Prognosis: slowly progressive condition over a median time of 2-3 years, poor prognosis.
Seizures Seizures in response to noise (often high-pitch noises):
Myoclonic seizures - an involuntary jerk of the legs and/or body that can be likened in appearance to a brief electric shock. This sign is a pre-requisite for a diagnosis of feline audiogenic reflex seizures (FARS).
Absence seizures (or 'petit mal') - a temporary loss of awareness, staring vacantly into space and being unresponsive.
Clinical signs exclusively appear in the second decade of life (median age 15 years).
Any breed of cat.
Approximately one third of affected cats are Birmans Birman suggesting a predisposition in this breed (notably in blue and seal points only).
Consideration should be given to concurrent disease in cats undergoing investigation under sedation or anesthesia due to the high prevalence of concurrent diseases in older cats.
Audiogenic seizures in mice have been found to be associated with deafness as a result of a mutation in GIPC3. This mutation has not been found in cats with audiogenic seizures.
Many of the sounds causing these seizures are relatively innocuous to us (ie they are quiet, high-pitched sounds).
Suggested that the reason they may cause seizures is that cats have an ultrasonic hearing range, allowing them to catch mice that communicate with each other in this ultrasonic frequency range.
These domestic noises that precipitate seizures have a high component of ultrasonic frequencies and so this may be a reason for the exaggerated response in these cats.
Signs usually develop in cats over 10 years old and slowly progress.
First observable signs are infrequent myoclonic (twitches) seizures which can progress to generalized tonic-clonic seizures and disorientation with time.
Volume of the sound is proportional to the severity of the seizures.
A persistent sound can cause repeated jerks (myoclonic seizures) of the body that may culminate in a full generalized tonic-clonic seizure - this phenomenon is known as audiogenic kindling in which lots of small sound stimuli culminate to produce a larger response, in this case a generalized tonic-clonic seizure.
Repetitive sound-induced seizures (audiogenic kindling) gradually induce the transference of epileptic activity from brainstem (or myoclonic seizures) to forebrain structures (generalized tonic-clonic seizures) along with behavioral changes in these cats.
Myoclonic seizures - a sudden, brief involuntary contraction of a muscle or muscle group. This sign is a pre-requisite for a diagnosis of FARS.
Generalized tonic-clonic seizures - one in which a cat will lose awareness whilst suddenly falling to the ground and show chomping of the jaw and chewing, foaming at the mouth, paddling of the legs, and sometimes the passing of urine or stools. They usually last no more than a few minutes. These occur in around 80% of cats with FARS.
Absence seizures (also known as 'petit mal') - the occurrence of an abrupt, transient apparent loss of consciousness with no motor activity. These occur in around 5% of cats with FARS.
Hearing loss or deafness.
Decreased cognitive function.
Incoordination/weakness later on in the disease.
Inability to jump.
Frequent twitching/jerks (NB: owners have not always made a connection with noise).
Avoiding precipitating sounds where possible. However, the noises provoking these seizures are everyday household sounds that tend to be high-pitched, relatively quiet sounds (eg computer keyboard tapping or mouse clicking; paper or plastic bags crinkling; the sound of cutlery on a ceramic plate when eating or preparing food; foil crinkling; and the clinking of keys) making avoidance challenging.
Levetiracetam Levetiracetam therapy (20-30 mg/kg q8-12h) has been shown to be beneficial in controlling myoclonic seizures and is more effective in doing this compared to phenobarbital Phenobarbital.
Both levetiracetam and phenobarbital (3 mg/kg q12h) have similar efficacy at controlling generalized tonic-clonic seizures associated with FARS.
Recent work suggests levetiracetam may help in retarding the progression of the disease and allows freedom from seizures allowing cats to be more alert and aware of their surroundings.
Hematology, biochemistry and urinalysis is advised every 3-6 months to monitor general health due to chronic administration of levetiracetam.
As many as 40% of cats with FARS have concurrent medical conditions including cardiac and renal disease. Older cats are prone to these conditions and enough cats have been diagnosed with FARS in the absence of these conditions to suggest that this is not a cause for noise-induced seizures in cats.
Expected response to treatment
Levetiracetam should reduce the number of myoclonic seizures significantly and will also help to control generalized tonic-clonic seizures.
Phenobarbital (3 mg/kg q12h) can be considered as an adjunct to treatment if levetiracetam is insufficient alone.
With time, and following numerous sound exposures, trains of myoclonic seizures can develop resulting in generalized tonic-clonic seizures. This occurs due to seizure discharges spreading from the brainstem to the forebrain and is referred to as audiogenic kindling.