McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

Clinically important drug–drug interactions If any of these drugs is taken with other CNS depressants or alcohol, a potential for increased CNS depression exists. Caution people to avoid alcohol while taking drugs for partial seizures or to take extreme precautions if such combinations cannot be avoided. In addition, numerous drug–drug interactions are associated with carbamazepine. Always consult a drug reference whenever a drug is added or withdrawn from a carbamazepine-containing regimen. Dose adjustments may be necessary. Oxcarbazepine is an inhibitor of CYP2C19. There­ fore, interactions could arise when co-administering high doses of Trileptal with medicinal products that are metabolised by CYP2C19 (e.g. phenobarbitone, phenytoin). ■ ■ Assess for contraindications and cautions: any known allergies to these drugs to avoid hypersensitivity reactions ; history of bone marrow suppression or renal stones, which could be exacerbated by these drugs ; history of renal or hepatic dysfunction that might interfere with drug metabolism and excretion ; and current status of pregnancy or breastfeeding, which are contraindicated or require caution when using these drugs . ■ ■ Perform a physical assessment to establish baseline data for determining the effectiveness of therapy and the occurrence of any potential adverse effects. ■ ■ Inspect the skin for colour and lesions to determine evidence of possible skin effects ; assess pulse and blood pressure and auscultate heart to evaluate for possible cardiac effects ; assess level of orientation, affect, reflexes and bilateral grip strength to evaluate any CNS effects ; monitor bowel sounds and urine output to determine possible gastrointestinal or genitourinary effects. ■ ■ Obtain a baseline electroencephalogram if appropriate to evaluate brain function. ■ ■ Assess the person’s renal and liver function, including renal and liver function tests, to determine the appropriateness of therapy and determine the need for possible dose adjustment. ■ ■ Monitor the results of laboratory tests such as urinalysis and FBC with differential to identify changes in bone marrow function. Care considerations for people receiving drugs to treat partial seizures Assessment: History and examination

alter the metabolism and excretion of the drugs ; and with renal stones, which could be exacerbated by the effects of some of these agents. Adverse effects The most frequently occurring adverse effects associ­ ated with the drugs used for partial seizures relate to the CNS depression that results. The following conditions may occur: drowsiness, fatigue, weakness, confusion, headache and insomnia; GI depression, with nausea, vomiting and anorexia; and upper respiratory infec­ tions. These antiepileptics can also be directly toxic to the liver and the bone marrow, causing dysfunction. The exact effects of each drug vary. People with renal dysfunction are more likely to experience toxic effects of levetiracetam, and the dose for these individuals needs to be decreased accordingly. The adverse effects most commonly seen with pregabalin are related to CNS depression—tremor, dizziness, somnolence and visual changes. This drug does have a controlled substance rating as Category V. It can cause feelings of wellbeing and euphoria. Because of this, its use should be limited in people who have a history of abuse of medications or alcohol. A reduced dose of topiramate is recommended for people with renal impairment. The drug has also been associated with marked CNS depression. Tiagabine has also been associated with serious skin rash.

Prototype summary: Carbamazepine Indications: Treatment of seizure disorders, including partial seizures with complex patterns; tonic–clonic seizures; mixed seizures; trigeminal neuralgia. Actions: Inhibits polysynaptic responses and blocks post-tetanic potentiations; mechanism of action is not understood; related to the tricyclic

antidepressants. Pharmacokinetics: Route

Onset

Peak

Oral

Slow 4–5 hours Slow 3–12 hours

Extended release oral

T 1/2 : 25 to 65 hours, then 12 to 17 hours; metabolised in the liver, excreted in the urine and faeces. Adverse effects: Drowsiness, ataxia, dizziness, nausea, vomiting, CV complications, hepatitis, haematological disorders, Stevens–Johnson syndrome.