Ophthalmology – Unexplained High-Anion-Gap Metabolic Acidosis

What the Disorder Represents

High-anion-gap metabolic acidosis (HAGMA) is a metabolic acid-base disturbance caused by accumulation of acids whose accompanying anions are not routinely measured on the standard electrolyte panel.

The basic calculation is:

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

The key clinical question is not simply whether bicarbonate is low, but:

Why has an unmeasured acid accumulated?

This can represent a life-threatening emergency from:

• Lactic acidosis
• Ketoacidosis
• Renal failure
• Toxic alcohol poisoning
• Salicylate poisoning
• Other severe toxic or metabolic disorders

Important Modern Correction: Metabolic Acidosis Is Not Defined by Bicarbonate Alone

A serum bicarbonate below 24 mEq/L does not by itself prove metabolic acidosis.

True metabolic acidosis is characterized by:

• Reduced serum bicarbonate
• Appropriate acidemia or compensatory respiratory response

The blood pH may occasionally be normal when another simultaneous acid-base disorder is present.

A blood gas and clinical context are therefore important.

Why an Anion Gap Exists

Plasma must remain electrically neutral.

Routine chemistry measures only some ions:

Measured Cation

• Sodium

Measured Anions

• Chloride
• Bicarbonate

Important unmeasured anions include:

• Albumin
• Phosphate
• Sulfate
• Organic acids

The normal apparent gap largely reflects:

Negatively charged albumin.

What Is a Normal Anion Gap?

The normal range depends on:

• Laboratory methodology
• Electrolyte analyzer
• Albumin concentration

With modern assays, a typical reference range without potassium is approximately:

8–12 mEq/L

rather than the older universal cutoff of <16 mEq/L.

Always use the:

Local laboratory reference range.

Correcting the Gap for Albumin

Hypoalbuminemia can conceal a dangerous high-anion-gap acidosis.

A commonly used correction is:

Corrected AG = measured AG + 2.5 × (4 − serum albumin in g/dL)

For example, an apparently normal gap in a severely hypoalbuminemic ICU patient may actually represent:

Significant accumulation of unmeasured acid.

The Modern Differential: GOLD MARK

The older MUDPILES mnemonic is historically useful but includes obsolete causes such as phenformin and paraldehyde.

A more useful modern mnemonic is:

GOLD MARK

• G – Glycols: ethylene glycol, propylene glycol
• O – Oxoproline (5-oxoproline/pyroglutamic acidosis)
• L – L-lactic acidosis
• D – D-lactic acidosis
• M – Methanol
• A – Aspirin/salicylates
• R – Renal failure
• K – Ketoacidosis

This should be combined with the patient’s clinical circumstances rather than used mechanically.

The Most Common Cause: Lactic Acidosis

Elevated lactate is among the most common causes of HAGMA.

Lactate accumulation may result from:

• Shock
• Sepsis
• Severe hypoxemia
• Tissue ischemia
• Cardiac arrest
• Generalized seizures
• Extreme agitation
• Severe anemia
• Mesenteric or limb ischemia

Drug- and toxin-related causes include:

• Metformin in susceptible patients
• Cyanide
• Carbon monoxide
• Propofol infusion syndrome
• Some antiretroviral drugs
• Beta-adrenergic stimulation

Type A vs Type B Lactic Acidosis

Type A

Caused primarily by impaired tissue oxygen delivery or utilization, such as:

• Shock
• Hypoxemia
• Severe ischemia

Type B

Occurs without obvious systemic hypoperfusion and may result from:

• Drugs
• Liver dysfunction
• Malignancy
• Thiamine deficiency
• Mitochondrial dysfunction

This distinction can help guide investigation.

Ketoacidosis

Important causes include:

• Diabetic ketoacidosis (DKA)
• Alcoholic ketoacidosis
• Starvation ketoacidosis
• SGLT2 inhibitor–associated euglycemic DKA

The most useful ketone assay is:

Serum beta-hydroxybutyrate.

Urine ketone strips primarily detect acetoacetate and may substantially underestimate early DKA.

Euglycemic Ketoacidosis

Do not exclude DKA solely because the glucose is not markedly elevated.

SGLT2 inhibitors can produce significant ketoacidosis with:

• Normal
• Mildly elevated

blood glucose.

A high gap with unexplained nausea, abdominal symptoms, or tachypnea in such a patient should prompt:

Beta-hydroxybutyrate testing.

Renal Failure

Advanced kidney failure causes accumulation of:

• Sulfate
• Phosphate
• Organic acids

producing:

High-anion-gap metabolic acidosis.

Earlier chronic kidney disease may instead produce a predominantly:

Normal-anion-gap acidosis.

Methanol Poisoning

Methanol is particularly important in ophthalmology because its toxic metabolite:

Formic acid

can damage the:

• Retina
• Optic nerve
• CNS

Methanol poisoning classically causes:

High-anion-gap metabolic acidosis + visual toxicity.

Ocular Clues to Methanol Poisoning

Patients may report:

• Blurred vision
• “Snowfield” vision
• Photophobia
• Central visual loss
• Dyschromatopsia
• Complete blindness in severe poisoning

Examination may reveal:

• Reduced acuity
• RAPD if asymmetric
• Optic disc hyperemia or edema early
• Subsequent optic atrophy

Unexplained visual symptoms combined with severe metabolic acidosis should immediately raise concern for:

Methanol exposure.

Ethylene Glycol Poisoning

Ethylene glycol is metabolized to toxic acids including:

• Glycolic acid
• Oxalic acid

It can cause:

• Severe metabolic acidosis
• CNS depression
• Hypocalcemia
• Acute kidney injury

Calcium oxalate crystals may appear in urine but:

Their absence does not exclude poisoning.

Salicylate Poisoning

Salicylate toxicity classically produces a:

Mixed acid-base disorder.

Early:

Respiratory alkalosis

from direct respiratory-center stimulation.

Later:

High-anion-gap metabolic acidosis

from organic acid accumulation.

Therefore a near-normal pH can conceal severe toxicity because the two processes may offset each other.

Salicylate Clinical Clues

Important features include:

• Tinnitus
• Tachypnea
• Nausea/vomiting
• Diaphoresis
• Fever
• Agitation
• Delirium

Severe poisoning can cause:

• Pulmonary edema
• Seizures
• Coma

A serum salicylate concentration should be measured when the cause of HAGMA is unexplained.

Isoniazid

Isoniazid overdose can produce:

• Refractory seizures
• Lactic acidosis
• Coma

The specific antidote is:

Pyridoxine (vitamin B6).

Persistent seizures after suspected isoniazid ingestion are a major diagnostic clue.

Iron Poisoning

Severe iron toxicity can produce:

• Vomiting
• Hematemesis
• Abdominal pain
• Shock
• HAGMA
• Hepatic failure

A serum iron concentration and toxicology consultation are appropriate when suspected.

Acetaminophen and the Anion Gap

Two different mechanisms are relevant.

Massive Acute Overdose

May produce early:

Lactic acidosis

from mitochondrial dysfunction.

Repeated or Chronic Exposure in Susceptible Patients

Can cause:

5-oxoproline (pyroglutamic) acidosis

particularly with:

• Malnutrition
• Sepsis
• Renal dysfunction
• Chronic illness

This is the O in GOLD MARK.

Toluene

Toluene exposure may cause acidosis, especially in solvent abuse.

However, the classic disturbance is often:

Hyperchloremic normal-anion-gap metabolic acidosis

because hippurate is rapidly excreted with sodium and potassium.

An elevated gap may occur earlier or with severe exposure.

Carbon Monoxide and Cyanide

Both can produce:

Lactic acidosis through impaired cellular oxygen utilization.

Carbon Monoxide

May cause:

• Headache
• Confusion
• Syncope
• Chest pain

Pulse oximetry can be misleading.

Diagnosis relies on:

Carboxyhemoglobin measurement by co-oximetry.

Cyanide

Can cause:

• Abrupt cardiovascular collapse
• Severe lactic acidosis
• Altered mental status

A very high unexplained lactate in the appropriate exposure setting should raise concern.

Do Not Depend on Characteristic Odors

Older descriptions emphasize:

• Bitter almonds for cyanide
• Rotten eggs for hydrogen sulfide

These are unreliable because:

• Many individuals cannot detect them
• Odor may disappear rapidly
• Exposure itself may impair olfaction

Diagnosis should not depend on smell.

The Osmolal Gap

The osmolal gap is useful when toxic alcohol exposure is suspected.

It is:

Measured serum osmolality − calculated serum osmolality.

A commonly used calculation in conventional US units is:

Calculated osmolality ≈ 2 × Na + glucose/18 + BUN/2.8 + ethanol/4.6

Exact formulas vary.

Why Toxic Alcohols Produce an Osmolal Gap

Methanol and ethylene glycol themselves are:

Osmotically active parent alcohols.

Soon after ingestion:

• Osmolal gap rises
• Anion gap may still be normal

As metabolism proceeds:

• Parent alcohol concentration falls
• Osmolal gap may normalize
• Toxic organic acids accumulate
• Anion gap rises

Thus the two gaps can evolve in opposite directions.

A Normal Osmolal Gap Does Not Exclude Toxic Alcohol Poisoning

This is a critical principle.

A patient presenting late after methanol or ethylene glycol ingestion may have:

• Severe HAGMA
• Organ toxicity
• Normal or minimally increased osmolal gap

because the parent alcohol has already been converted to acidic metabolites.

First-Line Laboratory Evaluation

In unexplained HAGMA, useful initial tests generally include:

• Repeat serum electrolytes
• Glucose
• Creatinine and BUN
• Serum lactate
• Serum beta-hydroxybutyrate
• Blood gas
• Serum osmolality
• Salicylate level
• Acetaminophen level

Additional testing should be guided by the clinical context.

Venous vs Arterial Blood Gas

For most metabolic acid-base assessment:

Venous blood gas is usually adequate

for evaluating:

• pH
• Bicarbonate
• PCO₂ trend

Arterial sampling is more useful when precise assessment of:

Oxygenation

is needed.

Toxic Alcohol Levels

When available, directly measure:

• Methanol
• Ethylene glycol

But treatment should not be delayed while awaiting these levels when clinical suspicion is high.

Urinalysis

Useful findings may include:

• Ketones
• Glucose
• Hematuria
• Myoglobin
• Oxalate crystals

But urinary calcium oxalate crystals in ethylene glycol poisoning have:

Insufficient sensitivity to exclude poisoning when absent.

ECG

Obtain an ECG when overdose is possible.

It can reveal:

• QRS prolongation
• QT prolongation
• Dysrhythmias
• Ischemic changes

that may identify otherwise unsuspected cardiotoxic ingestion.

Assessing Respiratory Compensation

A patient with metabolic acidosis should compensate by lowering PCO₂.

Expected PCO₂ can be estimated using:

Winter’s formula:

Expected PCO₂ = 1.5 × HCO₃⁻ + 8 ± 2

If measured PCO₂ is:

• Higher than expected → additional respiratory acidosis
• Lower than expected → additional respiratory alkalosis

This is especially important in poisoned patients.

Why Respiratory Compensation Matters

A severely acidotic patient may be sustaining life by maintaining:

Very high minute ventilation.

Loss of that compensation during:

• Sedation
• Intubation
• Mechanical ventilation

can cause a sudden rise in PCO₂ and a catastrophic fall in pH.

This is especially dangerous in:

Salicylate poisoning.

Delta Gap and Mixed Metabolic Disorders

A high anion gap does not exclude a second metabolic disorder.

Comparing the increase in anion gap with the fall in bicarbonate can identify:

• Concurrent normal-gap acidosis
• Concurrent metabolic alkalosis

A commonly used concept is the:

Delta ratio.

This is useful in complicated ICU or toxicology cases but should be interpreted alongside the clinical picture.

Initial Management Priorities

Management begins with:

1. Airway and breathing when necessary
2. Circulatory support
3. Correction of hypoxemia
4. Identification of the acid source
5. Specific antidotal therapy when indicated
6. Serial reassessment

The underlying cause must be treated rather than simply correcting the laboratory bicarbonate value.

Important Modern Correction: The “Coma Cocktail” Is Obsolete

Older protocols routinely gave:

• Oxygen
• Thiamine
• Glucose
• Naloxone

to any patient with altered mental status.

Modern therapy is targeted.

Use:

• Glucose for documented or strongly suspected hypoglycemia
• Naloxone for suspected opioid-induced respiratory depression
• Thiamine in patients at risk of deficiency
• Oxygen for hypoxemia or carbon monoxide poisoning

Do not delay glucose in a hypoglycemic patient while waiting to administer thiamine.

Fomepizole for Toxic Alcohol Poisoning

Fomepizole inhibits alcohol dehydrogenase and prevents conversion of:

• Methanol → formic acid
• Ethylene glycol → glycolic/oxalic acids

It is the preferred antidote when toxic alcohol poisoning is suspected.

Do Not Wait for Confirmation

If there is a credible toxic alcohol exposure plus findings such as:

• Unexplained HAGMA
• Elevated osmolal gap
• Visual symptoms
• Acute kidney injury

Start fomepizole while confirmatory testing is pending.

Delaying treatment can cause irreversible:

• Blindness
• Renal failure
• Death

Ethanol as an Antidote

Ethanol also competes for alcohol dehydrogenase.

It remains an alternative when:

Fomepizole is unavailable.

However, fomepizole is preferred because ethanol is harder to dose and causes:

• Intoxication
• Hypoglycemia
• CNS depression
• Monitoring difficulties

Hemodialysis for Toxic Alcohols

Hemodialysis rapidly removes:

• Parent toxic alcohol
• Toxic metabolites

and corrects severe acidosis.

It should be considered in methanol or ethylene glycol poisoning with features such as:

• Severe metabolic acidosis
• Significant end-organ toxicity
• Visual toxicity from methanol
• Acute kidney injury from ethylene glycol
• Very high toxic alcohol concentration
• Clinical deterioration despite antidote

Exact thresholds depend on current toxicology protocols.

Folate Therapy in Methanol Poisoning

Folinic acid or folic acid may be administered because it facilitates metabolism of:

Formate to nontoxic products.

It is adjunctive to:

• Fomepizole
• Acidosis management
• Dialysis when indicated

Pyridoxine and Thiamine in Ethylene Glycol

Pyridoxine and thiamine are sometimes given as adjuncts to promote metabolism toward:

Less toxic metabolites.

They do not replace:

• Fomepizole
• Dialysis when indicated

Sodium Bicarbonate

Bicarbonate is not routine treatment for every lactic acidosis.

The priority is correcting the underlying:

• Shock
• Hypoxia
• Sepsis
• Ischemia

However, bicarbonate has important roles in selected severe acid-base disorders and toxicologic emergencies.

When Bicarbonate Is Particularly Important

Examples include:

• Salicylate poisoning, where serum and urinary alkalinization reduces tissue penetration and enhances elimination
• Severe methanol poisoning with marked acidemia
• Severe ethylene glycol poisoning with marked acidemia
• Selected cases of profound acidemia with cardiovascular instability

Use should be guided by the specific disorder.

Salicylate Alkalinization

In significant salicylate toxicity:

IV sodium bicarbonate is used to alkalinize serum and urine.

Increasing blood pH reduces movement of salicylate into:

• Brain
• Other tissues

Urinary alkalinization also increases salicylate excretion.

Intubation in Salicylate Poisoning

Intubation can be dangerous because even brief hypoventilation causes:

• Rising PCO₂
• Falling pH
• Increased nonionized salicylate
• Increased CNS penetration

If intubation is unavoidable:

Maintain or exceed the patient’s pre-intubation minute ventilation as closely as possible.

Decontamination – Major Modern Correction

Induced emesis is not recommended.

Routine gastric lavage is also:

Not recommended.

Gastric lavage is now reserved for exceptional, potentially lethal ingestions when:

• Presentation is very early
• The airway is protected
• Expert toxicology guidance supports it

Activated Charcoal

A single dose of activated charcoal may be considered when:

• The ingestion is potentially toxic
• The substance binds charcoal
• Presentation is sufficiently early
• Airway protection is adequate

It is not useful for all toxicants.

Substances Poorly Adsorbed by Activated Charcoal

Activated charcoal is generally ineffective for substances such as:

• Methanol
• Ethylene glycol
• Iron
• Lithium
• Many caustics

Therefore charcoal should not delay specific treatment for these exposures.

Why Persistent HAGMA Requires Escalation

Persistent unexplained HAGMA may represent:

• Occult shock
• Mesenteric ischemia
• Toxic alcohol poisoning
• Salicylate toxicity
• Ketoacidosis
• Severe renal failure

A patient whose gap remains elevated or continues rising requires:

Repeated investigation rather than passive observation.

Serial Monitoring

Depending on severity, follow:

• Electrolytes
• Anion gap
• Blood gas
• Lactate
• Glucose
• Beta-hydroxybutyrate
• Renal function
• Osmolality

Toxin concentrations should be repeated when clinically appropriate.

When Critical Care Is Needed

ICU-level management should be considered for:

• Severe acidemia
• Hemodynamic instability
• Altered mental status
• Respiratory compromise
• Seizures
• Suspected toxic alcohol poisoning
• Severe salicylate poisoning
• Need for dialysis

Persistent unexplained HAGMA generally warrants:

Hospital admission and continued evaluation.

Common Diagnostic Pitfalls

Important errors include:

• Using a fixed AG >16 threshold regardless of laboratory reference range
• Failing to correct the gap for low albumin
• Assuming a normal osmolal gap excludes toxic alcohol poisoning
• Missing euglycemic DKA
• Failing to obtain a salicylate concentration
• Attributing all lactate elevation to sepsis without considering toxins or ischemia
• Treating the bicarbonate number instead of the underlying disorder
• Intubating a profoundly acidotic patient without preserving compensatory ventilation

Ophthalmic Red Flag

The most important ophthalmic association is:

Methanol toxicity.

The combination of:

• Unexplained high-anion-gap metabolic acidosis
• Possible toxic alcohol exposure
• Acute bilateral visual blurring or visual loss

should trigger immediate treatment for suspected methanol poisoning while definitive levels are pending.

Optic nerve injury can become:

Irreversible.

High-Yield Takeaways

• High-anion-gap metabolic acidosis reflects accumulation of unmeasured acids and can indicate a life-threatening metabolic or toxicologic emergency.
• Calculate the gap as Na − (Cl + HCO₃), but use the laboratory’s own normal range rather than an outdated universal cutoff of 16 mEq/L.
• Correct the anion gap for hypoalbuminemia, approximately adding 2.5 mEq/L for every 1 g/dL that albumin is below 4 g/dL.
• The modern differential is summarized by GOLD MARK: Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.
• Lactic acidosis, ketoacidosis, and renal failure are among the most common causes.
• Salicylate poisoning classically causes respiratory alkalosis plus high-anion-gap metabolic acidosis.
• Measure beta-hydroxybutyrate when ketoacidosis is suspected; urine ketones may underestimate early disease.
• SGLT2 inhibitors can cause euglycemic DKA, so a normal glucose does not exclude ketoacidosis.
• Methanol causes formate toxicity and optic neuropathy; visual symptoms plus HAGMA should be treated as methanol poisoning until proven otherwise.
• Ethylene glycol causes severe acidosis, hypocalcemia, and acute kidney injury; urinary oxalate crystals are supportive but not required.
• A high osmolal gap supports toxic alcohol exposure, but a normal osmolal gap does not exclude late methanol or ethylene glycol poisoning.
• Early toxic alcohol poisoning may have a high osmolal gap with little acidosis; later poisoning may have a high anion gap with a normalizing osmolal gap.
• Initial evaluation should usually include electrolytes, blood gas, lactate, beta-hydroxybutyrate, glucose, renal function, measured serum osmolality, salicylate level, and acetaminophen level.
• Use Winter’s formula to determine whether respiratory compensation is appropriate.
• Fomepizole is the preferred antidote for suspected methanol or ethylene glycol poisoning, and treatment should not wait for confirmatory levels when suspicion is high.
• Severe toxic alcohol poisoning may require urgent hemodialysis.
• Sodium bicarbonate is particularly important in salicylate toxicity and severe toxic-alcohol acidemia, but it is not routine treatment for every lactic acidosis.
• Induced emesis is obsolete, and gastric lavage is now extremely rarely indicated.
• Activated charcoal does not effectively adsorb methanol, ethylene glycol, iron, or lithium.
• The traditional indiscriminate “coma cocktail” is outdated; antidotes and supportive treatment should be targeted to the suspected problem.
• During intubation of a severely acidotic patient, preserve compensatory high minute ventilation; loss of hyperventilation can cause catastrophic acidemia, particularly in salicylate poisoning.
• Persistent, unexplained HAGMA warrants serial reassessment, hospital admission, and early toxicology/nephrology involvement when indicated.

High-Yield Takeaways