As the previous article showed, performing a good neurologic examination can be challenging and needs to be done in a relaxed
environment with enough time allowed for a thorough evaluation. Interpreting the findings can be difficult as well. In this
article, I help you correctly interpret the findings of a neurologic evaluation in order to answer the most important question
when dealing with a neurologic problem: Where is the lesion? Once a lesion's location is determined, a proper list of differential
diagnoses can be compiled based on the onset and progression of the clinical signs and the patient's signalment. Appropriate
diagnostic procedures can then be recommended.
SIGNS RELATED TO THE NERVOUS SYSTEM'S STRUCTURES
The central nervous system encompasses the cerebral cortex and thalamus, midbrain, pons, medulla oblongata, cerebellum, and
spinal cord. The peripheral nervous system includes the sensory, autonomic, and neuromuscular system. The neuromuscular system
is also called the lower motor neuron system and is composed of the nerve roots (including the cauda equina), peripheral nerves, neuromuscular junction, and muscles.
Cerebral cortex and thalamus (forebrain)
Together, these structures are responsible for personality, behavior (instinctual and learned), vision, and most reactions.
Signs of thalamocortical involvement include a change in personality or behavior, cortical blindness, compulsive behavior
(pacing, circling), mental confusion, and abnormal reactions (not reflexes, see "Skills Laboratory, Part 1: Performing a neurologic examination") on the neurologic examination. Seizures are also a common clinical sign.1-3 During the neurologic examination, clues of thalamocortical involvement will be obtained from evaluating the mental status
and everything that is a conscious reaction (menace reaction, reaction to a nasal sensation, conscious proprioception). The
signs and deficits are contralateral to the lesion except for circling, which is usually toward the side of the lesion.4 In cats, the most common presenting complaint of a brain tumor is a change in behavior.5 Although a change in behavior suggests cerebrocortical or thalamic involvement, it can also occur because of a change in
the pet's environment, stress, training, or other factors.
Midbrain, pons, and medulla (brainstem)
Together, these structures are also responsible for mental status, primarily alertness, through the large and diffuse reticular
formation. These areas also contain the nuclei from the third to 12th cranial nerves, the nuclei of most of the upper motor
neurons responsible for motor function, and the ascending sensory and proprioceptive tracts traveling through to the thalamus
and cerebrum. Signs of involvement of these areas include a change in mental status (mental depression, stupor, coma), ipsilateral
cranial nerve dysfunction, and motor (upper motor neuron paresis) and proprioceptive deficits (ipsilateral to the lesion).
Because the reticular formation is so large and diffuse, a brainstem lesion will virtually always cause at least some mental
alteration. To identify this mental alteration, it is important to consider your own assessment but also any changes reported
by the owner (e.g. suddenly seems older, is not as excited as it used to be, is not greeting the owner at the door anymore). If there is no
alteration in the mental status, the lesion is unlikely to be in the brainstem.
Keep in mind that a cranial nerve deficit does not always indicate a brainstem disease. Cranial nerve deficits can be seen
with a cranial neuropathy (e.g. trigeminal neuritis, idiopathic facial paralysis) or a neuromuscular problem (e.g. pharyngeal paresis associated with myasthenia gravis). Also, although proprioceptive ataxia and upper motor neuron tetraparesis
can be signs of brainstem disease, they are more often signs of a cervical spinal cord problem. The same is true for a vestibular
syndrome, which can be secondary to a brainstem disease or a peripheral vestibular system involvement (inner ear or vestibulocochlear
nerve) as well as a cerebellar disease. This is one example of why it is so important to do a complete neurologic examination
before attempting to localize the lesion.