A number of neurologic conditions comprise what is known as "Wobbler's Syndrome" in horses. The conditions have similar presentations, but stem from varying causes.
The term "wobbler" is used to describe a horse that exhibits a wobbling gait. The abnormal gait arises from the horse's inability to sense the location of the limbs (i.e. compromised sensory input) and/or the inability to provide adequate neurogenic information (i.e. compromised motor input) to the muscles controlling the limbs. Abnormal sensory and motor function results from compression and/or inflammation of the brain and/or spinal cord. Depending on the degree and location of the compromise, horses can exhibit a variety of abnormal gait characteristics.
Additionally, clinical signs may be temporary or permanent. Trauma, infectious disease, and parasite migration can affect any or multiple areas of the nervous system and result in a clinical signs consistent with wobbler's syndrome. Furthermore, any disease causing severe metabolic debilitation or high fever (Lyme's Disease for example) can result in similar signs on a temporary basis.
Four primary areas can be affected in wobbler's syndrome. These include the brain and brain stem, the neck, the back and pelvis, or a combination of any of the above. During neurologic evaluation, veterinarians perform a
variety of tests on the horse to help determine the location and degree of the problem. Nerve function starts at the brain and moves towards the back end of the horse, so the location of the problem is typically easy to determine. The spinal cord leaves the brain and travels through a canal formed by the vertebrae (the spinal canal). Along the way, various nerves branch off of the spinal cord and head to specific areas of the body to perform their function. The nerve branches that assist in controlling the limbs are the ones affected in wobbler's syndrome.
1) Problems affecting the brain and brain stem.
Since the brain is where nerve function begins, wobblers with brain lesions usually exhibit abnormal movements in all four limbs. Lesions located here manifest through abnormal cranial nerve signs. Such signs would include impaired vision, inability to eat, drink or swallow, head tilt, etc. The primary causes of brain lesions include infectious disease (e.g. rabies, herpes, EPM), trauma (e.g. flipping over backwards), parasite migration, and rarely neoplasia (tumors).
Diagnostic imaging modalities used to examine the head include radiography, MRI, cat scanning, and videoendoscopy.
2) Problems affecting the cervical (neck) spinal cord.
Horses with neck lesions typically exhibit abnormalities in all four limbs. Since the neurons providing input to the back limbs reside outside (i.e. more superficial) in the spinal cord than those providing input to the fore limbs, they are the first to be affected. Consequently, wobblers with neck lesions will typically be worse (more wobbly) behind than they will be up front.
The most typical neck problem is cervical vertebral stenosis (CVS). CVS occurs as a result of the spinal canal not growing large enough or growing abnormally, resulting in compression of the cord residing within. Cervical vertebral malformation (CVM) or deformity of the vetebrae, malalignment of the vertebrae, exceptionally small vertebral canal, or changes associated with the articular facets (joints) or end plates of the vertebral bodies are problems that can occur in young horses (between birth and 2 years of age). Osteoarthropathy (degenerative joint disease) associated with the articular facets can affect older horses (typically over 5 years of age) and result in cord compression through synovial enlargement.
CVS is typically diagnosed via radiographic examination (x-rays). Suspicious areas of compression can be confirmed as problematic by performing a myelogram, which uses a radiographically-visible media to highlight the spinal cord within the bony vertebral canal. More than one level can be affected simultaneously.
Treatment options include rest, surgical intervention (cervical fusion) and intraarticular injection in the cases of arthrosis (joint problems).
Laminectomy is a common surgical procedure utilized to treat similar problems in people, but is extremely difficult to perform in horses and is therefore not considered as a viable option nowadays. Ventral stabilization through the use of a Seattle Slew implant has proved to be extremely effective and can be performed at multiple levels (as many as three levels have been successfully fused in racehorses). The Seattle Slew implant is a threaded cylinder that twists into place. Once the vertebral articulation is stabilized, no new bone is layed down and existing bone atrophies thereby decompressing the spinal cord over time.
3) Problems affecting the back and pelvis.
Equine Degenerative Myelopathy (EDM) is usually observed in horses less than 2 years of age and primarily affects proprioception (the ability of the horse to know where the feet are in space). The pelvic limbs tend to be most severely affected. Neurologic disease is progressive (consistently gets worse) and is difficult to diagnose. Clinical signs probably occur as a result of an oxidant injury to the nervous system. Consequently, some research studies suggest that horses with EDM require a higher level of vitamin E than normal horses. Vitamin E supplementation in horses with EDM can arrest progression of the disease in some cases. Unfortunately, diagnosis of EDM can only be confirmed through post mortem examination.
Radiographic examination of the spine is often used to assess the severity and location of back injuries (trauma) resulting in spinal cord compression.
4) Diffuse problems (i.e. problems affecting more than one area).
Equine Protozoal Myeloencephalitis (EPM) is a sporadic disease transmitted by its intermediate host, the opossum. Clinical signs generally depend on where the protozoan enters the horse's body. Most commonly, horses exhibit asymmetric ataxia (wobbling) with muscle atrophy. However, horses with EPM can exhibit a variety of symptoms including cranial nerve deficits, weight loss, lameness, and single limb deficits. EPM diagnosis is generally made after blood analysis (positive western blot titer for the protozoa Sarcocystis neurona), cerebral spinal fluid analysis (positive western blot titer and PCR), and/or positive response to empirical treatment. In many cases, negative radiographic examination of the neck is required to rule out cervical stenosis prior to confirming a diagnosis of EPM.
At present, there are four approved treatment strategies available for EPM. Rebalance (Phoenix Laboratories; pyrimethamine and sulfadiazine), Marquis (Bayer DVM; ponazuril), Navigator (Idexx Laboratories, nitazoxanide), and diclazuril which is FDA-approved but not yet on the market. All of the medications are considered to be comparable in regard to effectiveness. The horse is the dead-end host for EPM, so resistance to medication is only a problem for individuals and not for the population.
Learn more about EPM treatment HERE.
Equine herpesvirus myelopathy results from inflammation of blood vessels (vasculitis) that have been damaged from infection with equine herpesvirus type-1 (EHV-1). This problem can usually be diagnosed based on history.
EHV-1 usually affects multiple horses from a group. Clinical signs characteristically involve paralysis that starts in the back limbs and works its way forward. Diagnosis is generally confirmed through cerebral spinal fluid (CSF) tap analysis. The fluid will be absent of cells, have a very high protein content, and a characteristic orange-yellow color due to xanthachromia. Isolation of the virus from a nasal swab can also be supportive in the diagnosis of EHV-1.
Horses affected with EHV-1 within a group should be isolated to avoid exposure to other individuals. Valcyclovir is very effective at treating EHV-1, especially when implemented at the onset of disease. Acyclovir is less expensive but also less effective due to malabsorption from the GI tract in the horse.
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