HOW TO DIAGNOSE POOR PERFORMANCE IN THE EQUINE ATHLETE


Table 2. Current Ataxia Grading Scale Used in Horses. Modified Mayhew Scale8 Grade 0


No neurologic deficits


1 2 3


4 5


Neurological deficits just detected at normal gait, but worsened by backing, turning, loin pressure, or neck extension Neurologic deficits easily detected at the walk and exaggerated by backing, turning, loin pressure, or neck extension


Neurologic deficits prominent at the walk with a tendency to buckle or fall with backing, turning, loin pressure, or neck extension. Postural deficits noted at rest


Stumbling, tripping, and falling spontaneously at a normal gait Horse recumbent


Description


rologic deficits and grade 4 is assigned to horses that stumble, trip, and fall spontaneously.7,8 Grade 5 is reserved for horses that are recumbent. The neurologic examination is usually completed


with the horse at a walk; however, the author will have most horses trot out and back at least once to determine whether any other gait deficits become apparent at this gait. Most horses with spinal ataxia, grades 1 to 3, will be able to do this. A more compre- hensive lameness evaluation is needed in horses that have obvious lameness in conjunction with neurologic disease, and in horses that have subtle gait deficits with components that can be produced by disorders affecting the neurologic system and/or the musculosk- eletal disease, such as paresis.


Ancillary Diagnostics


Diagnostics that will improve our understanding of the horse’s neurologic system include imaging of the neck through cervical radiographs and cervical ar- ticular process ultrasonography, and cerebrospinal fluid analysis. More complex diagnostic tools that can be valuable to study the cervical spinal cord include myelography and computed tomography (CT) myelography. The neuromuscular system can be further studied using muscle and/or nerve biop- sies and through clinical neurophysiological studies. In particular, electromyography and nerve conduc- tion studies can be useful to evaluate lesions of the lower motor neuron or motor unit.7 In most areas of North America, EPM should be


one of the main differential diagnoses for horses identified with disease of the central nervous sys- tem. Unfortunately, definitive diagnosis of this disease requires postmortem confirmation of proto- zoal infection of the central nervous system. Nu- merous studies have evaluated laboratory tests for the antemortem diagnosis of EPM, and most recent research has focused on the surface antigen ELISAs, which are quantitative (end-point titer) tests based on Sarcocystis neurona surface antigens.9–11 For highest accuracy in antemortem diagnosis of EPM, the American College of Veterinary Internal Medi- cine has recommended the following steps that are outlined in the most recent consensus statement on this topic.12 First, the presence of clinical signs consistent with EPM should be confirmed by con- ducting a thorough neurologic examination. Sec-


206 2018  Vol. 64  AAEP PROCEEDINGS


ond, other potential causes should be ruled out using available tools (e.g., cervical radiography). Third, immunodiagnostic testing of serum and ce- rebrospinal fluid should be conducted to confirm intrathecal antibody production against Sarcocystis neurona or Neospora hughesi. The ratio of antibody in serum to cerebrospinal fluid (CSF) will reveal in- trathecal antibodies in most cases of EPM. The SnSAG2, 4/3 ELISA serum:CSF titer ratio and NhSAG1 ELISA serum:CSF titer ratio are the only tests currently offered commercially that provide in- formation regarding intrathecal antibody production based on serum and CSF titers.


3. Results


Undoubtedly the two most common neurologic dis- orders that can impair a horse’s gait and lead to poor performance in the United States are cervical verte- bral compressive myelopathy and equine protozoal myeloenceaphitis. Both of these conditions can af- fect the horse in a significant enough fashion such that there is obvious neurologic disease and when appropriate testing algorithms are followed the di- agnosis is made. However, both of these conditions can also lead to subtle disease accompanied by sub- tle gait deficits. Additionally, neck lesions can be a cause for thoracic limb lameness when pain cannot be localized to the limb.13 It is unclear what as- pects of the neck are painful in those situations. There are no reports in the literature of nerve root impingement causing radicular or referred pain in the horse. However, there is a report in which tra- ditional myelography was compared with post-mortem contrast enhanced computed tomography in 6 horses that demonstrated the presence of nerve root compres- sion by a malformed articular facet, at the level of the proximal intervertebral foramen in 2 horses.14 This seems to be a condition that is recognized clinically with increasing frequency and we expect that with advanced imaging and use of electrodiagnostics some of these syndromes can be diagnosed and better ex- plained in the near future. Other nervous system disorders associated with poor performance include peripheral neuropathies, space-occupying lesions, and infectious/inflammatory diseases such as neuroborre- liosis.15


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