10–16% of patients — most present with only 1–2 features; a high index of suspicion is essential
- Pathology: hemorrhagic necrosis of mamillary bodies (pathognomonic), periaqueductal gray matter, medial thalami, tectal plate, floor of 4th ventricle
- MRI: T2/FLAIR hyperintensity and diffusion restriction in mamillary bodies, periaqueductal gray, medial thalami
- Causes: alcoholism (most common), malnutrition, hyperemesis gravidarum, bariatric surgery, prolonged TPN without supplementation, refeeding syndrome
Critical Treatment Principle
- ALWAYS give thiamine BEFORE glucose — glucose metabolism requires thiamine (pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase are all thiamine-dependent)
- Administering glucose without thiamine can precipitate or worsen Wernicke encephalopathy by depleting remaining thiamine stores
- Give IV thiamine 500 mg TID for 2–3 days, then 250 mg IV/IM daily for 3–5 days (Royal College/EFNS guidance)
Korsakoff Syndrome
- Chronic sequela of untreated/undertreated Wernicke encephalopathy
- Anterograde amnesia (inability to form new memories) > retrograde amnesia
- Confabulation (fabrication of memories without intent to deceive) is characteristic
- Due to damage to mamillary bodies + anterior and dorsomedial thalamic nuclei + mammillothalamic tract (part of the Papez circuit); the anterior thalamic nuclei are arguably most important for the amnestic syndrome
- Usually irreversible once established; only ~20% recover significantly with treatment
Beriberi
- Dry beriberi: symmetric peripheral neuropathy (distal sensorimotor, length-dependent)
- Wet beriberi: high-output cardiac failure + peripheral edema (due to impaired cardiac energy metabolism)
- Infantile beriberi: seen in breastfed infants of thiamine-deficient mothers; acute cardiac failure and lactic acidosis
Board Pearl
Thiamine before glucose is a cardinal rule in emergency medicine and a board favorite. Glucose metabolism depletes thiamine, and giving dextrose to a malnourished patient without first giving thiamine can precipitate fulminant Wernicke encephalopathy. The classic triad (confusion, ophthalmoplegia, ataxia) is present in only ~10–16% of cases — any one feature in a malnourished or alcoholic patient should prompt empiric thiamine treatment.
Vitamin B12 (Cobalamin) Deficiency
Subacute Combined Degeneration
- Posterior columns (vibration and proprioception loss, sensory ataxia, positive Romberg) + lateral corticospinal tracts (spasticity, hyperreflexia, Babinski sign) = combined upper and lower motor neuron findings
- The "subacute" onset over weeks to months distinguishes it from acute myelopathies
- MRI: T2 hyperintensity in dorsal columns of spinal cord — "inverted V" sign (or "inverted rabbit ears") on axial imaging
- Also causes: peripheral neuropathy, cognitive decline ("megaloblastic madness"), depression, psychosis
- Megaloblastic anemia (macrocytic) and hypersegmented neutrophils — but neurologic disease can occur without anemia
Causes
- Pernicious anemia (autoimmune destruction of gastric parietal cells → loss of intrinsic factor) — most common cause
- Strict vegan diet (no animal products); malabsorption (celiac disease, Crohn disease, ileal resection)
- Nitrous oxide (N2O) — oxidizes the cobalt of cobalamin, irreversibly inactivating methionine synthase, depleting the active cofactor pool (does not destroy all body B12 stores); recreational use ("whippets") or chronic occupational exposure
- Medications: metformin, proton pump inhibitors (long-term)
Laboratory Findings
- Low serum B12 (<200 pg/mL)
- Elevated methylmalonic acid (MMA) — most specific test for B12 deficiency
- Elevated homocysteine — also elevated in folate deficiency (less specific)
- Both MMA and homocysteine elevated = B12 deficiency; homocysteine alone elevated = could be folate deficiency
Clinical Pearl
Nitrous oxide is an increasingly recognized cause of B12 deficiency, especially in young patients presenting with myelopathy. N2O oxidizes the cobalt of cobalamin, irreversibly inactivating methionine synthase and depleting the active cofactor pool (it does not destroy all body B12 stores). Recreational use of "whippets" (N2O canisters) can cause acute subacute combined degeneration even in patients with previously normal B12 stores. Consider this diagnosis in a young patient with progressive myeloneuropathy and check B12, MMA, and homocysteine.
Vitamin B6 (Pyridoxine)
B6 Deficiency
- Neonates/infants: pyridoxine-dependent epilepsy — refractory neonatal seizures that respond to IV pyridoxine; due to ALDH7A1 (antiquitin) gene mutations
- B6 is a cofactor for glutamic acid decarboxylase (GAD), which converts glutamate → GABA; deficiency reduces GABA synthesis → lowered seizure threshold
- Peripheral neuropathy (sensorimotor, length-dependent) with chronic deficiency
- Isoniazid (INH) is the most common drug cause — inhibits pyridoxine metabolism; supplement B6 with INH to prevent neuropathy
- INH overdose → refractory seizures + metabolic acidosis; treated with gram-for-gram IV pyridoxine
B6 Excess (Toxicity)
- Megadose supplementation (>200 mg/day, often in "health food" enthusiasts)
- Causes pure sensory neuropathy/sensory ganglionopathy (dorsal root ganglion involvement)
- Large-fiber modalities affected → sensory ataxia, loss of vibration and proprioception, areflexia
- One of very few vitamins where excess causes neurologic disease
- Usually improves after discontinuation, but recovery may be incomplete
Other Vitamin Deficiencies
Comparison Table
| Vitamin | Deficiency or Toxicity Syndrome | Key Neurologic Features | Board-Yield Points |
|---|---|---|---|
| Vitamin E (alpha-tocopherol) | Spinocerebellar ataxia | Ataxia, loss of proprioception/vibration, areflexia, retinitis pigmentosa — mimics Friedreich ataxia | Fat-soluble vitamin; seen in abetalipoproteinemia (Bassen-Kornzweig), fat malabsorption, cholestatic liver disease; acanthocytes on blood smear |
| Niacin (B3) | Pellagra | 3 Ds: Dermatitis (sun-exposed areas), Diarrhea, Dementia; 4th D = Death if untreated | Seen in alcoholism, carcinoid syndrome (tryptophan diverted to serotonin), Hartnup disease (tryptophan malabsorption), INH use |
| Folate (B9) | Neural tube defects; megaloblastic anemia | Does NOT cause subacute combined degeneration (unlike B12); elevated homocysteine but normal MMA (vs. B12 where both are elevated) | Neural tube defects in pregnancy; supplementation 0.4 mg/day (4 mg/day if prior NTD); megaloblastic anemia identical to B12 |
| Vitamin A (excess) | Pseudotumor cerebri | Idiopathic intracranial hypertension (IIH) — headache, papilledema, visual obscurations, CN VI palsy | Also caused by isotretinoin (Accutane), tetracyclines, excess vitamin A intake; teratogenic (avoid in pregnancy) |
| Vitamin D | Osteomalacia; proximal myopathy; hypocalcemia | Proximal muscle weakness/myopathy and osteomalacia (bone pain, waddling gait) are primary features; hypocalcemic tetany (Chvostek/Trousseau signs), paresthesias, seizures, and laryngospasm with severe deficiency | Seen in renal failure, malabsorption, lack of sun exposure; treat underlying cause + calcium + vitamin D replacement |
Board Pearl
Vitamin E deficiency mimics Friedreich ataxia clinically (spinocerebellar ataxia, posterior column dysfunction, areflexia). The key difference: vitamin E deficiency is treatable and reversible with supplementation, while Friedreich ataxia is a progressive genetic disorder (GAA repeat expansion in FXN gene). Always check vitamin E levels in a patient with a Friedreich-like presentation, especially with fat malabsorption history or acanthocytes on blood smear (abetalipoproteinemia).
Copper Deficiency
Clinical Presentation & Diagnosis
- Mimics B12 deficiency: myelopathy (posterior column + corticospinal tract involvement) + peripheral neuropathy + cytopenias (anemia, neutropenia)
- Can produce an identical picture to subacute combined degeneration with normal B12 levels
- Also causes optic neuropathy (rare)
Causes
- Zinc excess — most important cause; zinc induces metallothionein in enterocytes, which binds copper and prevents absorption
- Denture cream containing zinc — classic board-style vignette (excessive use of zinc-containing denture adhesive)
- Gastric bypass surgery, gastrectomy
- Malabsorption syndromes
- Excessive zinc supplementation
Diagnosis & Management
- Low serum copper and low ceruloplasmin; elevated zinc level (if zinc-induced)
- MRI may show dorsal column T2 hyperintensity (identical to B12 deficiency)
- Check copper levels in any patient with a "B12-like" myeloneuropathy and normal B12
- Treatment: oral copper supplementation (copper gluconate 2–8 mg/day); remove zinc source
Clinical Pearl
Copper deficiency from denture cream is a classic board vignette: an elderly patient with progressive myeloneuropathy, cytopenias, and normal B12 who uses excessive zinc-containing denture adhesive. Zinc induces metallothionein, which sequesters copper in enterocytes and prevents absorption. Always ask about denture cream use and check serum copper/zinc levels in patients with unexplained myeloneuropathy.
Alcohol-Related Neurologic Disease
Spectrum of Alcohol Neurotoxicity
| Condition | Mechanism | Key Features |
|---|---|---|
| Wernicke-Korsakoff | Thiamine (B1) deficiency | See B1 section above; mamillary body necrosis; confusion → ophthalmoplegia → ataxia → amnesia + confabulation |
| Alcoholic cerebellar degeneration | Direct toxicity + nutritional deficiency | Anterior vermis atrophy; predominantly gait ataxia (legs > arms); limb coordination relatively preserved; develops over weeks to months |
| Alcoholic peripheral neuropathy | Direct toxicity + B1/B12 deficiency | Length-dependent, painful, sensorimotor neuropathy; burning feet; distal sensory loss; reduced ankle reflexes |
| Marchiafava-Bignami disease | Corpus callosum demyelination/necrosis | Rare; acute confusion, seizures, interhemispheric disconnection; MRI shows corpus callosum demyelination/necrosis (especially body > genu/splenium) |
| Central pontine myelinolysis (osmotic demyelination) | Rapid correction of hyponatremia | "Locked-in"-like syndrome (quadriplegia, anarthria, preserved consciousness); can also occur extrapontine (basal ganglia, thalami); limit Na correction to ≤8 mEq/L per 24h in high-risk patients (alcoholism, malnutrition, hypokalemia, liver disease, chronic hyponatremia); ≤10 mEq/L in low-risk cases |
| Hepatic encephalopathy | Hyperammonemia, portosystemic shunting | Asterixis (negative myoclonus); confusion → coma; T1 hyperintensity in globus pallidus on MRI (manganese deposition from impaired hepatic clearance) |
| Fetal alcohol syndrome | Prenatal alcohol exposure | Smooth philtrum, thin vermilion border, short palpebral fissures; microcephaly; intellectual disability; cardiac defects; leading preventable cause of intellectual disability |
Board Pearl
Osmotic demyelination syndrome (central pontine myelinolysis) results from overly rapid correction of hyponatremia — limit sodium correction to ≤8 mEq/L per 24h in high-risk patients (alcoholism, malnutrition, hypokalemia, liver disease, chronic hyponatremia) and ≤10 mEq/L per 24h in low-risk cases. The classic presentation is a "locked-in"-like state (quadriplegia, anarthria, preserved vertical eye movements and consciousness). It is most common in alcoholics and malnourished patients but can occur in anyone. MRI shows pontine demyelination with sparing of ventrolateral pons and tegmental neurons. If overcorrection occurs, consider re-lowering sodium with DDAVP + D5W.
Radiation-Induced Neurotoxicity
Temporal Phases of Radiation Injury
| Phase | Timing | Features |
|---|---|---|
| Acute | During radiotherapy | Cerebral edema, headache, nausea, worsening focal deficits; responds to corticosteroids |
| Early-delayed | Weeks to months after RT | Somnolence syndrome (especially in children after whole-brain RT); transient demyelination; Lhermitte sign after spinal RT; usually self-limited |
| Late-delayed | >6 months to years | Radiation necrosis, leukoencephalopathy, cognitive decline (especially memory and executive function), secondary tumors (meningiomas, gliomas, sarcomas), vasculopathy (large-vessel stenosis, Moyamoya, cavernous malformations), endocrinopathy (hypopituitarism after sellar RT) |
SMART Syndrome
- Stroke-like Migraine Attacks after Radiation Therapy — rare late complication of cranial irradiation
- Recurrent episodes of migraine + transient focal neurologic deficits (aphasia, hemiparesis, visual disturbance) ± seizures
- MRI: unilateral cortical gyriform enhancement in the previously irradiated field; usually reversible
- Mostly self-resolving over days to weeks; treat with anticonvulsants and migraine therapy
Radiation Necrosis vs Tumor Progression
- Standard MRI cannot reliably distinguish radiation necrosis from recurrent tumor — both show enhancement and edema
- Helpful adjuncts: MR spectroscopy (necrosis: low Cho, low NAA, lipid-lactate peak; tumor: high Cho/NAA ratio), FDG-PET or amino acid PET (necrosis: hypometabolic; tumor: hypermetabolic), perfusion MRI (necrosis: low rCBV; tumor: high rCBV)
- Treatment of radiation necrosis: corticosteroids, bevacizumab (VEGF inhibitor — effective for radiation necrosis), hyperbaric oxygen, surgical resection in refractory cases
Board Pearl
SMART syndrome = Stroke-like Migraine Attacks after Radiation Therapy. Look for a patient with prior cranial RT who develops recurrent migraine-like episodes with reversible focal deficits ± seizures and unilateral cortical gyriform enhancement on MRI in the irradiated field. Bevacizumab is the most effective treatment for radiation necrosis itself.
Heat Stroke, Hypothermia & Metabolic Encephalopathies
Heat Stroke
- Hyperthermia >40°C plus CNS dysfunction (encephalopathy, seizures, coma)
- Classic (non-exertional, elderly, environmental) vs exertional (young athletes, military) heat stroke
- Cerebellar Purkinje cells are selectively vulnerable → chronic cerebellar ataxia is a known sequela
- Multiorgan failure: rhabdomyolysis, DIC, acute liver failure, AKI
- Treatment: rapid cooling (cold water immersion is fastest); supportive care; benzodiazepines for shivering
Hypothermia
- Core temperature <35°C; CNS depression progresses with falling temperature
- J wave (Osborn wave) on ECG — positive deflection at the J-point
- Coagulopathy, bradycardia, hypoventilation; pupils fixed and dilated when severe
- "Not dead until warm and dead" — resuscitation continued until rewarmed to ~32–35°C
- Risk of rewarming arrhythmias (ventricular fibrillation); rewarm gently in severe cases (ECMO/CPB for unstable)
Hypoglycemic Encephalopathy
- Selective neuronal vulnerability: hippocampal CA1, cortical layer III (and deeper laminae), dentate gyrus, striatum (caudate/putamen)
- Spares the cerebellum — unlike global anoxia (a useful distinguishing point)
- MRI: bilateral cortical, hippocampal, and basal ganglia DWI restriction; splenium of corpus callosum may be involved
- Prolonged severe hypoglycemia (<40 mg/dL) can produce permanent injury and coma
- Treat with IV dextrose; in malnourished/alcoholic patients give thiamine first
Anoxic-Ischemic Encephalopathy
- Selective neuronal vulnerability: hippocampus CA1, cerebellar Purkinje cells, cortical layers III/V, basal ganglia (striatum, globus pallidus)
- Watershed (border-zone) infarcts — ACA/MCA and MCA/PCA junctions — classic with prolonged hypotension
- Myoclonic status epilepticus within 24h of cardiac arrest is a poor prognostic sign
- Lance-Adams syndrome: chronic post-hypoxic action myoclonus in survivors
- Targeted temperature management 33–36°C for 24h post-arrest improves neurologic outcomes
- Prognostication tools: clinical exam at 72h, EEG (myoclonic SE, burst-suppression), MRI (diffuse cortical/basal ganglia DWI), SSEP (bilateral absent N20), NSE (>60 ng/mL)
Board Pearl
Hypoglycemia vs anoxia — cerebellum is the discriminator. Both injure hippocampus CA1, cortex, and basal ganglia. Anoxia damages cerebellar Purkinje cells; hypoglycemia spares the cerebellum. Heat stroke also selectively damages Purkinje cells — chronic cerebellar ataxia after recovery is a known sequela.
Neuroanatomical Toxin Pattern Reference
Selective Regional Vulnerability — Toxin/Insult Pattern Map
| Anatomic Region | Toxins / Insults That Selectively Injure It |
|---|---|
| Globus pallidus (necrosis) | Carbon monoxide, manganese, hypoglycemia, kernicterus |
| Putamen (necrosis) | Methanol, Wilson disease, mitochondrial disorders (e.g., Leigh) |
| Hippocampus (CA1) | Anoxia/ischemia, hypoglycemia, kainic acid, status epilepticus, transient global amnesia |
| Corpus callosum | Marchiafava-Bignami, MS, callosotomy/callosal disconnection, lymphoma |
| Cerebellar vermis | Chronic alcohol, phenytoin (chronic) |
| Cerebellar Purkinje cells | Anoxia, heat stroke, lithium toxicity, phenytoin |
| Mammillary bodies | Wernicke (thiamine deficiency) |
| Periaqueductal gray + medial thalami | Wernicke (thiamine deficiency) |
| Dentate nuclei | Metronidazole (T2/FLAIR hyperintensity) |
| Substantia nigra | MPTP, idiopathic Parkinson disease |
| Dorsal columns + lateral corticospinal tracts | B12 deficiency, copper deficiency, nitrous oxide (subacute combined degeneration) |
| Calcarine (occipital) cortex | Organic mercury (Minamata) |
| Optic nerve | Methanol, ethambutol, linezolid, amiodarone, metronidazole, B12 deficiency |
Board Pearl
Bilateral globus pallidus lesions on MRI? Think carbon monoxide, manganese, hypoglycemia, kernicterus. Bilateral putaminal necrosis? Think methanol, Wilson disease, mitochondrial (Leigh). Mammillary body + medial thalamus + periaqueductal gray? Wernicke. Dentate nucleus T2 hyperintensity? Metronidazole.
Quick Reference Table
Neurotoxicology & Nutritional Deficiencies — At a Glance
| Condition | Key Finding / Buzzword | Board-Yield Feature |
|---|---|---|
| Lead toxicity | Basophilic stippling; Burton lines | Wrist drop (adults); encephalopathy (children) |
| Arsenic toxicity | Mees lines; rain-drop pigmentation | Painful neuropathy + GI symptoms |
| Mercury (organic) | Minamata disease; calcarine cortex | Visual field constriction + ataxia |
| Manganese toxicity | T1 hyperintensity in globus pallidus | Parkinsonism (levodopa-unresponsive); cock-walk gait |
| Thallium toxicity | Alopecia; Mees lines | Painful neuropathy; Prussian blue treatment |
| Carbon monoxide | Bilateral GP necrosis on MRI | Delayed neuropsychiatric syndrome |
| Organophosphates | DUMBELS mnemonic | 3 phases: cholinergic → intermediate → OPIDN |
| Methanol | Putaminal necrosis; visual loss | Treat with fomepizole (blocks alcohol dehydrogenase) |
| B1 (thiamine) deficiency | Mamillary body necrosis | Thiamine before glucose; triad present in only ~10–16% |
| B12 deficiency | Inverted V sign on MRI | Elevated MMA + homocysteine; nitrous oxide inactivates B12 |
| B6 excess | Sensory ganglionopathy | Toxicity (not deficiency) causes neuropathy |
| Vitamin E deficiency | Friedreich ataxia mimic | Abetalipoproteinemia; treatable with supplementation |
| Niacin (B3) deficiency | Pellagra (3 Ds + Death) | Dermatitis, Diarrhea, Dementia |
| Copper deficiency | B12 mimic with normal B12 | Zinc excess (denture cream); check copper levels |
| Osmotic demyelination | Pontine myelinolysis | Rapid Na correction; limit ≤8 mEq/L per 24h (high-risk) / ≤10 mEq/L per 24h (low-risk) |
| Marchiafava-Bignami | Corpus callosum necrosis | Alcoholism; interhemispheric disconnection |
References
- Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 12th ed. McGraw-Hill; 2023.
- Aminoff MJ, Greenberg DA, Simon RP. Clinical Neurology. 11th ed. McGraw-Hill; 2021.
- Dobbs MR. Clinical Neurotoxicology: Syndromes, Substances, Environments. Saunders Elsevier; 2009.
- Kumar N. Nutritional neuropathies. Neurol Clin. 2007;25(1):209–255.
- Kumar N. Copper deficiency myelopathy (human swayback). Mayo Clin Proc. 2006;81(10):1371–1384.
- Sechi G, Serra A. Wernicke’s encephalopathy: new clinical settings and recent advances in diagnosis and management. Lancet Neurol. 2007;6(5):442–455.
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
- Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. Am Fam Physician. 2017;96(6):384–389.
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