By Dr. Melissa Logan | PHD, DVM, DACVIM (Neurology)
Cerebrovascular accidents (CVA) are increasing in recognition in veterinary medicine. A CVA, more commonly known as a “stroke” can be classified into two different groups. One type is secondary to obstruction of the blood vessels, leading to an infarction and the other type is due to rupture of the blood vessel and hemorrhage. In my experience, infarctions tend to be more common than hemorrhagic strokes in our patients. The prognosis for animals tends to better than what is seen in humans due to the fact that animals do not have a prominent pyramidal system.
The brain requires a constant supply of oxygen and glucose due to high energy consumption. The blood supply to the dog’s brain is from the basilar and internal carotid arteries, while the cat’s major supply is the maxillary artery. Any disease that affects this blood flow can lead to damage of the brain tissue and subsequent neurologic impairment.
Unfortunately, the brain relies solely on aerobic metabolism and does not have energy stores when blood supplies are not adequate. Cerebral blood flow requires 20% of the cardiac output and 15% of oxygen consumption in the resting state. This is disproportional to the fact that the brain itself only makes up 2% of the animal’s body weight.
Cerebral infarction can result from either disease of the arterial or venous system. In the larger vessels, a CVA can lead to a global infarction. If a small vessel is involved, this is considered a regional infarction. The infarction is a result of an obstruction from either thrombosis of the vessel or due to an embolism. In many cases, there is an underlying metabolic or endocrine disorder that predisposes an animal to thrombosis. The most common causes include atherosclerosis from conditions like hypothyroidism, diabetes mellitus and hyperadrenocorticism. When dealing with embolic disease, the most common causes include sepsis, neoplasia, Dirofilaria immitis, and heart disease.
Cerebral hemorrhage is less common in our veterinary patients. It is classified as epidural, subdural, subarachnoid, intraparenchymal, or intraventricular. Essentially a hematoma is formed and creates a mass effect. The presence of the hematoma also induces edema, fluid accumulation and neuronal damage due to the release of osmotically active proteins that are released from the blood clot. Cerebral hemorrhage is classified into primary, such as due to hypertension, or secondary from neoplasia, coagulopathy, and even vascular malformations.
Clinical signs of a CVA include an acute onset of focal, asymmetric, and non-progressive neurologic dysfunction. In some cases of hemorrhage, the signs can worsen over a period of time due to the mass effect and edema. The neurologic signs will obviously depend on the location of the CVA. Disruption of blood flow in the cerebrum could lead to signs such as seizures, circling to the ipsilateral side, contralateral central blindness, contralateral proprioceptive deficits, and even head turns. In my experience, I commonly observe signs of vestibular dysfunction in animals with lesions in the thalamus and the brainstem. Obviously, other cranial nerve abnormalities may be detected depending on the location of the CVA.
There is a subset of CVA that has clinical signs that are short lived and these are called transient ischemic attacks (TIAs). These are considered transient in that the signs resolve within 24 hours of onset. In my experience, I feel these animals present with acute vestibular dysfunction that resolves in many cases very quickly. Although the signs are short lived, the concern is that a TIA may make an animal at higher risk for a more profound CVA or global ischemic event.
Diagnosis of a CVA is typically made through MRI or CT imaging and clinical suspicion. CT can detect ischemia and infarction, as well as hemorrhage. That being said, MRI is more sensitive in evaluating for smaller lesions and the detection of edema. The lesions with cerebral infarction appear as well-demarcated regions with mild to no mass effect. Obviously, hemorrhagic lesions can be expansile and have a mass effect noted on imaging. They can appear similar to a neoplastic lesion. There is a special imaging sequence on MRI called gradient echo (GRE) that can identify hemorrhage. There are also certain MRI characteristics of a hematoma over time. In addition to advanced imaging, baseline blood work as well as thyroid testing, urinalysis, coagulation profiles, and blood pressures are very useful in patients with a suspected CVA. This will help to discern for a potential underlying etiology for the CVA. In a majority of cases, however, an underlying etiology is not identified. This can be frustrating for the clients and the veterinarian alike.
There is no specific treatment for a CVA aside from treatment of the underlying etiology if identified. In cases of infarction, I have prescribed clopidogrel to help prevent additional CVAs in the future. This treatment is not necessarily proven to be effective, but in most cases carries little risk and minimal side effects.
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