Large Animal Neurology. Joe Mayhew

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Figure 3.17 Brain 3T MR images of a horse with presumptive equine protozoal myeloencephalitis {EPM]. Rostral is to the left in panel C and towards the top of panel D; the left side of the brain is to the right of images A, B & D. The images in A, B & C are T2‐weighted sequences and D is a short‐T1 inversion recovery (STIR) sequence. The transverse images A & B were obtained at the planes indicated by a & b in C. Image C is a sagittal section at plane c in A. D is a ventral view of a dorsal plane just ventral to the mesencephalic aqueduct.This 15‐year‐old Holstein hunter gelding had a 2‐week history of somnolence, right‐sided central blindness, and predominantly right‐sided limb ataxia and weakness. snSAG serum:CSF ratio was 6 (values <100 consistent with EPM). The horse was given nonstandard 7x doses of ponazuril on 2 successive days. The day before presentation, he suddenly developed mild right‐sided head tilt and facial paresis. There are at least 3 areas of increased signal intensity in the basilar regions: (i) left midbrain extending from the junction with the pons well into the diencephalon (thalamus/hypothalamus; white arrows); (ii) right midbrain (yellow arrows); and (iii) left rostral thalamus (blue arrows). These areas did not enhance with gadolinium. The visible lesions may account for the blind right eye (left thalamus), for persistent somnolence (sleep centers in the reticular formations of the midbrain and thalamus) and forelimb weakness and conscious proprioceptive deficits but spinal/cerebellar ataxia, facial paresis and head tilt may have resulted from additional un‐sectioned medullary, cerebellar or spinal cord foci. Unfortunately, the horse was unable to regain its feet after the MRI procedure and was euthanized. Histopathologic examination revealed moderate to severe lymphoplasmacytic and histiocytic encephalitis of the midbrain and thalamus, with multifocal neuronal degeneration and gliosis. Protozoal stages were not seen, possibly because of previous antiprotozoal treatment. The authors have successfully treated several other horses with similar MR lesions consistent with EPM.

Thermography

      Infrared, electronic thermography is a completely noninvasive method of determining skin temperature.^174,175 Thermography should be well suited for horses because of their short, even hair coat and because radiography of the thoracolumbar vertebral column, which is so useful in smaller patients, often contributes less to the neurologic workup of large patients.

      Superficial temperature primarily depends on cutaneous blood flow. Because many neurologic disorders can be associated with local alterations in blood flow, this diagnostic modality can help localize neuromuscular lesions.^174–177 In this manner, exercise‐exacerbated focal, thoracolumbar myopathies with associated pelvic limb gait abnormalities have been corroborated by focal and asymmetric thermographic patterns before and after exercise. Also, neurologic and disuse muscle atrophy have been associated with a lower overlying superficial temperature, when compared with the normal, opposite side. However, any such changes in skin temperature are very variable and not specific such that cutaneous thermography is not regarded as very useful for diagnosis and monitoring of spinal cord and nerve root diseases in humans^178,179 and large animals. Its utility in cases of back pain in horses^{175,177, 180} is also dubious.

      Because loss of sympathetic innervation in the horse causes demarcated cutaneous vasodilation and hyperhidrosis, thermography can be of great assistance in localizing any lesion affecting the sympathetic nervous system, particularly those involving peripheral nerves that contain sympathetic fibers. For example, the well‐known facial hyperthermia of Horner syndrome in the horse produces a characteristic, abnormal thermographic pattern.^180–186

Scintigraphy

      The use of scintigraphy in diagnostic neurology has been generally restricted to attempting to confirm cervical and thoracolumbar vertebral degenerative and inflammatory lesions,^{177,183, 185,187–191} in corroborating diagnoses of aorto–iliac–femoral arterial thrombosis^192–194 and identifying the presence of portosystemic shunting¹⁹⁵ as a cause of liver failure.

Ultrasonography

      Soft tissue lesions that cause signs of neuromuscular disease and are accessible to ultrasound beams can be imaged and have included space‐occupying brain lesions,^196,197 hydrocephalus,¹⁹⁸ aortic–iliac–femoral thrombosis^{193,199–202} and musculotendinous lesions^203,204 and muscle atrophy.²⁰⁵ This imaging modality can also be used to confirm the site of vertebral arthritis and discospondylitis^177,187 and the presence of clinical or subclinical otitis media in calves.²⁰⁶ Recently, ultrasonographic examinations performed per rectum have been used to find abnormalities of lumbosacral and L5–6 intervertebral discs and foramina and associated lumbosacral nerves of horses.^207,208 The technique also had favor in confirming the presence of enlarged vertebral articular processes seen radiographically and in guiding the administration of intra‐ and periarticular injections of medicaments at these sites,^209–214 whether they are indicated or not.

Spinal endoscopy

      The advent of microcameras has allowed the interesting but orphan technique of epidural and subarachnoid flexible endoscopy to be developed,^215,216 but these techniques are really awaiting disorders for evaluation.²¹⁷

Brain biopsy

      Sounding heroic in large animal neurology circles, brain biopsy is actually a relatively straightforward surgical exercise in human and small animal neurology,^218,219 and the goal of obtaining a histologic diagnosis is laudable. But of course the damage caused in attaining a biopsy for histologic evaluation must be kept minimal, and clinically vital regions of the brain must be avoided. However, with training,²²⁰ the application of new imaging techniques with neuronavigational guidance procedures will allow diagnostic brain biopsies to be obtained efficiently.²²¹

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