Neuro Exam of The Horse - Pathology

It is possible to test a horse’s conscious proprioception by altering the position of the limbs in space.
- Horses should normally replace their limbs to the normal position within a few seconds.
  - There is a degress of individual variability, however, and the test can be difficult to interpret.
Tests can also be performed by walking or trotting a horse and then suddenly stopping.
- The animal should be observed to see how quickly it replaces its legs in a normal position.

Gait is a combination of higher control of motor function and unconscious proprioception.
During assessment of gait, the examiner is looking primarily for evidence of ataxia and/ or weakness.
- Each leg should be assessed in turn.
Defects may be graded on a five-point scale (the Mayhew scale).
- 0 is normal and 5 is recumbent.

Defects in unconscious proprioceptive pathways result in ataxia.
- These pathways originate in muscle spindle stretch receptors and golgi tendon organs.
- Impulses are relayed via peripheral nerves to the spinal cord.
- Signals then ascend in pathways to the cerebellum.
  - Primary cerebellar diseases in horses are rare, but classically cause incoordination of the head and limbs.
    - Weakness is not a prominent feature.
    - The cerebellum communicates with the vestibular nuclei, and so cerebellar diseases may sometimes present with vestibular signs.
Manifestations of ataxia:
- Exaggerated movements - hypermetria
- Reduced movements - stilted hypometric movements
- Both exaggerated and reduced movements - dysmetria.
Animals may also
- Sway
- Place their feet abnormally
  - Horses cross or weave their feet, with abnormal abduction or adduction.
Signs of spinal ataxia may be exaggerated by lifting the horse's head.
- This is beacuse it removes the horizon.
- Similar responses can be generated by walking up and down slopes.
Circling the horse can also reveal ataxia.
- The horse may plant its feet and rotate around them without placing them properly.
- Outward rotation of the pelvic limbs is also a common sign.
  - The term for this is "circumduction".
Unconscious proprioception may also be tested by backing the horse.
- Normal horses back with a diagonal gait.
- Horses with spinal ataxia will often pace when backing.

Weakness usually manifests as toe dragging, stumbling, or knuckling over.
- It is a good idea to look closely at the horse’s hooves to see if any are abnormally worn.
Weakness may be caused by dysfunction of either the upper or lower motorneurons.
- Lower motor neuron weakness.
  - Accompanied by:
    - Muscle flaccidity
    - Muscle atrophy if chronic
  - May sometimes appear similar to lameness.
- Upper motor neuron weakness.
  - Presents as a lack of control of limb muscle movement.
  - Tested most easily tested by pulling the tail during walking.
    - Pulling the tail while stationary probably primarily tests lower motor neuron control and muscular strength of the pelvic limb.

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.