Tox: Methanol & Ethylene Glycol (Toxic Alcohols) for MRCP Part 1
- Crack Medicine
- 8 hours ago
- 4 min read
TL;DR:
Tox: Methanol & Ethylene Glycol (Toxic Alcohols) is a high-yield toxicology topic in MRCP Part 1 because it combines acid-base interpretation, renal medicine, neurology, and emergency management. Methanol classically causes visual toxicity, while ethylene glycol leads to renal failure and calcium oxalate crystal deposition. Candidates should focus on toxic metabolites, high anion gap metabolic acidosis, osmolar gap interpretation, antidotes, and indications for haemodialysis.
Why toxic alcohols matter in MRCP Part 1
Toxic alcohol poisoning is a classic exam topic because it tests multiple disciplines simultaneously:
Biochemistry
Acid-base disorders
Renal medicine
Toxicology
Emergency management
Questions often present patients with confusion, severe metabolic acidosis, visual symptoms, or acute kidney injury. The challenge is recognising the pattern quickly and identifying the correct toxin or next management step.
Core concepts you must know
1. Methanol metabolism
Methanol itself is not the main toxic agent. Toxicity occurs after metabolism by alcohol dehydrogenase.
Metabolic pathway
Methanol → Formaldehyde → Formic acid
Why formic acid matters
Formic acid causes:
Optic nerve toxicity
Metabolic acidosis
CNS depression
High-yield clue
Visual symptoms strongly suggest methanol poisoning.
Patients may describe:
Blurred vision
“Snowstorm” vision
Central scotoma
Progressive blindness
2. Ethylene glycol metabolism
Ethylene glycol is commonly found in antifreeze.
Metabolic pathway
Ethylene glycol → Glycolic acid → Glyoxylic acid → Oxalic acid
Why oxalic acid matters
Oxalic acid combines with calcium to form calcium oxalate crystals, leading to:
Acute kidney injury
Tubular damage
Hypocalcaemia
Classic exam clue
Urinalysis showing calcium oxalate crystals is highly suggestive.
3. High anion gap metabolic acidosis
This is one of the most heavily tested concepts in MRCP Part 1 toxicology.
Typical pattern
Early poisoning
Raised osmolar gap
Mild acidosis
Later poisoning
Severe high anion gap metabolic acidosis
Reduced osmolar gap as alcohol is metabolised
Important exam trap
A normal osmolar gap does not exclude toxic alcohol poisoning.
By the time the patient presents, the parent alcohol may already have been converted into toxic acids.
4. Key laboratory findings
Feature | Methanol | Ethylene Glycol |
Main organ toxicity | Optic nerve | Kidneys |
Characteristic symptom | Visual disturbance | Flank pain/AKI |
Acid-base finding | High AG metabolic acidosis | High AG metabolic acidosis |
Osmolar gap | Elevated early | Elevated early |
Urinary crystals | Absent | Calcium oxalate |
Hypocalcaemia | Uncommon | Common |
5. Commonly tested investigations
Candidates should be comfortable interpreting:
Arterial blood gases
Anion gap
Osmolar gap
Serum bicarbonate
Serum osmolality
Urea & electrolytes
Additional findings
Methanol
Bilateral putaminal necrosis on CT/MRI
Visual impairment
Ethylene glycol
Calcium oxalate crystals
Acute kidney injury
Hypocalcaemia
For more acid-base revision, see related notes on metabolic acidosis in the <a href="https://www.crackmedicine.com/lectures notes section</a>.
Management essentials
Management questions are very common in MRCP Part 1.
Immediate priorities
ABC assessment
Correct acidosis
Prevent further toxic metabolism
Consider haemodialysis
6. Antidotes
Fomepizole
Mechanism
Fomepizole inhibits alcohol dehydrogenase.
Why it works
This prevents production of toxic metabolites.
High-yield point
Fomepizole is preferred over ethanol in modern practice.
7. Ethanol therapy
Ethanol can also be used because it competes for alcohol dehydrogenase.
Although older and less predictable, it remains a classic exam association.
8. Adjunctive treatments
Methanol poisoning
Folinic acid or folic acid
These help metabolise formic acid into non-toxic products.
Ethylene glycol poisoning
Thiamine
Pyridoxine
These support alternative non-toxic metabolic pathways.
9. Indications for haemodialysis
This is a favourite MRCP question area.
Dialysis indications include:
Severe metabolic acidosis
Visual symptoms
Renal failure
Significant electrolyte disturbance
Clinical deterioration
Very high toxic alcohol levels
Why dialysis is effective
Dialysis removes:
Parent alcohol
Toxic metabolites
It also rapidly corrects acidosis.
The 10 highest-yield facts to memorise
Methanol causes visual toxicity
Ethylene glycol causes renal toxicity
Both produce high anion gap metabolic acidosis
Osmolar gap rises early
Formic acid damages the optic nerve
Oxalic acid forms calcium oxalate crystals
Fomepizole inhibits alcohol dehydrogenase
Ethanol is an alternative antidote
Haemodialysis removes both toxins and metabolites
A normal osmolar gap does not exclude poisoning
Mini-case MCQ
A 39-year-old man presents with vomiting, confusion, blurred vision, and tachypnoea after drinking illicit alcohol. Blood gas analysis shows severe metabolic acidosis with a high anion gap. Serum osmolar gap is elevated.
Which toxin is the most likely cause?
A. IsopropanolB. EthanolC. MethanolD. Carbon monoxideE. Salicylate
Answer: C. Methanol
Explanation
Methanol poisoning classically presents with:
Visual disturbance
Raised osmolar gap
High anion gap metabolic acidosis
The toxic metabolite is formic acid, which causes optic nerve injury.
Practise more exam-style questions in the <a href=https://www.crackmedicine.com/mock-tests mock tests</a> section.
The 5 most tested subtopics
1. High anion gap metabolic acidosis
Always differentiate from:
Diabetic ketoacidosis
Lactic acidosis
Salicylate poisoning
Uraemia
2. Osmolar gap interpretation
Candidates frequently forget that the osmolar gap may normalise later in poisoning.
3. Visual symptoms in methanol poisoning
This is one of the strongest exam clues.
4. Calcium oxalate crystals
Strongly associated with ethylene glycol poisoning.
5. Fomepizole mechanism
You should know that it inhibits alcohol dehydrogenase.
Common pitfalls
Assuming a normal osmolar gap excludes toxic alcohol poisoning
Forgetting that metabolites cause the toxicity
Missing visual symptoms as a clue to methanol poisoning
Confusing calcium oxalate crystals with uric acid crystals
Delaying dialysis despite severe acidosis or organ failure
Practical MRCP Part 1 study checklist
Before the exam, ensure you can:
Differentiate methanol from ethylene glycol rapidly
Interpret osmolar and anion gaps
Recognise calcium oxalate crystals
Recall the mechanism of fomepizole
State indications for haemodialysis
Identify causes of high anion gap metabolic acidosis
Recognise visual symptoms of methanol poisoning
Identify hypocalcaemia in ethylene glycol poisoning
To consolidate revision, use the <a href=https://www.crackmedicine.com/lectures/lectures</a> alongside the <a href= bank</a>.https://www.crackmedicine.com/qbank
FAQs
What is the key difference between methanol and ethylene glycol poisoning?
Methanol primarily causes optic nerve toxicity and visual symptoms, while ethylene glycol mainly damages the kidneys through calcium oxalate crystal deposition.
Why is the osmolar gap elevated in toxic alcohol poisoning?
The parent alcohol molecules increase serum osmolality early in poisoning before being converted into acidic metabolites.
What is the preferred antidote for methanol poisoning?
Fomepizole is preferred because it inhibits alcohol dehydrogenase and prevents formation of toxic metabolites.
Why does ethylene glycol cause hypocalcaemia?
Oxalic acid binds calcium to form calcium oxalate crystals, reducing serum calcium levels.
When is haemodialysis required?
Haemodialysis is indicated in severe metabolic acidosis, renal failure, visual symptoms, electrolyte abnormalities, or very high toxic alcohol concentrations.
Ready to start?
Strengthen your preparation with structured revision via the MRCP Part 1 overview. Practise actively using the Free MRCP MCQs and simulate exam conditions with a Start a mock test.
For deeper understanding, combine this guide with lecture-based revision at:https://www.crackmedicine.com/lectures/
Sources
MRCP(UK) Examination: <a href="https://www.mrcpuk.org/mrcpuk-examinations/part-1">https://www.mrcpuk.org/mrcpuk-examinations/part-1</a>
National Poisons Information Service (NPIS): <a href="https://www.npis.org/">https://www.npis.org/</a>
NICE Clinical Knowledge Summaries: <a href="https://cks.nice.org.uk/">https://cks.nice.org.uk/</a>
British National Formulary: <a href="https://bnf.nice.org.uk/">https://bnf.nice.org.uk/</a>
Royal College of Physicians: <a href="https://www.rcplondon.ac.uk/">https://www.rcplondon.ac.uk/</a>
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