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Neurology at theVSCAN
Meningoencephalitis of unknown origin (MUO):
Meningoencephalitis is characterized by inflammation of the brain and meninges. The meninges is a thin membranous layer which cover the delicate tissues of the brain and spinal cord. MUO is an immune-mediated condition in which the white blood cells of the body attack the central nervous system – the brain, spinal cord, and occasionally the optic nerves. Magnetic resonance imaging (MRI) of the affected area and cerebrospinal fluid (CSF) evaluation are common and effective tests used to diagnose this condition. Because there may be infectious causes of meningoencephalitis (bacteria, viruses, fungal) infectious agent testing may be submitted as part of the diagnostic workup.
There are three main types of MUO: granulomatous meningoencephalitis (GME), necrotizing meningoencephalitis (NME), and necrotizing leukoencephalitis (NLE). Despite the type of MUO, the treatment is the same and it begins with immunosuppression to stop the body’s white blood cells from attacking the neural tissues. Prednisone is the mainstay therapy and it is usually used in conjunction with other immunosuppressive medications to provide better control of the disease, depending on the needs of the individual patient.
Myasthenia gravis (MG):
Myasthenia gravis is a disease of the neuromuscular junction that results in generalized weakness in dogs and cats. Acquired myasthenia gravis is an autoimmune disease that can occur spontaneously and acutely. Antibodies produced by the body’s white blood cells attack receptors for acetylcholine (a neurotransmitter responsible for muscle contraction) on the muscle. This results in a decreased number of functional acetylcholine receptors and prevents the muscles from functioning properly, leading to weakness. The clinical signs include increased lethargy, exercise intolerance, inability to walk, facial weakness, inability to blink, or even collapse. This condition often leads to esophageal dilation (megaesophagus) in dogs, which can result in regurgitation of food from the stomach. Aspiration pneumonia is a severe secondary effect of regurgitation.
To diagnose this condition, chest x-rays should be performed to check for signs of myasthenia gravis (such as megaesophagus) and to ensure that there is no evidence of aspiration pneumonia. A Tensilon test and electrodiagnostics, such as repetitive nerve stimulation, can also be performed. The most definitive test for diagnosing myasthenia gravis is a laboratory blood test called an antibody titer against acetylcholine receptor.
Steroid responsive meningitis arteritis (SRMA):
SRMA is an autoimmune disease that results in meningitis. Meningitis is a condition in which there is inflammation of the meninges, which is a thin membrane that covers the brain and spinal cord. Clinical signs include fever, with pain of the head, neck, and/or back. This condition usually occurs in young dogs and breeds that are predisposed to this condition such as the German Shorthaired Pointer, Boxer, Bernese Mountain Dog, and the Beagle. Diagnostics include imaging of the spine (x-ray and MRI) and a cerebrospinal fluid (CSF) analysis. Measurement of the immunoglobulin A (IgA) antibody in the CSF is highly indicative of SRMA. Infectious agent testing may also be performed to rule out bacterial, viral, or fungal infection. The ideal treatment for SRMA is a long-term course of immunosuppressive corticosteroids.
Immune-mediated polyarthritis (IMPA):
As the name of the condition suggests, this is an immune-mediated disorder resulting in joint effusion and pain. It is important to understand that other conditions such as infections, conditions resulting in vasculitis, severe degenerative joint disease, and cancer can also result in joint effusion as well as other immune-mediated processes.
IMPA is most common in young dogs and occurs when the body produces antibodies against membranes in the joint. Certain breeds predisposed to such conditions include Siberian Huskies, husky crossbreeds, and Bernese Mountain Dogs. Diagnosis of IMPA starts with x-rays, followed by joint taps of the affected joints. The joint tap collects samples of the synovial fluid which is submitted to a laboratory for cytology and possibly culture to rule out infection. Treatment for IMPA is with long-term immunosuppressive corticosteroids. Occasionally, adjunctive immunosuppressive medications may be required.
Idiopathic epilepsy, also known as primary epilepsy, is the most common cause of seizures in dogs. Many of these have a genetic basis for epilepsy. Several breeds are commonly present with idiopathic epilepsy and these are the Labrador Retriever, German Shepherd, and Border Collie breeds and their cross breeds. The age of onset in dogs is usually from 1 to 6 years of age. While idiopathic epilepsy may be the most common cause of epilepsy in dogs, the diagnosis is reached by ruling out other underlying causes of seizures. Bloodwork, magnetic resonance imaging (MRI) of the brain, and a cerebrospinal fluid (CSF) analysis are standard tests used to diagnose this condition. Electroencephalogram (EEG) may be recommended for the diagnostic workup in some patients to map the electrical connectivity of the brain. Examples of other diseases that can cause seizures include structural malformations of the brain (intracranial arachnoid cysts, lissencephaly. porencephaly), metabolic diseases (e.g. liver disease, kidney disease, low blood sugar, hypothyroidism, hyperlipidemia, etc.), infection, and cancer. For some patients, infectious agent testing may also be recommended.
Because seizure disorders can become very severe and difficult to control, especially in breeds predisposed to such conditions, a very close relationship with your neurologist or veterinarian is important. Keeping a journal of each seizure episode, descriptions of the event, when adjustments to medications are made, and regular rechecks of the patients bloodwork results are all pertinent for appropriate seizure management.
Intervertebral Disk Disease
Occasionally, intervertebral disk disease is managed medically and not surgically. The most pertinent factor for the success of medical therapy is cage rest with appropriate pain medications provided for comfort. Patients on cage rest should only be allowed to come out of the cage to use the bathroom and eat, and then they are to go back to cage confinement. They are not allowed to walk up/down stairs, jump up/down surfaces, engage in rambunctious play, or walk on slick surfaces. Should they come out of their crate, they are required to be on a body harness and leash at all times, including when they go out in the yard to use the bathroom or when they are allowed outside their crate to eat or to interact with their family members. Should there not be compliance to strict cage rest, this can result in decline of neurologic function and failure of conservative treatment. Despite successful medical management and initial neurologic improvement, 50% of patients that do not undergo surgery to decompress the spinal cord after suffering from intervertebral disk disease will experience recurrence in clinical signs due to issues at that same offending site. When clinical signs recur, this could be mild or worse than the initial clinical signs.
The vestibular system controls the sense of balance and maintains the posture of the body in space. A very simple analogy would be: without the vestibular system intact, one will be unable to move around without stumbling like a drunkard, let alone hold yoga postures. The vestibular system consists of structures located outside and inside of the brain. Specifically, these structures are the inner ear, the vestibulocochlear nerve, vestibular nuclei in the brainstem, and the cerebellum. Several other structures in the brain transmit these vestibular signals and allow the brain to perceive vestibular sense. Disruption of any of these structures will result in vestibular disease.
Vestibular disease can be caused by inner ear disease, cerebrovascular disease (stroke), transient ischemic attack (TIA), meningoencephalitis (brain inflammation or infection), structural abnormalities, hypertension, hypothyroidism, hemorrhage, intoxication, side effects from medications, and cancer. Some vestibular disease may be diagnosed as ͞idiopathic vestibular disease͟ when no underlying cause is found.
One of the main challenges is to determine if the vestibular disease is outside the brain (peripheral vestibular disease) or inside the brain (central vestibular disease). Occasionally, the neurologic examination is frustrating at best. This puts important emphasis on advanced imaging either with computed tomography (CT) or magnetic resonance imaging (MRI). A cerebrospinal fluid (CSF) analysis may be a necessary addition to the diagnostic process. Combining the information obtained from the patient's clinical signs, neurologic examination, imaging results, and CSF analysis we are able to diagnosis and create a directed treatment plan specific to the needs of each patient.
Neuropathy / Neuritis
Neuropathy and neuritis are conditions of the nerves. Depending on the disease process a single nerve or multiple nerves may be involved. Infection, immune-mediated causes, endocrinopathies (e.g. hypothyroidism or Cushing’s disease), degenerative diseases, nerve compression from disk disease, or cancer can all affect the nerves. Since multiple diseases can cause neuropathy or neuritis, it is important to utilize the proper diagnostic tools in order to reach the appropriate diagnosis and to institute directed therapy. Diagnostics include bloodwork, x-rays, electrodiagnostics, magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) analysis, and possibly a nerve/muscle biopsy.
Degenerative Myelopathy (DM)
Degenerative myelopathy is a neurodegenerative disease that is progressive and invariably fatal. It occurs most commonly in middle-aged to older German Shepherd Dogs, Pembroke Welsh Corgis, and Boxers. This condition is the result of a genetic mutation in the gene that encodes for the superoxide dismutase (SOD1) enzyme. SOD1 genetic mutation has also been identified as one of the genes implicated in amyotrophic lateral sclerosis (ALS) in humans. With DM the central and peripheral nervous systems are affected with the thoracolumbar spinal cord usually being the first to be affected. The disease progresses to involve the lumbar and cervical spinal cord eventually extending to the peripheral nervous system. Many other diseases can mimic DM such as intervertebral disk disease, lumbosacral disease, and cancer. Diagnosis involves first ruling out these other underlying causes for neurologic abnormalities, followed by confirmation with a genetic test for the SOD1 mutation. A patient is presumed to have DM when there are no other causes for the clinical signs and the patient tests positive for DM.