Page 84 - Canine Lameness

56 4 The Neurologic Examination

4.3.5.2 Hopping Reaction
The hopping reaction of the thoracic limb is tested with the animal facing in the same direction as
the examiner and one thoracic limb lifted off the ground (Video 4.2). As the weight is increased on
the tested limb, the animal’s ability to maintain full limb extension is observed (evaluating muscle
tone/strength). The patient’s weight is then shifted laterally and the appropriateness of reposition-
ing the foot (i.e. hopping) is interpreted. Important components of the response that are evaluated
include initiation, movement, and support throughout the test. Testing in the pelvic limbs is com-
pleted in a similar fashion but with the examiner facing away from the patient (Video 4.2).
Hopping response testing is more sensitive than other postural reactions, especially when minor
deficits are present. Not only is proprioception tested – indicated by how quickly the patient moves
the limb as the examiner moves the shoulder or hip laterally – but strength and muscle tone are
assessed as the patient bears weight on the limb. Poor initiation of correction suggests sensory defi-
cits (i.e. proprioceptive) while weak follow‐through suggests motor deficits (i.e. paresis).
If pain is a component of the clinical picture, it may affect postural reactions that rely on muscle
strength such as hemiwalking, wheelbarrowing (Video 4.2), and hopping. In these situations, the
results should not be overinterpreted.

4.3.6 Muscle Mass and Tone
Muscle mass will be affected differently depending on the underlying disease. If innervation to the
muscle is compromised, there is disruption in trophic factors and neurogenic (i.e. denervation)
atrophy occurs. These changes take place very rapidly, starting within several hours of injury, and
become most noticeable in 7–10 days. Conversely, mechanical unloading of the muscles, seen with
either musculoskeletal or UMN disease, can cause disuse atrophy. This atrophy occurs very slowly
and becomes apparent only after several weeks to months following onset. Knowing how rapid
muscle atrophy occurred can greatly influence the list of differential diagnoses. For example, rapid
focal muscle atrophy occurring in the thoracic limb with an associated lameness should alert the
clinician to a higher likelihood of a neurogenic origin, such as a nerve sheath tumor.
Muscle tone, along with gait assessment, will help differentiate UMN from LMN lesions. Through
passive manipulation of the limb, the degree of muscle tone is assessed, especially of extensor
muscles. Dysfunction in the UMN system can cause muscle hypertonicity appearing as a spastic
paresis/paralysis; however, normal muscle tone is present frequently. LMN dysfunction causes
muscle hypotonicity and a flaccid paresis/paralysis. Decreased muscle tone is a key feature specifi-
cally localizing to the LMN system.

4.3.7 Spinal Reflexes
Normally during gait and posture, spinal reflexes (also called segmental spinal reflexes) maintain
the limbs in extension to support the animal’s weight. Evaluation of these reflexes should be con-
sidered a continuum of gait evaluation and postural reaction testing; not as a sole entity. Disruption
of sensory input, the associated spinal cord segments, or LMN output will result in decrease or loss
of reflex activity. Spinal reflexes are a dominant factor in differentiating UMN from LMN disease.
A normal sensorium (consciousness) is not required to elicit these reflexes; they will remain intact
as long as the local reflex arc (i.e. the cell bodies and sensory and motor nerves) is intact, even if the
spinal cord cranial to the lesion is completely transected.
Two types of reflexes are evaluated in the limbs (Table 4.3): myotatic and flexor reflexes. Myotatic
reflexes are monosynaptic (two neuron pathways; i.e. one synapse) stretch reflexes and include the