Clare Rusbridge BVMS DipECVN MRCVS
1) The dog whose seizures have never been controlled
The majority of dogs with idiopathic epilepsy
are treated with anticonvulsant drugs such as phenobarbitone and primidone.
Although most patients will have a satisfactory reduction in seizures,
some will continue to have fits causing frustration for owners and veterinary
surgeons alike. However, many patients can be controlled. The
most common reason for inadequate seizure control is that the dog is not
receiving enough anticonvulsant. In order to prevent seizures, an
anticonvulsant must be given in a sufficient amount and manner to ensure
that a "therapeutic concentration" is achieved in the blood and ultimately
in the brain.
The anticonvulsant must be administered at regular
time intervals. For example, phenobarbitone is usually given every
12 hours. If the first is given at 7am then the second dose should
be given at 7pm. A delay or early administration will lead to a period
of increased susceptibility to seizures. Obviously, forgetting to
administer the drug altogether is very dangerous. At best this results
in poor control; at worse life threatening seizures can occur.
The anticonvulsant must be given adequate time
to work. It takes two weeks for phenobarbitone to achieve a steady
concentration in the blood. This means that the dog should be on
the new dosage for at least two weeks before a decision can be made as
to the anticonvulsant's effectiveness. Understandably, many owners
assume that the drug cannot be working if the seizures do not stop immediately.
Owners can also become concerned about the sedation of the pet from drugs
such as phenobarbitone and primidone. The owner becomes concerned
that the personality of the dog has been changed and that the seizures
are still not under control. It is important to realise that although
unpleasant, the sedation is not permanent and is usually only seen during
the first two weeks of therapy.
The most common reason for failure of control
is that the dosage of anticonvulsant is too low, If your veterinary
surgeon suspects this, he/she may measure the concentration of phenobarbitone
in the dog's blood. If the concentration is below the therapeutic
range, then the dosage should be increased. Evaluating the concentration
of the drug in the blood is also an invaluable method to ensure that the
dog is not receiving an overdose.
If a blood sample reveals that the phenobarbitone
concentration is within the therapeutic range, but the dog is still experiencing
frequent seizures, it does not necessarily mean that the anticonvulsant
is ineffective for the patient. As long as the blood concentration
remains within the therapeutic range the dosage can be increased.
The frequency or number of seizures will usually decrease. If satisfactory
seizure control has not been achieved at this point then your veterinary
surgeon may recommend the addition of another anticonvulsant such as potassium
bromide.
An obvious reason for inadequate control is an
incorrect diagnosis - i.e. the dog does not have true idiopathic epilepsy.
Idiopathic epilepsy is the term used for dogs with recurrent seizures that
are not secondary to an underlying disease process. There are many
other causes of seizures. To rule out underlying disease, your vet
will take a blood sample to evaluate general organ function. He may
also recommend that a cerebrospinal fluid sample is taken and analysed
and/or that the dog is referred for a CT or MRI scan. These tests
reveal inflammation in the brain and structural changes such as brain tumours.
Unfortunately there is always a proportion of
dogs that continue to have frequent seizures despite adequate anticonvulsant
therapy. One study suggested that 30% of canine epileptics cannot
be controlled. Until further methods of treating epilepsy are developed
there is no satisfactory method of managing these dogs.
2) The dog whose seizures were controlled,
but are now out of control
The most common reason for an increase in seizure
frequency is the phenomenon of drug tolerance. Anticonvulsants such
as phenobarbitone and primidone are capable of stimulating the production
of the enzymes that can break them down. This means that the liver
can break the anticonvulsant down more quickly resulting in an inadequate
concentration of drug in the blood and in the brain. If the phenobarbitone
concentration in the blood is evaluated it will be below the therapeutic
range. Simply increasing the dosage should re-establish control.
The blood concentration is usually rechecked two weeks after changing the
dosage to ensure the new concentration is adequate. Many veterinary
surgeons now check the epileptic dog's blood phenobarbitone concentration
every six months to ensure their patient is receiving the correct dose.
Another cause of recurrence of seizures are the
fluctuations of hormone levels found around the time of oestrus in the
female dog - i.e. coming "on heat". For this reason your veterinary
surgeon will probably recommend that the female epileptic dog is spayed.
Concurrent disease or stress can also cause a temporary recurrence of seizures.
Vomiting and diarrhoea can lead to poor absorption of the anticonvulsant
from the intestine. Usually when the dog recovers from the illness,
the seizure frequency decreases. If seizures are associated with
an identifiable cause of stress, the dosage of anticonvulsant can be temporarily
increased twelve hours prior to the stressful event. Obviously, to
ensure the safety of your dog, it is vital to consult your veterinary surgeon
before making any changes in anticonvulsant dosage.
The most distressing but fortunately less common
cause of a recurrence of seizures is development of a second seizure causing
disease. If your veterinary surgeon suspects this he/she may recommend
performing further tests such as taking blood tests, a cerebrospinal fluid
sample or brain scan.
Clare Rusbridge is in private practice at The
Stone Lion Veterinary Centre, 41 High Street, Wimbledon Common, London,
SW19 5AU Tel: 0181-946-4228/Fax: 0181 944 0871.