Site of Spinal Lesions Determination - Horse

• If there are no brain or brainstem signs, an animal with ataxia or weakness is most likely to have a lesion in the spinal cord.
• The spinal reflexes used in small animals to assess specific segments are not very useful in adult horses.
  • However, withdrawal reflexes are useful in recumbent horses with severe lesions.
    • A stimulus is applied to the distal limb and the reflex results in limb withdrawal.

Cervical Lesions

• Cervical lesions occur from C1 to C5
  • C6 and C7 may also be included.
• Horses with defects in all four legs are most likely to have a cervical lesion.
• The pelvic limbs tend to be more severely affected than the thoracic limbs.
  • This is due to the topographical arrangement of the white matter tracts in the spinal cord.

Effects on Reflexes

• Cervicofacial reflex
  • The cervicofacial reflex is tested by lightly stimulating the skin of the lateral neck.
    • In an intact reflex movement of the lips should be observed.
    • This reflex varies widely between individual horses.
    • This is not intact in cervical lesions.
• Withdrawal reflexes
  • Only of use in recumbent horses.
  • In cervical lesions, reflexes in both thoracic and pelvic limbs may be exaggerated.
    • This is due to a loss of upper motor neuron inhibition.

Brachial Lesions

• Brachial lesions occur from C6 to T2.
• Signs of a spinal lesion at this level differ in the thoracic and pelvic limbs.
  • Thoracic limbs
    • Lower motor neuron signs
      • Weakness
        • May be mild
        • May manifest unwillingness to take the weight on the other thoracic limb when one is picked up.
      • Muscle atrophy
    • Withdrawal reflex in recumbent animals may be reduced.
  • Pelvic limbs
    • Ataxia
    • Upper motor neuon weakness
      • For example, swaying and stumbling.
    • Withdrawal reflex in recumbent animals may be exaggerated.

Thoraco-lumbar Lesions

• Thoraco-lumbar lesions occur in the segments T3 to L3 of the spinal cord.
• In an animal that has pelvic limb ataxia and weakness but no thoracic limb involvement, the lesion will be between T2 and L3.
  • I.e. caudal to the brachial plexus.
  • The animal should be closely examined for symmetry over the gluteals and pelvic limbs.
    • If there is symmetry, the lesion is likely to be at this level.
    • If there is no symmetry, the lesion is likely to be more caudal, and involve the pelvic outflow.
• Asymmetry may be detected in the cutaneous trunic reflex.
  • This is tested by lightly stimulating the skin of the lateral thorax.
• The withdrawal reflex in the pelvic limb may be exaggerated.
• Flexibility of the thoracolumbar spine may be assessed.
  • The skin of the longissimus musculature is firmly stimulated, and observed for:
    • A lordotic movement over the thorax and cranial lumbar region, and
    • A kyphotic movement over the caudal lumbar region and gluteals.

Lumbosacral Lesions

• Lumbosacral lesions occur between the segments L4 and S3.
• Horses with such lesions may:
  • Exhibit atrophy of one or more muscles of the pelvic region, and so
  • Be easy to pull over when standing, and
  • Be unwilling or unable to take weight on the affected side when the other limb is lifted.
• More caudal lesions may:
  • Affect the anal and perineal reflexes.
    • These are tested by lightly stimulating the skin surrounding the anus and observing for a reflex contraction of the anal sphincter.
  • Cause paresis of the tail.
• The withdrawal reflex may be reduced or absent in the pelvic limb.

Sacral Lesions

• Damage to the savral spinal cord can affect bladder function and control of defaecation.

Bladder Incontinence

• The bladder is reflexely controlled, via higher order neurones in the caudal brainstem and cerebrum, by the:
  • Pelvic nerve
    • Parasympathetic fibres leave the spinal cord in the sacral segments and combine to form the pelvic nerve.
      • These fibres innervate the smooth detrusor muscle of the bladder wall.
  • Pudendal nerve
    • The pudendal nerve innervates the striated muscle of the bladder neck.
• During micturition, the bladder is evacuated as:
  • The striated muscle is relaxed.
  • Stimulation of the pelvic nerve causes the detrusor smooth muscle to contract.

Faecal Incompetence

• Lesions in the sacral region may also be manifest as faecal incompetence.
  • In horses, problems are usually caused by defects in the lower motor neurons in the pelvic nerve.
    • Faeces is retained.