Neuro-diagnostic tests

If your pet is unwell it can be a confusing time trying to make sense of what your vet is doing and why. There are many tests commonly used in veterinary practice that help your vet to work out what is wrong with your pet. This information sheet explains what we are looking for when they perform tests to investigate an animal with a disease affecting the nervous system. Some of these tests can be done in general practice, but others are more difficult to perform or interpret and your pet may need to be referred to a specialist for these.

Your vet will be able to get a lot of information about what is wrong with your pet by giving it a thorough examination. However, in most cases examination only provides clues to where the problem is and allows your vet to produce a list of likely causes of the problem. In order to work out exactly what is wrong with your pet, so that the best possible treatment can be recommended, your vet will need to do other tests to identify the exact cause.

If your pet has a neurological disease your vet may want to take blood or urine samples to check that your pet’s kidneys, liver and other organs are functioning well. Sometimes changes in the blood such as low sugar level, abnormal salt balance, toxins not cleared by a failing liver, or an under active thyroid gland can cause neurological signs. A simple blood test will sometimes provide the answer to your pets problem.

The CSF (cerebrospinal fluid) is the fluid that bathes the brain and the spinal cord. CSF collection (also known as a spinal tap) is indicated in most dogs with nervous system disease. This fluid can be collected from the back of the neck or in the lower back. The procedure can only be carried out under general anaesthesia and is associated with rare but significant risks.

Normal CSF contains very few cells and a small quantity of protein. Many neurological diseases, in particular inflammation of the brain, spinal cord and their covering (encephalitis, meningitis and myelitis), can cause changes in the CSF. A laboratory may find increased numbers of cells, changes in the type of cells, elevation in the quantity of protein and these changes may indicate what is wrong.

On rare occasions, certain types of cancer (such as lymphoma) can be detected by examination of the CSF. Unfortunately, taken on their own CSF changes are rarely typical of a specific disease and the results of CSF tests must be interpreted alongside the clinician’s suspicion and results of other tests (blood test, myelogram, MRI or CT-scan).

Spinal X-rays (radiographs) are commonly used in animals with neck or back pain, wobbliness and paralysis. They can reveal fractures, dislocations, infection or cancers of the spine. Spinal X-rays can sometimes indicate a slipped disc but cannot be used alone to confirm such a problem. Unfortunately, because X-rays are much better for looking at bone than soft tissues, spinal X-rays are likely to be normal in conditions affecting the spinal cord such as ischaemic myelopathy, myelitis or meningitis.

Myelography is a special kind of spinal X-ray taken after injection of a dye into the fluid that surrounds the spinal cord. This dye allows the shape of the spinal cord to be outlined so that it shows up on X-rays. Conditions such as slipped disc will cause changes in shape of the spinal cord and are frequently diagnosed using myelography. Although myelography provides accurate information about the shape of the spinal cord (and whether there is pressure on it), it does not allow your vet to look at the spinal cord tissue itself as an MRI scan do.

MRI stands for Magnetic Resonance Imaging. One of the main advantages of MRI over X-rays or CT-scans is that MRI allows a vet to see, in great detail, the brain and spinal cord tissue. MRI has revolutionised the investigation of neurological disease. MRI is non-invasive and painless although the patient does need a general anaesthetic for the procedure.

Contrary to popular belief, it is only possible to scan a small portion of the body at any one time using MRI. Each patient must therefore receive a full examination to ensure not only that an MRI is needed for diagnosis, but also that the correct part of the animal is scanned.

MRI is an essential tool in the diagnosis of brain disease. However, changes observed in the brain may not always be typical of a specific condition changes on MRI may be very similar for different diseases such as encephalitis, tumour or stroke. Other tests such as CSF analysis may help to identify what is causing the changes seen on MRI.

CT stands for computed tomography and is a special kind of X-ray which allows 3-dimensional pictures to be created. CT gives better details of bones (skull, spine, and joints) than MRI, but MRI is better for looking at soft tissues such as the brain or spinal cord.

EMG stands for electromyography. A very fine needle is inserted in a muscle to detect any abnormal electrical activity. The test is used to look for muscle disease (myopathy) or nerve disease (neuropathy). Although an EMG cannot say what disease is causing the problem, it is a useful test to show which muscles and nerves are affected. EMGs must be performed under general anaesthesia.

Biopsy (or tissue sampling) is an important tool to determine the exact type of cells within abnormal tissue. Most neuro-diagnostic tests are very good at detecting abnormality but not as good as finding out what is causing it.

For example, MRI may reveal a mass within the brain but it cannot say whether this is a cancer, abscess, inflammation, or bleeding. Although in many cases the mass will be a cancer, the only way to confirm this is to take a sample of this mass either by biopsy (using guidance of a CT-scan or ultrasound-scan) or after it has been surgically removed.

Samples of tissue from inside the brain are only rarely taken in veterinary practice. In many practices samples of muscle and nerve tissues are routinely sampled.