Large Animal Neurology. Joe Mayhew

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СКАЧАТЬ innocuous testing for transcranial magnetic and electrical motor evoked potentials (Figure 3.10) now has received attention in large animal neurology,^85,86 and it appears to be a very sensitive and quite specific electrophysiologic test for the disruption of somatic motor pathways in disease states.^87–89 This and the additional use of more elaborate but error‐prone quantitative EMG investigations^58,60,62,70 should allow more accurate identification of neuromuscular disorders.

      Finally, measuring conduction velocity across the cauda equina can be simply performed by using one stimulating electrode placed over the dura mater at the lumbosacral space and another stimulating electrode in the sacrocaudal cauda equina, with anode in adjacent skin. Induced motor action potentials are recorded using electrodes in a ventral coccygeal muscle. No conduction from the cranial stimulating site, with some potentials recorded from the caudal site early in a case of a fractured sacrum, can reasonably be taken as the cauda equina being totally severed. In this context, measuring the maximal bladder contraction pressure and maximal urethral closure pressure is a technique⁹⁰ that should assist in better defining the site of a lesion and should particularly assist in monitoring the use of drugs that can be used to treat patients suffering from urinary incontinence. Of note is the fact that there appears to be discrepancies in reported normal values.⁹⁰ However, the measurement of intravesicular and urethral pressure profiles could well be useful in monitoring horses with urinary incontinence.^91,92

Neuroimaging

       Plain and contrast radiography

Radiography of the calvaria and vertebral column (Figure 3.11) is indispensable for identifying normal variations, bony malformations, fractures and osteomyelitis,^93–104 and even evidence of reactivity to parasites.¹⁰⁵ Undoubtedly, the use of digital radiography will continue to be used more widely allowing better routine radiographic imaging and decreased radiation exposure.

Almost certainly, the cervical region is the area most often radiographed in neurologic large animal patients. However, even though the radiographic anatomy of equine cervical vertebrae is well described, many normal variations and inconsequential findings must still be realized in interpreting such radiographs.^{99,104,106,107} The frequent finding in <5% of Thoroughbred and Warmblood horses of transposition of ventral processes of C₆ onto C₅ and more often onto C₇ on one or both sides must be considered when vertebrae are being identified on radiographs.^104,108 This can also occur occasionally in other breeds including ponies.

Figure 3.9 Brainstem auditory evoked potential (BAEP) recording is a minimally invasive procedure as being performed here in a patient suspected as having vestibulocochlear (CN VIII) nerve disease. Broad band clicks are generated (1) and passed to insert earphones. The electroencephalogram is recorded with subcutaneous electrodes and passed through a preamplifier (2) to be recorded in an electrodiagnostic unit (3). By signal averaging many individual electrical responses, a waveform is produced that represents electric activity in the brainstem because of the clicks, which can be printed out as a BAEP trace (shown below). This horse indeed is deaf in the left ear as shown by there being no brainstem electrical activity resulting when the left ear is stimulated (top two traces) compared to the normal responses recorded from the left (third trace down) and right (bottom trace) sides of the brain when the right ear is stimulated.

      Notable though usually inconsequential radiographic findings on equine cervical radiographs include the following:

       Variations and asymmetry in the shape of the intervertebral notch or orifice at the cranial border of C2.

       Irregularities to the dorsal aspect of the caudal physes of C2–C7 projecting into the intervertebral space.

       Separate ossification of the caudal projection of the transverse processes of C6.

       One or two transverse processes of C6 sometimes transferred onto C5 or onto C7.

       Serpentine lucent vascular channels on the spine of C2.

       Relative lucencies of the pedicles of the arches of the vertebral canal of C1 and C2.

       Irregular border to the caudal aspect of the spine of C2.

       Circular 3‐20 mm, cyst‐like lucencies occasionally present in the arches or bodies of all cervical vertebrae

       Variable size of, and irregular dorsal border to spines of C3‐6

       large and cranially‐projecting, irregularly‐mineralized, dorsal spinous process on C7 and T1

Figure 3.10 Magnetic motor evoked potential (mMEP) recording can be undertaken in the fully conscious cooperative patient or with the patient sedated with alpha‐2 and synthetic narcotic drugs. A magnetic stimulator (1) creates a very brief magnetic pulse in the attached coil (2) held centrally over the forebrain of the patient. At the onset of the stimulus, the generator triggers an electrodiagnostic unit (not shown—see Figure 3.9) to begin recording from the skin clip electrodes (3) placed over muscles such as cranial tibial (CT) and extensor carpi radialis (ECR) limb muscles. One or a few traces can be summated to produce mMEP waveforms that indicate the time for impulses to be produced in the brain motor centers and propagated along the central motor pathways to the peripheral nerves and ultimately the muscles from which a summated motor unit action potential can be recorded. Inset (4) shows a printout of the mMEP waveforms recorded from the ECR (top) and CT (bottom) muscles. Conduction times from stimulus till muscle response for these recordings were 24 ms to the ECR muscle and 46 ms to the CT muscle.

Figure 3.11 Radiographic evidence of a chronic lesion in the area of the occipital bursa such as the irregular mass of mineralized tissue seen here (arrows) has been suggested to be one cause for headshaking in horses. This horse was not a headshaker, and the radiographs were taken for other reasons.

Obtaining high‐quality radiographs is absolutely paramount to the accurate quantification of any measurements such as minimal sagittal diameter of the vertebral canal (Figure 3.12).^96,109,110 Standardized oblique radiographic views (Figure 3.13) can be useful in lateralizing changes seen on vertebral (and skull) radiographs especially where DV views are difficult or impossible to obtain.⁹⁷ Interpretation of plain radiographs of other СКАЧАТЬ