Acetazolamide

Overview

Acetazolamide (Diamox®) is a carbonic anhydrase inhibitor (CAI) with diuretic properties and the ability to reduce aqueous humor production. In veterinary medicine, it is used primarily for the management of metabolic alkalosis and as adjunctive therapy for glaucoma, as well as for specific neurologic and electrolyte-related conditions.

By inhibiting carbonic anhydrase, acetazolamide alters acid-base balance and electrolyte handling, leading to increased renal excretion of bicarbonate, sodium, potassium, and water. These effects result in urine alkalinization and a mild diuretic action. Within the eye, inhibition of carbonic anhydrase decreases aqueous humor formation, thereby lowering intraocular pressure.

In small animals, acetazolamide is most commonly used as adjunctive therapy for glaucoma and, less commonly, for metabolic alkalosis or increased cerebrospinal fluid pressure in select neurologic conditions.

Mechanism of Action (MOA): Acetazolamide produces its clinical effects through reversible, noncompetitive inhibition of carbonic anhydrase. This reduces the conversion of carbonic acid to hydrogen and bicarbonate ions, limiting their availability for secretion and transport. The resulting physiologic effects include decreased aqueous humor production, altered renal tubular electrolyte handling, metabolic acidosis, and mild diuresis. The drug also possesses anticonvulsant activity that appears to be independent of its diuretic effects.

Indications

Acetazolamide is used in veterinary medicine for its effects on acid-base balance, aqueous humor production, and electrolyte handling. Its clinical applications vary by species and are generally adjunctive rather than first-line therapies.

• Glaucoma (adjunctive therapy): Used to reduce intraocular pressure by decreasing aqueous humor production, typically in combination with topical antiglaucoma medications.
• Metabolic alkalosis: Utilized in small animals to promote bicarbonate excretion and correction of alkalosis when underlying causes are being addressed.
• Increased cerebrospinal fluid pressure: May be used as adjunctive treatment for increased CSF pressure associated with conditions such as syringomyelia in dogs, although clinical efficacy may be variable.
• Neurologic adjunctive use: Has anticonvulsant properties and has been used experimentally or adjunctively in select neurologic conditions.
• Episodic falling syndrome in Cavalier King Charles Spaniels (and other paroxysmal dyskinesias): 4-8 mg/kg PO q8-12h. Used when collapse episodes are refractory to first-line therapies (clonazepam, diazepam). BSAVA specifies a 2-week trial at q12h dosing; if no favourable response after 2 weeks of use q12h, the drug should be stopped. May be beneficial for other paroxysmal dyskinesias.

Dosage (Reference)

Dog

In dogs, acetazolamide is used as adjunctive therapy for specific conditions such as glaucoma and metabolic alkalosis. It is not considered a first-line drug, and dosing should be tailored to the individual patient with close monitoring of electrolytes and acid-base status.

Cat

Cats are particularly susceptible to the adverse effects of systemic carbonic anhydrase inhibitors; avoid in this species.

Warnings & Precautions

Acetazolamide alters acid-base balance and electrolyte homeostasis and should be used cautiously, particularly with repeated or high-dose therapy. Careful patient selection and monitoring are essential to minimize adverse effects.

• Hepatic disease: Contraindicated in patients with significant hepatic dysfunction, as carbonic anhydrase inhibition may precipitate hepatic encephalopathy or coma.
• Renal or adrenocortical insufficiency: Contraindicated due to impaired drug handling and increased risk of electrolyte disturbances.
• Electrolyte imbalances: Do not use in patients with hyponatremia, hypokalemia, hyperchloremic acidosis, or other significant electrolyte abnormalities.
• Acid–base disturbances: Use cautiously in patients with severe respiratory acidosis or conditions that limit compensation for metabolic acidosis.
• Pulmonary disease: Contraindicated in patients with severe pulmonary obstruction that cannot increase alveolar ventilation, as worsening acidosis may occur.
• Hematologic abnormalities: Use with caution in patients with pre-existing hematologic disorders due to the risk of bone marrow suppression.
• Chronic or high-dose therapy: May lead to decreased exercise tolerance, hypercapnia, and respiratory acidosis.
• Gastrointestinal tolerance: Gastrointestinal upset is common; oral doses may be administered with food to improve tolerance.
• Glaucoma management: When used for glaucoma, intraocular pressure should be monitored regularly using tonometry to assess efficacy and adjust therapy.
• Sulfonamide structure: Although acetazolamide is a sulfonamide, cross-reactivity with antibacterial sulfonamides or furosemide appears unlikely.
• Medication errors: Avoid confusion with similarly named drugs such as acetohexamide or acetaminophen.
• Pregnancy & lactation: Acetazolamide crosses the placenta in unknown quantities and has caused limb defects, reduced fetal weight, and incisor development changes in laboratory animals. Fetal toxicity has been noted in pregnant humans. Excreted in milk — if unavoidable, use milk replacer instead of allowing offspring to nurse. Use only when maternal benefits outweigh potential fetal risks.

Drug Interactions

Drug interactions with acetazolamide are primarily related to its effects on electrolyte balance, urinary alkalinization, and interference with folate metabolism or renal excretion of other drugs. When combinations cannot be avoided, additional monitoring is recommended.

• Cyclosporine: Acetazolamide may increase cyclosporine concentrations; monitor for toxicity.
• Digoxin: Acetazolamide-induced hypokalemia may increase the risk of digoxin toxicity.
• Drugs affecting potassium balance: Concomitant use may exacerbate potassium depletion.
• Folic acid antagonists: May augment folic acid antagonism and increase the risk of hematologic toxicity.
• Insulin: Rarely, carbonic anhydrase inhibitors may interfere with the hypoglycemic effects of insulin.
• Methenamine compounds: Alkalinization of urine may negate the antibacterial effects of methenamine.
• Methotrexate: May enhance folic acid antagonism and increase toxicity risk.
• Phenobarbital: Alkaline urine may increase urinary excretion and reduce therapeutic efficacy.
• Primidone: May reduce absorption and serum concentrations of primidone.
• Procainamide: Decreased urinary excretion in alkaline urine may increase the risk of procainamide toxicity.
• Quinidine: Reduced urinary excretion may lead to increased quinidine concentrations and toxicity.
• Salicylates: Increased risk of acetazolamide accumulation, metabolic acidosis, and toxicity.
• Sodium bicarbonate: Concurrent use increases the risk of renal calculi formation.
• Tricyclic antidepressants: Urinary alkalinization may decrease drug excretion and increase systemic exposure.

Side Effects & Overdose

Side Effects

Adverse effects of acetazolamide are related to its effects on carbonic anhydrase inhibition, electrolyte balance, and acid-base status. The likelihood of adverse effects increases with higher doses or chronic administration.

• Gastrointestinal effects: Nausea, vomiting, diarrhea, anorexia, and abdominal discomfort are commonly reported.
• Central nervous system effects: Sedation, depression, weakness, excitement, paresthesias, or tremors may occur; mild tremors have been reported in dogs after high-dose IV administration.
• Electrolyte disturbances: Hypokalemia, hyponatremia, hyperchloremia, and metabolic acidosis may develop, especially with repeated dosing.
• Renal effects: Polyuria, crystalluria, dysuria, renal colic, and altered urine pH may occur.
• Hematologic effects: Bone marrow suppression and other hematologic abnormalities have been reported.
• Metabolic effects: Hyperglycemia and hyperuricemia may occur.
• Hepatic effects: Hepatic insufficiency has been reported, particularly in predisposed patients.
• Dermatologic and hypersensitivity reactions: Rash, hypersensitivity reactions, Stevens-Johnson syndrome, and toxic epidermal necrolysis have been reported.

Overdose

Information regarding acute overdose of acetazolamide in veterinary patients is limited. Toxicity is expected to primarily reflect exaggerated pharmacologic effects.

• Expected findings: Electrolyte abnormalities, metabolic acidosis, dehydration, and central nervous system depression.
• Monitoring: Serum electrolytes, venous blood gases (including pH), hydration status, and neurologic status should be closely monitored.
• Management: Treatment is supportive and directed at correction of electrolyte and acid-base disturbances and maintenance of hydration.
• Dialysis: Acetazolamide is dialyzable and may be removed by dialysis in severe cases.
• Consultation: Consultation with a veterinary poison control center is recommended for suspected or confirmed overdose cases.

Key Notes

Additional practical considerations for the clinical use of acetazolamide that are not emphasized elsewhere in this monograph:

• Adjunctive role: Acetazolamide is rarely sufficient as monotherapy; outcomes are optimized when underlying disease processes are addressed concurrently.
• Variable response: Clinical response, particularly for glaucoma and neurologic indications, can be inconsistent between patients; reassess benefit early after initiation.
• Short duration of action: Effects on intraocular pressure and acid-base balance are relatively short-lived: IOP-reducing onset is approximately 30 minutes, maximal effects occur after 2-4 hours, and the effect duration is 4-6 hours. This kinetic profile is why divided daily dosing (typically q8-12h) is required for sustained benefit.
• Oral bioavailability: Clinical response should guide dosing adjustments.
• Urine alkalinization effects: Changes in urine pH may influence the elimination of concurrently administered drugs and the formation of urinary crystals.
• Diagnostic interference: Altered urine chemistry and thyroid-related laboratory values may complicate interpretation of diagnostic tests during therapy.
• Client education: Emphasize adherence to monitoring schedules, as laboratory follow-up is critical to safe continued use.