Phenobarbitone monitoring — Epilepsy in dogs and cats
Phenobarbitone monitoring
Phenobarbitone is an anticonvulsant drug. It is indicated in cases of acute life-threatening seizures, and also in the management of seizures over a prolonged period. Phenobarbitone is most commonly the first-choice drug in such cases and acts well regardless of the primary cause of the seizures, which may be a consequence of poisoning, infectious conditions, epilepsy and/or neoplasia of the brain.
In cases of long-term therapy for the control of seizures, regular monitoring of the blood concentration of phenobarbitone is important for patient health and welfare. Therapeutic concentrations above the upper value (150.0 µmol/L) lead to irreversible liver cell damage.
It is not possible in any way to predict the blood concentration that phenobarbitone will achieve from the oral dose the patient receives. This relationship varies from patient to patient and depends on many factors. One of these is the timing of food intake in relation to when therapy is administered.
Monitoring and adjustment of the therapeutic phenobarbitone concentration is necessary at the start of introducing this drug. In addition to several protocols for monitoring blood concentration, the most important factor is observation of the clinical picture, namely the degree of seizure frequency.
ACVIM recommendations are as follows
- The first sample should be tested 14 days after starting therapy.
- The second sample should be tested 6 weeks after starting therapy.
- Blood concentration should be checked every 6 months, provided the patient has had at least 2 seizures in that period.
- Blood concentration determination should be repeated 14 days after any changes in the therapeutic dose.
The time taken for phenobarbitone to be metabolised in the patient's liver varies between 24 and 72 hours. Oral therapy should therefore be given every 12 to 24 hours. Blood concentration varies throughout the day. Phenobarbitone reaches its peak concentration 2 to 6 hours after administration. After this, the concentration falls until the next dose is given. For determining phenobarbitone concentration, sampling immediately before therapy administration is recommended, as this is theoretically the lowest concentration the drug achieves in the blood.
Since there is a large individual variation between patients in the metabolism of phenobarbitone, in addition to monitoring blood concentration it is necessary to take into account the clinical picture, physical examination findings, haematological and biochemical test results and the results of other diagnostic procedures.
Due to a certain degree of toxicity in cases of long-term therapy, it is necessary to monitor the general condition of the organism using the phenobarbitone monitoring profile, a general profile and the bile acid stimulation test.
DVM, PhD, MS, Strahinja Medić