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 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.

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.

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 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.

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

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.

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.