4 Neurologic Exam in Horses

Neurologic Examination in Horses

• Recognise neurologic dysfunction and perform relevant examinations
• Describe the relationship between neurologic deficit and neuroanatomic localization with emphasis on differentiating cortical, brainstem, cerebellar, and spinal cord dysfunction
• Distinguish between upper and lower motor neuron deficits
• Identify focal, multifocal, and diffuse lesions

History and Observation

• Key information includes age, breed, gender, disease progression, and environment
• Observe the horse in unfamiliar surroundings to assess mentation and vision

Neuroanatomic Localization

• Neuroanatomic localization is based upon loss of neurologic function
• Cortical dysfunction can manifest as abnormal mentation (altered consciousness), seizures, head pressing, and blindness with a normal pupillary light reflex
• Brainstem issues may present as cranial nerve deficits, altered consciousness, and gait deficits
• Cerebellar dysfunction affects depth and direction of motion, potentially causing intention tremor, opisthotonos, and absent menace response, though strength is typically preserved
• Spinal cord issues typically cause ataxia, weakness, and spasticity of the limbs
• Determining if lesions are diffuse, focal, or multifocal is key for differential diagnoses

Cranial Nerve Examination

• CN1 (olfactory): Assess interest in feed
• CN5 (trigeminal): Check for facial symmetry, as it innervates muscles of mastication
• CN7 (facial): Responsible for muscles of facial expression
• Menace Response: Involves CN2 (optic) as the afferent and CN7 (facial) as the efferent
• Pupillary Light Reflex: Involves CN2 (optic) as the afferent and CN3 (oculomotor) as the efferent
• Palpebral Response: Involves CN5 (trigeminal) as the afferent and CN7 (facial) as the efferent
• Strabismus: Indicates abnormal ocular position
• Elevation of the head can improve detection of certain deficits
• CN3 (Oculomotor) function is ventrolateral
• CN4 (Trochlear) function is dorsal deviation of the medial angle
• CN6 (Abducens) function is medial
• CN8 (Vestibulocochlear) function is ventrolateral and has UMN input
• Nystagmus involves rapid, rhythmic eye movements
• Physiologic nystagmus (Doll's eye response) requires normal function of cranial nerves III, IV, VI, and VIII
• Pathologic nystagmus indicates CN8 (vestibulocochlear) or cerebellar dysfunction
  • Peripheral damage leads to horizontal nystagmus with a fast phase away from the lesion
  • Central damage can cause horizontal, vertical, or rotary nystagmus, with the direction changing based on head position
• Facial sensation is mediated by CN5 (trigeminal) in the nasal passages
• Gag reflex involves CN9 (glossopharyngeal) and CN10 (vagus)
• Tongue tone is assessed via CN12 (hypoglossal)

Horner's Syndrome

• Sympathetic disruption can cause miosis, ptosis, 3rd eyelid prolapse, enophthalmus, sweating of the head and neck, and poor airflow through the nostril on the affected side
• The sympathetic nerve pathway includes the intermediate grey column T1 to T3, vagosympathetic trunk, cervical thoracic ganglion, cranial cervical ganglion, traveling caudomedial to the tympanic bulla and with the ophthalmic branch of CN5
• The nerve innervates smooth muscle of the periorbital area, eyelids (Mueller's muscle), third eyelid, iris, blood vessels, and sweat glands of the head and neck

Limb Assessment

• Assess limbs for weakness, spasticity, and ataxia
• Manipulation is used to test proprioceptive positioning like crossing limbs, placing limbs wide, or on the dorsal hoof wall
• Circling can indicate ataxia through circumduction of limbs or posting of a pivotal limb
• Tail pull assesses weakness when inclining
• Weakness, spasticity, and ataxia are graded from 0 to 5, noting symmetry
  • Grade 0: no deficits detected
  • Grade 1: mild deficits detected by a trained eye
  • Grade 2: deficits detected by most observers
  • Grade 3: prominent deficits detected by all observers
  • Grade 4: marked deficits, potentially causing the horse to fall during examination
  • Grade 5: The horse is unable to rise

Diagnostic Evaluation through CSF Analysis

• CSF should be obtained from the lumbosacral (LS) site in horses with spinal cord dysfunction
• CSF flows from cranial to caudal, a lumbosacral sample accurately reflects spinal cord pathology better than an atlanto-occipital (A-O) sample
• LS CSF taps are performed in standing, sedated animals
• If CSF flow is poor, the handler can occlude the jugular veins to increase CSF pressure (Quakenstat maneuver)
• If brain/brainstem dysfunction is observed, an A-O tap is indicated, requiring general anesthesia
• Damaging the spinal cord during an A-O tap is possible if you insert the needle too far

CSF Analysis Specifics

• Normal nucleated cell count is 0-5 cells/µl (small lymphocytes), and normal CSF protein concentration is < 80 mg/dl
• Hemorrhagic CSF can be iatrogenic or pathologic
  • Iatrogenic hemorrhage shows blood swirling with CSF during aspiration, clears with subsequent aspiration, clear CSF after centrifugation, and platelets identified on cytologic examination
  • Pathologic hemorrhage shows a homogenous distribution of blood during aspiration, doesn't clear with subsequent aspiration, presents a yellow supernatant after centrifugation, and no platelets are identified on cytologic examination
• Xanthochromic (yellow) CSF indicates bilirubin in CSF from RBC destruction, CNS hemorrhage happened 1 to 10 days prior
• Pandy test is a qualitative assessment of CSF immunoglobulin, layering CSF over phenol causes proteins to precipitate at the interface

Radiographic Examination

• Cervical spine radiographs assess malformation of the cervical vertebrae and vertebral canal diameter, vertebral trauma, vertebral body osteomyelitis, or tumors
• Myelographic examination uses positive contrast to identify spinal cord compression and is injected at the A-O site under general anesthesia
• Skull radiographs can detect fractures or tumors
• CT scans are used to detect fractures, tumors, or abscesses

Unusual Gaits

• Stringhalt involves exaggerated hindlimb advancement, unilaterally or bilaterally
  • Etiology is unknown in the US, but in Australia, it is related to a plant toxin
  • Pathophysiology is unknown
• Shivers involves mm fasciculation of hindlimbs when asked to back or pick up a hoof, common in draft horses and warmblood breeds, pathophysiology is unknown
• Sweeny involves suprascapular nerve damage
  • Pathophysiology is suprascapular nerve trauma at the cranial aspect of the scapula, where the nerve traverses medial to lateral to innervate the supraspinatus and infraspinatus mm
  • Neurologic examination reveals neurogenic atrophy of the supraspinatus and infraspinatus mm, a prominent scapular spine, subluxation of the scapulohumeral joint during weight-bearing (loss of rotator cuff function), and circumduction of the limb (decreased anterior phase)