Canine brain disorders fall into three main categories: Encephalitis (inflammation or infection in the brain), Cerebral Degeneration (where the cells of the dog’s brain die), and Brain Tumors (a mass growing within the dog’s brain). Brain tumors can affect young dogs, but are more commonly seen in older dogs, likely due to a diminished immune system.
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Infection and inflammation of the central nervous system (CNS) is referred to as Encephalitis. The CNS consists of the brain, spinal cord, and meninges (the tough fibrous membrane that surrounds the brain and spinal cord). Bacteria, viruses, parasites, fungi and tick-borne diseases are the primary causes of encephalitis. Bacterial encephalitis is caused by organisms that enter the brain through the circulatory system resulting from a sinus infection, deep inner ear infection, or abscess in the head. Meningitis is a bacterial infection on the surface membrane of the brain and spinal canal caused by infected bite wounds around the head and neck as well as bacterial infections that travel to the brain from the sinuses or ears.
Types of Encephalitis
Idiopathic encephalitis is a general term used for cases in which an infectious cause or other direct cause cannot be found. In many cases, an underlying immune-medicated cause is suspected because these animals improve after suppression of the immune system. Immune-mediated diseases occur when the body’s white blood cells mistakenly attack normal tissue, in this case the brain or spinal cord. High-dose therapy with the prednisone steroid is the most common treatment.
Granulomatous Meningoencephalitis (GME)
This common inflammatory brain disease is usually seen in female dogs of small breeds, especially Terriors, Dachshunds, and Poodles. Although GME can occur at any age, dogs 3-6 years of age are most at risk. It is typically characterized by fever, depressions, uncoordinated gait, behavior changes (particularly aggression), and seizures.
Necrotizing Meningoencephalitis (NME)
This inflammatory brain disease affects the forebrain predominately and is characterized by seizures. Young dogs 6 months – 7 years of age are affected. Breeds that are commonly predisposed to this disease include Pugs, Maltese, Chihuahuas, Papillion, Shih-tz, and Boston Terriors.
Necrotizing Leukoencephalitis (NLE)
This is a multifocal, localized, chronic progressive disease of the forebrain and brainstem with the central vestibular disease being common. It primarily affects Yorkshire Terrior, Chihuahua, and Shih-tzu dogs within the first 10 years of their life.
Clinical signs of encephalitis reflect the area of the brain that is affected. Most animals will develop neurologic abnormalities over a short period of time (days to weeks).
Forebrain (cerebral) disease:
- Behavior changes
- Head tilt
- Facial paralysis
The diagnosis of encephalitis or meningitis is usually based on analysis of cerebrospinal fluid obtained by a spinal tap collected at the base of the skull. A significant increase in white blood cells in the spinal fluid indicates encephalitis. The brain may be imaged with an MRI or CT scan before a spinal tap to look for signs of elevated intracranial pressure and to rule out other causes of neurological disease.
Once an animal has been diagnosed, antibiotics are usually started to treat for the common infectious diseases. If the animal has seizures, anticonvulsants like phenobarbital are started. Low doses of steroids may be used if there is significant inflammation in the spine. Dogs who recover from encephalitis may develop seizure disorders and other neurological symptoms and will require continuous treatment.
In severe cases of recurrent encephalitis, more potent immunosuppressive medications are used along with prednisone. Most of these medications are types of chemotherapy and require periodic blood tests to avoid toxicity.
Cerebellar degeneration in dogs occurs when the cells within the cerebellum die. Infection with canine herpes may cause cerebellar degeneration in dogs. A genetic predisposition for the condition is possible in some breeds including Irish Setters, Wire-Hair Fox Terriors, Samoyeds, Chow Chows, Rough-Coated Collies, Boarder Collies, Bullmastiffs, Labrador Retrievers, Beagles, Kerry Blue Terriors, Bern Running Dogs, English Pointers, Gordon Setters, Brittany Spaniels, American Staffordshire Terriors, and English Bulldogs.
- Loss of coordination
- Abnormal posturing
- Muscle tremors
An MRI may reveal a smaller than normal cerebellum but a biopsy is the most definitive means of diagnosis. Routine blood and urine testing may be necessary to rule out other disease conditions which may appear similar.
There is no curative treatment, the medications such as Amantidine, Buspirone, Co-enzyme Q10, and Acetyl-L-Carnitine have shown some promise. This type of brain disease in dogs can lead to poor coordination and decision making by the dog. Restrict the dog’s activity to safe areas of the household to avoid stairs, sharp objects, and swimming pools. With loss of coordination, dogs with cerebellar degeneration may require physical aid in eating.
A brain tumor is a cancerous mass inside the cranial cavity and can either be primary (arising from the cells of the brain and its lining), or secondary (arising elsewhere and spreading to the brain).
Primary Brain Tumors
This is the most common primary brain tumor in dogs. It arises from the arachnoid mater of the meninges. These tumors occur more commonly in long-nosed breeds of dog such as the Golden Retriever. Meningiomas usually grow slowly and are amenable to treatment, although more malignant forms do occur.
Gliomas arise from the supporting cells of the brain and are most common in breeds with short noses such as the Boxer, Bostom Terrior, and the French and English Bulldog. Gliomas can range in malignancy from low grade and slow growing, to high grade.
Choroid Plexus Papilloma
Tumors of the choroid plexus are relatively common in dogs. Due to their cell of origin, they tend to arise within the ventricular system and can block drainage of cerebral spinal fluid, thus a small tumor can cause very severe neurologic signs
Pituitary Adenoma (or Adenocarcinoma)
The pituitary gland lies beneath the forebrain and is connected by a stalk to the area of the brain called the hypothalamus. Pituitary tumors (adenomas) are common in dogs and cause “Cushing’s disease” (hyperadrenocorticism). Usually they do not cause any other neurological signs and remain outside the actual cranial cavity, but in some cases they expand rapidly and compress the brain. These tumors are known as pituitary macroadenomas (or adenocarcinomas when more malignant).
Secondary Brain Tumors
Secondary tumors represent the spread (metastasis) of another tumor to the brain from elsewhere in the body. Examples of this are hemangiosarcoma, mammary carcinoma, and melanoma. These tumors carry a very poor prognosis because they have already spread through the body. It is routine practice to take radiographs of the thorax and even to ultrasound the abdomen to check if there is evidence of cancer elsewhere in the body.
Brain tumors cause signs by compressing or invading the brain. The resulting signs relate to the area of the brain affected:
- Loss of learned behaviors
- Increased or decreased appetite and thirst
- Constant pacing or circling
- Loss of appetite or vomiting
- Head tilt (disturbance of balance)
- Circling/leaning/falling to the side of the head tilt
- Nystagmus (involuntary flicking of the eyes)
- Uncoordinated gait (characterized by dramatic goose-stepping)
- Head tremors
- Wide based stance
The brain can be imaged using an MRI or CT scan following a complete physical and neurological examination and routine bloodwork. Thoratic radiographs check to see if there is evidence of spreading (metastasis) of the cancer to the lungs. Tumor type can be suspected from the appearacnce of the mass on CT or MRI, but can only be definitively identified by taking a sample of the tumor, wither at surgery or biopsy. Not only will this identify the tumor type, but it will also grade the malignancy of the tumor.
The aim of surgical removal of a brain tumor is either to cure the disease by complete removal or to alleviate the clinical signs by decompressing the brain. This can be lifesaving if the mass is very large. Brain tumors can be removed surgically if they are located in an area that can be safely reached. Meningiomas tend to be located on the surface of the brain and are therefore the best candidates for surgical removal, while Gliomas are more difficult to remove because they lie deep within the substance of the brain.
We do know that radiation will slow the rate of growth of most types of brain tumor. The full dose of radiation is administered in fractions, the number and timing of which vary between veterinary institutions. It is common to see a change in hair color in the area irradiated. In order to complete a course of radiation safely, the animal must be healthy enough to have a general anesthetic for each dose of radiation.
The brain is protected from circulating substances in the blood by the blood brain barrier, and this barrier limits the effectiveness of chemotheraphy agents. As a result, chemotherapy has not been advocated all that often for treatment of canine brain tumors. Alkylating drugs such as lomustine (CCNU), carmustine (BCNU) and temozolomide can cross this barrier and may be effective when treating gliomas. There are dose limiting side effects on the liver and bone marrow with these drugs so organ function must be monitoried closely during treatment.
Any dog with a brain tumor that has seizures will be placed on an anti-epileptic drug such as Phenobarbital. Tumors tend to cause the accumulation of fluid (edema) around them and this can be treated with a corticosteroid such as prednisone. As many of the clinical signs can be due to the edema, some animals show a dramatic improvement within 24 hours of starting treatment with prednisone. This response is often short lived as the tumor itself is not being treated by this drug.
This material is provided for educational purposes only and is not intended to diagnose or treat any disease or condition. All specific treatment decisions must be made by you and your local, attending veterinarian.