Topamax

Action

Blocks sodium channels, enhancing the action of gamma-amino butyrate (a neurotransmitter); also inhibits amino acid excitatory receptors

Availability

Sprinkle capsules: 15 mg, 25 mg

Tablets: 25 mg, 50 mg, 100 mg, 200 mg

Indications and dosages

➣ Adjunct in partial-onset seizures, primary generalized tonic-clonic seizures, and seizures associated with Lennox-Gastaut syndrome

Adults and children older than age 17: Initially, 25 to 50 mg P.O. daily. To achieve adequate response, may increase by 25 to 50 mg weekly, up to 200 mg b.i.d.

Children ages 2 to 16: Initially, up to 25 mg P.O. daily; increase at 1- or 2-week intervals in increments of 1 to 3 mg/kg/day given in two divided doses to achieve adequate response.

➣ Migraine prophylaxis

Adults: Dosage titrated to 100 mg P.O. daily as follows: 25 mg/day during week 1, 25 mg b.i.d. during week 2, 25 mg in morning and 50 mg in evening during week 3, and 50 mg b.i.d. during week 4

➣ Monotherapy for epilepsy

Adults and children ages 10 and older: Initially, 50 mg P.O. daily in two divided dosages. Increase dosage weekly by increments of 50 mg for first 4 weeks, then 100 mg for weeks 5 and 6. Maximum dosage is 400 mg/day in two divided doses.

Children ages 2 to younger than 10: Initially, 25 mg/day P.O. nightly for first week. Titrate dosage over 5 to 7 weeks, with total maximum daily dosage based on weight.

Dosage adjustment

• Renal impairment

Off-label uses

• Cluster headaches

• Infantile spasms

• Mood stabilization

Contraindications

None

Precautions

Use cautiously in:

• renal or hepatic impairment, dehydration, urolithiasis, glaucoma, myopia, patients at increased risk for hyperammonemia (such as those with inborn errors of metabolism or reduced mitochondrial activity)

• concurrent use of other carbonic anhydrase inhibitors, other drugs causing metabolic acidosis, or patients on ketogenic diet (avoid use)

• concurrent use of other drugs that predispose patients to heat-related disorders (such as carbonic anhydrase inhibitors, drugs with anticholinergic activity)

• pregnant or breastfeeding patients.

• children younger than age 2 (safety and efficacy not established).

Administration

• Give without regard to meals.

• Don't break tablets, because of bitter taste.

• Administer capsules either whole or by opening capsule carefully and sprinkling entire contents into small amount of soft food. Instruct patient to swallow mixture immediately without chewing sprinkles.

Don't stop therapy suddenly. Dosage must be tapered.

Adverse reactions

CNS: dizziness, drowsiness, fatigue, malaise, poor memory and concentration, nervousness, psychomotor slowing, speech and language problems, aggressive reaction, agitation, anxiety, confusion, depression, irritability, ataxia, paresthesia, hyperesthesia, tremor, suicide attempt, increased seizures

EENT: abnormal vision, diplopia, nystagmus, acute myopia, secondary angle-closure glaucoma, decreased hearing, rhinitis, sinusitis, epistaxis, pharyngitis

GI: nausea, constipation, abdominal pain, dry mouth, gastroenteritis, increased salivation (in children), anorexia

GU: renal calculi, urinary incontinence, leukorrhea

Hematologic: purpura, leukopenia, thrombocytopenia

Metabolic: hypocalcemia, hyperchloremia, hypernatremia, hyponatremia, hypophosphatemia, hyperammonemia, metabolic acidosis, hypoglycemia

Musculoskeletal: myalgia, back pain, leg pain

Respiratory: pneumonia

Skin: rash, skin disorder, alopecia, dermatitis, hypertrichosis, eczema, seborrhea, skin discoloration

Other: altered taste, weight loss, thirst, fever, flulike symptoms, hot flashes, infection, edema, hypothermia (when used in conjunction with valproic acid), hyperthermia, decreased sweating, allergic reaction

Interactions

Drug-drug. Carbamazepine: decreased topiramate blood level and effects

Carbonic anhydrase inhibitors (such as acetazolamide): increased risk of renal calculi

CNS depressants: increased risk of CNS depression and other adverse cognitive or neuropsychiatric reactions

Hormonal contraceptives: decreased contraceptive efficacy

Phenytoin: increased phenytoin blood level and effects, decreased topiramate blood level and effects

Valproic acid: decreased effects of both drugs

Drug-diagnostic tests. Alanine aminotransferase, alkaline phosphatase, ammonia, aspartate aminotransferase, creatinine: increased levels

Calcium, cholesterol, glucose, phosphate: decreased levels

Sodium: increased or decreased level

Drug-behaviors. Alcohol use: increased CNS depression

Patient monitoring

Monitor seizure type and pattern. Report new seizure types or worsening seizure pattern.

• Assess neurologic status closely. Report significant adverse reactions.

Watch for and immediately report signs and symptoms of depression or suicidal ideation.

• Monitor fluid intake and output. Report indications of urinary tract infection, urinary incontinence, or renal calculi.

Monitor vision. If patient becomes acutely nearsighted with symptoms of angle-closure glaucoma (cloudy vision, eye pain), stop drug and contact prescriber right away.

Monitor patient for hyperammonemia and encephalopathy with or without concomitant valproic acid use; measure ammonia level if encephalopathic symptoms occur.

Monitor patient for metabolic acidosis; obtain baseline and periodic measurements of serum bicarbonate. Consider dosage reduction or drug discontinuation if clinically appropriate.

Patient teaching

• Tell patient he may take with or without food.

• Caution patient not to crush or break tablets.

• If patient takes capsules, tell him he may open them, sprinkle contents onto small amount of soft food, and consume immediately. Tell him not to store this mixture.

Caution patient not to stop drug suddenly. Dosage must be tapered.

• Instruct patient to drink plenty of fluids to reduce risk of kidney stones.

Tell patient drug may cause new seizure types or worsen seizure pattern. Instruct him to report these developments immediately.

Instruct patient (and significant other as appropriate) to immediately report signs or symptoms of depression or suicidal thoughts.

Instruct patient how to recognize and immediately report signs and symptoms of hyperammonemia and metabolic acidosis.

Advise patient to immediately report vision changes, especially nearsightedness, cloudy vision, or eye pain.

• Caution patient not to drive or perform other hazardous activities.

• Tell patient not to drink alcohol during drug therapy.

• Advise female patient to notify prescriber of suspected pregnancy.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, and behaviors mentioned above.