It is likely in this world of technology, that a computer program will soon be available to guide the veterinarian toward lesion localisation. No matter how sophisticated the program, the neurological examination will remain pivotal. The veterinarian will need to be able to perform the tests and more importantly interpret the responses. The time allowed for this presentation does not permit covering the entire examination. Only the tests commonly performed poorly, misinterpreted or simply forgotten are reviewed.
The neurological examination is composed of six parts: the evaluation of the mental status, gait and posture, cranial nerves reflexes, postural reactions, spinal reflexes and pain perception. Of these, it is the evaluation of the mental status and gait and posture that are the most important to localize the lesion. The mental status dictates if the lesion is intra or extra-cranial. The evaluation of the gait and posture is crucial in determining if the nature of the gait abnormalities is related to the nervous system or to a musculo-skeletal problem.
1. Mental status
The mental status is the most important clue in differentiating intra- from extracranial disease in the animal with cranial nerve abnormalities. The abnormalities of the mental status relate to the brainstem or to the thalamocortex. The mental status cannot be evaluated in the examination room, even if the animal has had time to relax. In most cases, the evaluation is obtained from the history, not from observation of the animal. Dogs and cats are excited and/or fearful in the hospital environment. This adrenaline surge hides the mental changes, especially if subtle. The history is crucial and must be thoroughly performed.
a. Brainstem: State of consciousness
The bulk of the brainstem parenchyma is made of the reticular formation or the so-called Ascending Reticular Activating System (ARAS). This formation is responsible for the arousal of the cerebrum. When this system is affected, the animal becomes somnolent, lethargic, stuporous or comatose. However, he is aware of his surrounding because his intelligence (cerebrum and thalamus) is not affected. The owner must be asked specific questions regarding the mental status of the animal: Is the animal as playful? Does he sleep more than before? Is he quieter, lethargic? If the animal is historically somnolent and this is accompanied by cranial nerve deficits, the disease is intracranial and involves the brainstem.
b. Thalamocortex: Behaviour
The thalamocortex is the site of the intelligence, and goal-directed behaviours. The abnormalities can be obvious such as with patients circling compulsively or head pressing, but this is rare. In most cases, the changes are subtle especially with chronic illness, relying only on the history to bring them out. The animal is unable to relate normally with his environment. There is a decrease or an absence of the animal’s awareness. Beware of the animal that bumps into objects. Patients that are blind because of ocular diseases are cautious while walking and rarely do bump into objects. The blindness in the patient that collides with objects is central and relates to a brain disease. Another example is the older dog becoming deaf. This may be the fact. However, ensure that the animal is aware of his surroundings. Even a deaf dog can "feel" the garage door opening. If the animal no longer greets the owner at the door, there may be a more serious problem. The brain may not register what the eye sees or the ear hears.
2. Trigeminal Nerve
The trigeminal nerve has three branches: (1) Ophthalmic (sensory) (2) Maxillary (sensory) and (3) Mandibular (sensory and motor). This large nerve innervates the masticatory muscles and provides sensation for most of the head. The masticatory muscles are palpated for symmetry and atrophy. The sensory part of the nerve is assessed mainly through the palpebral reflexes. In these reflexes, the trigeminal nerve is the afferent limb (sensory), while the facial nerve (VII) is the efferent limb (motor). The evaluation of the ophthalmic branch is assessed by touching the medial canthus (medial corner of the eye where the lid margins meet) and observing for a blink reflex. Touching the lateral canthus and observing for a blink reflex assesses the maxillary branch. Touching the base of the ear and observing for a blink reflex assesses the mandibular branch. The indicated areas to touch are the only ones that consistently give a blink reflex in the normal animal. Touching the canthi and the ears assesses the reflex arc, i.e., the cranial nerves V and VII but DOES NOT assess if the information gets to the cerebral cortex. This is evaluated by stimulating the nasal septum. The animal may or may not blink but the head will be pulled away as a pain response. This test should be done gently using cotton swabs (Q-tips) to reveal subtle differences between sides. Start with a gentle stimulus and gradually increase the strength of the stimulus going from side to side, until a consistent response (normal or not) is obtained. The nasal septum is the only area that consistently elicits a pain response in the domestic species. This latter test provides an evaluation of the cortical response to pain.
3. Gait and posture
The evaluation of the gait is important in lesion localization. For the small cat or dog, the examiner lets the animal walk freely on the floor of the examination room while taking the history. Most hospital rooms do not allow a good examination of the gait because of their small size. It is preferable that the medium to large size dog be leash-walked by the owner in the parking lot, back yard or sidewalk. Use a non-slippery floor such as concrete, grass, asphalt, etc. The dog is evaluated at different paces, towards and away from the examiner. The clinician observes the front limb gait as the animal is coming toward him/her and the hind limb gait as the animal is going away from him/her. Particular attention is given to the foot placement when the animal turns or changes speed. A series of questions are then answered. The questions should not be left unanswered. If unsure, look at the gait again and again.
1. Is the animal able to walk?
2. If the animal is walking, is the gait normal?
3. Which limb(s) is/are affected: one limb, both hind limbs, the hind and front limbs, or the ipsilateral limbs only?
4. Is there ataxia?
5. If there is ataxia, of which type: vestibular, cerebellar or proprioceptive?
4. Proprioceptive positioning
Performed appropriately, proprioceptive positioning (knuckling) is an invaluable tool to evaluate proprioception. To evaluate proprioceptive positioning of the hind limbs, the clinician is positioned behind the animal. The animal's weight must be supported with a hand (and forearm in the large dog) between and behind the hind limbs. For the evaluation of the front limbs, the examiner's hand is placed between the limbs and behind or in front of the limbs. The paw is then slowly knuckled over. The flexion of the toes stimulates hundreds of proprioceptors located in the tendons and joints of the toes. The test must be done with care to enhance subtleties. A pet that knuckles does not necessarily have proprioceptive deficits. Orthopaedic problems, generalized weakness and patient’s personality may all have an effect on the test. The test results must be taken in the light of the entire examination.
5. Patellar reflexes
The patellar reflex (tendinous or monosynaptic reflex) is elicited by tapping the patellar tendon. There are two components to the reflex: the ascending sensory pathway and the descending pathway or lower motor neuron. Both must be intact for the reflex to be present. To elicit the reflex, the limb is positioned in a relaxed flexion to tighten the patellar tendon. This reflex is the most often erroneously found to be absent. It is important to pay attention to details when eliciting it to ensure its absence is real and not the result of poor technique. If the reflex is absent yet the animal can walk and/or stand, the lesion likely involves the sensory pathway and not the lower motor neuron.
6. Withdrawal or flexor reflexes
The withdrawal (flexor) reflexes are difficult to examine objectively because in most of our patients, purposeful movements are still present (so pain perception is as well) and the animal inhibits the response, leading to a falsely decreased reflex. A normal animal should be able to bring his body to the examiner's hand while flexing the limb. The front limb reflexes can be used as a comparison for the hind limbs and vice versa. Musculo-skeletal disorders may affect the nature of the flexor and patellar reflexes. The patellar reflex may be absent or decreased in a dog with a cruciate rupture because the tension in the tendon cannot be raised. Hip dysplasia, polyarthritis and muscle diseases may lead to decreased flexor reflexes because the animal is too weak to withdraw his limb with force.
The neurological examination should be done in a methodical and stringent manner every single time paying attention to details. A neurological form should always be filled. Although the examination when performed on a regular basis becomes a routine, the form serves as a "check list" ensuring that all parameters have been evaluated. More importantly, it is an invaluable document in the follow up and monitoring of progressive and protracted nervous system diseases.