December 2004 Neurology Joane Parent DMV, MVetSc, ACVIM Neurology University of Guelph A Practical And Simple Approach To The Neurological Examination It is likely that a computer program will soon be available to guide the veterinarian toward lesion localization. No matter how sophisticated the program, a neurological examination will have to be done and the veterinarian will have to be able to perform the tests and more importantly interpret the results. The examination presented herein is the summation of twenty years of neurological practice. Only tests that give consistent responses in all normal dogs and cats have been kept. There are 6 parts to the neurological examination: Mental status Cranial nerves Gait and posture Postural reactions Spinal reflexes Response to pain Of these, (1) the evaluation of the mental status and (2) the evaluation of the gait and posture are the two most important in terms of lesion localization. Mental status In the neurological patient, the mental status evaluation offers the most important clue in differentiating intracranial from extracranial disease. Abnormalities of the mental status relate to the brainstem, or to the thalamocortex. The mental status is not evaluated in the examination room. It is obtained from the history. One cannot overemphasize the importance of history taking. The animal rarely displays in the hospital setting the abnormalities that exist in the home environment as the adrenaline rush created by coming to the veterinary facility improves the animal's behaviour. The owner must be asked specific questions regarding the mental status of the animal. Is the animal as playful? Does the animal sleeps more than before? Is the animal more quiet or lethargic? Is it deaf? Does it greet the owner at the door? Etc. Brainstem: State of consciousness The bulk of the brainstem matter 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's arousal state is affected. The dog or cat becomes somnolent. This may progress to a stupor or comatose state. In brainstem disease, when the animal can be aroused (somnolence, stupor), its behaviour is appropriate since its intelligence (cerebrum and thalamus) is unaffected. When questioning about the arousal state in the home environment, it is important to enquire about the animal's behaviour when it is non-stimulated. Dogs in early stages of brainstem disease respond to commands or sound such a refrigerator door opening or a leash being picked up. These stimuli are strong stimuli for a dog. It is more important to know if the dog or cat follows the owner from room to room or if he is as playful. Does the animal initiate play? If a cranial nerve deficit is associated with historical somnolence, the disease is localized in the brainstem. Thalamocortex: Behaviour This is the site of intelligence, of goal directed behaviours. Rarely, the abnormalities are obvious such as with patients circling compulsively or head pressing while being examined. Most commonly, the abnormalities are unnoticed in the hospital environment relying only on the history to bring them out. In thalamo-cortical diseases, there is a decrease to an absence of awareness. The animal does not relate normally to its environment. There is a disinterest, a disconnection with the surroundings. Be aware 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 is likely secondary to thalamo-cortical diseases in these cases. Another example is the older dog becoming deaf. This may be the fact but substantiate the deafness reported by the owner. Even a deaf dog should "feel" the garage door opening. If the animal does not anymore greet the owner at the door, there may be a more serious problem. The old dog that must be touched or heavily stimulated to wake up is in most cases cognitively impaired. It is not hearing that is deficient. In these cases, it is the visual and auditory cerebrum that does not register what the eye sees or the ear hears. Owners explain their animal's behaviour based on their life experience. From the veterinarian's point of view, it is frequently difficult to separate mental status abnormalities related to the cerebrum from those related to the brainstem. The information gathered during history taking must be critically appraised keeping in mind the function of each of these two major parts of the brain. Cranial nerves There are twelve pairs of cranial nerves numbered I to XII, from the most rostral to the most caudal. Counting mentally from 1 to 12 as the exam is performed ensures that all nerves are examined. Cranial nerves I (olfactory) and II (optic) are located in the rostral part of the brain, at the base of the thalamocortex (rostral fossa). The nuclei of nerves III (oculomotor) to XII (hypoglossal) are located in the brainstem. THALAMOCORTEX: OLFACTORY N. OPTIC N. MIDBRAIN: OCULOMOTOR N. TROCHLEAR N. PONS: V. TRIGEMINAL MOTOR N. ROSTRAL MEDULLA: TRIGEMINAL SENSORY N. ABDUCENT N. FACIAL N. VESTIBULO COCHLEAR N. CAUDAL MEDULLA: GLOSSOPHARYNGEAL N. VAGAL N. ACCESSORY N. HYPOGLOSSAL N. In brainstem diseases, the cranial nerve deficits dictate the location of the lesion within the brainstem. Knowing the location of the nuclei of each of the cranial nerves within the brainstem helps in lesion localization. Just as an absent spinal reflex indicates the location of the lesion within the spinal cord, the cranial nerve deficits do the same for the brainstem. No reliable clinical tests exist to evaluate nerve I (olfactory). Nerve XI (accessory) has axonal input from cervical spinal nerves 1, 2, and 3. Deficits from nerve XI are masked if the other nerves are intact. As a consequence, nerves XI and I are not examined. II. Optic nerve and visual pathways (Menace response) The role of the optic nerve is to carry the image from the retina to the visual cortex. The menace responses evaluate the visual pathways as a whole (retinas, optic nerves, optic chiasm, optic tracts, optic radiations, occipital cortices), not only the optic nerves. It also requires cognitive function (the brain must be capable of registering the "menacing" gesture), facial nerves and cerebellum. All these structures must be intact to obtain normal responses. The visual pathways include two parts: a lower extra-parenchymal part made of the retinas, optic nerves and optic chiasm; and a second intra-parenchymal part made of the optic tracts, optic radiations and occipital cortices. The menace gesture must be performed adequately to obtain a reliable response. This is not a reflex because it involves the cerebrum. How to perform the test: The "menacing" gesture must be done at least one and a half feet from the animal's head. Avoid fanning the animal. The hand that does not menace holds the animal's head while covering the eye that is not examined. The responses from each side are compared. III. Oculomotor nerve: pupillary light reflexes. The retinas, optic nerves and optic chiasm represent the afferent pathways of the pupillary light reflexes, while the oculomotor nerves (III) are the efferent pathways. The menace gesture when present, assuming that frontal lobes, facial nerve and cerebellum are intact, indicates integrity of the visual pathways. If the pupil is dilated and non-responsive to light, then it is nerve III (oculomotor) that is affected. If the menace gesture is absent, assuming the animal can blink and there are no cerebellar signs, there is dysfunction somewhere along the visual pathways. If the pupillary light reflexes are present, the lesion is above the optic chiasm, at the level of the thalamocortex. How to perform the test: The pupils are observed for (1) size at rest, (2) symmetry and (3) pupillary light reflexes. (1) Size at rest: The normal animal has larger than normal pupils upon examination. In the cat they are at times frankly widely dilated. The adrenaline rush created by the hospital environment is the reason for this dilation. In progressive cerebral space-occupying lesions, the pupils may be small. This finding should not be overlooked, as it may be an indication of increased intracranial pressure. (2) Symmetry: The pupils are evaluated for symmetry prior to stimulation with a light source. Be aware that in the cat, if there is a slightly stronger light source on one side, the pupil on that side will be smaller. To avoid this problem, the evaluation for symmetry is done in a dark room using a direct ophthalmoscope. The examiner holds the animal's head at arms reach directing the light source equally toward the animal's eyes. The pupil size is examined through the ophthalmoscope. The impression is as "a deer in the headlight". (3) Pupillary light reflexes: A strong light source must be used to overcome the stress response of the pupils. The use of a transilluminator is strongly recommended. The light beam is directed in a naso-temporal direction toward the temporal region of the retina, where the concentration of rods and cones is at its highest. III. Oculomotor nerve IV. Trochlear nerve VI. Abducent nerve These three nerves are examined as a functional unit. Look first for strabismus (permanent abnormal position of the eyeball). The oculomotor nerve is the major nerve controlling eye movements. In my experience, the "down and out" strabismus associated with nerve III paralysis is a rare occurrence. Instead, the eyeball on the affected side appears immobile in the orbit when the physiological nystagmus is induced. How to perform the test: Nerves III, IV and VI are evaluated directly by observing for presence of a strabismus and indirectly by inducing the physiological nystagmus. Physiological Nystagmus In most dogs, the test is performed with the examiner facing the animal and moving the animal's head horizontally from side to side, observing for coordinated conjugate eye movements. In miniature dogs and all cats, the examiner picks up the animal face to face, holding it from under the underarms. The index fingers of the examiner immobilize the animal's head at the same time. The animal is rotated 3601 in each direction. The nystagmus should be brisk. The physiological nystagmus involves the eye muscles, cranial nerves III, IV and VI, the medial longitudinal fasciculus (MLF) and cranial nerves VIII. The MLF is made of two bundles of fibres (one on each side) situated just medial to the fourth ventricle close to the midline. It is by the MLF that the information travels from the vestibular apparatus and cranial nerves VIII (proprioception for the head) to nerves III, IV and VI. All the listed structures above must be intact for coordinated conjugate eye movements to occur. V. Trigeminal nerve: ophthalmic, maxillary and mandibular branches The trigeminal nerve has two functions: Innervation of the masticatory muscles (mandibular n.) Sensation for most of the head. How to perform the test: The masticatory muscles are palpated for symmetry and atrophy. The sensory part of the nerve is assessed through the palpebral reflexes. In these reflexes, the trigeminal nerve is the afferent limb (sensory), while the facial nerve, the efferent limb (motor). The ophthalmic branch is assessed by touching the medial canthus (medial corner of the eye where the lids meet) and observing for a blink reflex. The maxillary branch is assessed by touching the lateral canthus and observing for a blink reflex. The mandibular branch is assessed by touching the base of the ear and observing for a blink reflex. The indicated areas to touch are the only ones that consistently give a blink reflex in the normal animal. The canthus is the region where the lid margins meet. Touching the canthi and the ears assesses the reflex but DOES NOT assess if the information reaches the cerebral cortex. This is evaluated by stimulating the nasal septum. The animal may or may not blink but the head is pulled away as a pain response. This test should be done gently using cotton swabs (Q-tips) so as 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 cortical response in the domestic species. VII. Facial nerve At this stage of the examination, the examiner has some knowledge of the function of the facial nerves because the nerves were involved in the menace responses and the palpebral reflexes. How the test is performed: The ears, eyelids, lip commissures and nostrils are observed for symmetry and function. It is helpful to lift the animal's head with a finger between the rami of the mandible, just behind the chin, to evaluate the symmetry of the lip commissures. Another way is to look at the commissures while the animal is panting. Then the strength of the palpebral closures is evaluated. The normal animal closes his eyelids completely upon the first gentle stroke of a finger on the upper and lower lids at once. The evaluation of nerves VIII (vestibulo-cochlear) to XII (hypoglossal) do not require special tests but only observation of the animal. The examiner observes for presence of specific abnormalities which if present, indicate involvement of the nerve being evaluated. VIII. Vestibulo-Cochlear nerve The cochlear part of the nerve can be objectively assessed only with the use of electrodiagnostic testing (Brain Auditory Evoked Responses). The vestibular part is evaluated by observing for presence of a head tilt (salient feature of vestibular dysfunction) and abnormal nystagmus. The presence of a head tilt is best assessed as the animal is walking away from the examiner or, when the animal is facing the examiner, by pulling an imaginary line through both eyes. The line should be horizontal. IX. Glosso-pharyngeal and X. Vagal nerves These nerves are evaluated together as a functional unit. Their abnormalities are disclosed from history taking. Airway compromise (larynx) leads to difficulty breathing, dyspnea, snoring, voice change. Pharyngeal problems lead to dysphagia such as choking while eating, or drinking. XII. Hypoglossal nerve At this stage, an experienced examiner has already observed the animal licking its lips during the examination. If not, open the mouth of the animal and observe the tongue. Look for deviation and atrophy. Gait and posture Most hospital rooms do not allow a good examination of the gait because of their small size. If necessary, the dog is walked in the parking lot, yard or sidewalk. Use a non-slippery floor such as concrete, grass, asphalt, etc. The gait is evaluated with the owner leash-walking the dog at different paces, back and forth, in direction of 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 as the animal turns or changes direction and speed. A series of questions are then answered. Is the animal able to walk? Is the gait normal? Which limb(s) is/are affected? One limb, both hind limbs, the hind and front limbs, or the ipsilateral limbs? Is there presence of ataxia? If there is ataxia, of which type? Vestibular, cerebellar or proprioceptive? Is there postural abnormality? Hunching of the back, low head carriage, opisthotonus, etc. The clinician should not leave these questions unanswered. The evaluation of the gait is extremely important in lesion localization. If unsure, look at the gait one more time. Cats often do not readily perform and may blatantly refuse to walk. It is occasionally helpful to have the owner video the animal in its environment, especially if the complaint relates to the gait. The examiner leaves the cat or small dog, free on the floor of the examination room, while the history is taken. It helps not only in evaluating the gait but also in evaluating the animal's behaviour. There are three types of ataxia: Vestibular, Cerebellar and Proprioceptive: Vestibular ataxia This ataxia is ALWAYS associated with a head tilt. With this ataxia, the animal leans, falls or rolls toward the side the head is tilted. Be aware of the animal that circles without a head tilt. The lesion involves then the thalamocortex and not the vestibular system. With bilateral vestibular dysfunction, the head tilt is often subtle on the side that is more affected. The head carriage in these cases is striking. The head moves with wide excursions. As mentioned previously, the presence of a head tilt is evaluated from the back of the animal observing is the ear on the side of the tilt is lower to the ground, or by pulling an imaginary line across both eyes. The line should be horizontal. Cerebellar ataxia Strength is preserved and there are no proprioceptive deficits. The ascending proprioceptive pathways from the limbs to the cerebrum, and the descending motor pathways (upper motor neurons) from the cerebrum to the limbs (lower motor neurons) are intact. Consequently with cerebellar ataxia, there are no proprioceptive deficits (no knuckling), nor weakness. All four limbs are affected if the lesion is bilateral, or the ipsilateral front and rear limbs if the disease is unilateral. Clinically, diffuse involvement of the cerebellum is more frequent than unilateral involvement. Proprioceptive ataxia This ataxia is also called sensory (loose term as vestibular ataxia is also a type of sensory ataxia) or spinal ataxia (rather restrictive as brainstem disease can also lead to proprioceptive ataxia but the ataxia is much milder). The ataxia is secondary to damage of the ascending proprioceptive pathways. There is however a concomitant weakness because of the simultaneous involvement of the descending motor pathways (UMN). It is the presence of this weakness that helps in differentiating this ataxia from cerebellar ataxia. It is the most difficult of the three types of ataxia to recognize. Ruling out the two others is often easier. Postural reactions The postural reactions are of little localizing value. Their usefulness is in identifying asymmetry between sides. Proprioceptive positioning (knuckling) and hopping are the only two postural reactions that I perform in the dog. Lateral tactile placing of the front and hind limbs is added in the cats that will not let the examiner do proprioceptive positioning. Proprioceptive positioning and lateral placing are tests that evaluate proprioception. Performed appropriately, they are invaluable. Hopping is unspecific. How to perform the tests: 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. It is often difficult to perform this reaction effectively in the cat. The lateral placing can be used instead of the proprioceptive positioning reaction to reveal subtle proprioceptive deficits in this species. The cat is brought laterally to the side of the table. The lateral aspect of each paw is gently brought to the edge of the table. The normal cat places its paw readily on the table. Placing tactile is more consistent with the front limbs. Many cats do not react when it is performed with the hind limbs. Hopping is evaluated with each limb. There is no need to lift the animal off the ground. In fact, it is preferable and easier if the animal stays on the same plane. With the animal standing between the legs of the examiner (playing the role of a pivot), the animal's right front limb is hopped laterally away from the centre, while the examiner holds the left front limb off the ground and pushes the animal gently toward the right side. The left limb is held off the ground with the animal's shoulder joint. Avoid performing the test folding the limb over, as it is painful for the arthritic patient. The left front limb is evaluated in a reverse manner. For the hind limbs, the front end of the animal acts naturally as a pivot. The examiner is positioned behind the patient. The right hind limb is hopped laterally pushing the animal sideways with its left hind limb. This left limb is held off the ground by the tibia while at the same time the examiner pushes the animal toward the right side. The left hind limb is evaluated in a reverse manner. This test is not to be done medially. Although it has been said that subtle deficits are disclosed in this manner, it has definitely not been my experience. If the test is done medially, it is easy for the animal to "rest" in the examiner's hand. Spinal reflexes Flexor and extensor reflexes are evaluated for the front and the hind limbs. The extensor reflexes of the front limbs are evaluated by observing if the animal can bear weight on its front limbs. In the medium to large size dogs, the other reflexes are examined with the animal in lateral recumbency, the side being evaluated in the up position. For cats and small dogs, the other reflexes are evaluated with the owner holding the animal by its chest, the animal facing the examiner. The rear end of the animal is not supported. The patellar reflex (tendinous, monosynaptic reflex) is elicited by tapping the patellar tendon. An extension of the stifle is observed as the patellar tendon is tapped. The limb should be in a relaxed flexion. The flexion should be just enough so the tendon is tight. The tendon is first palpated. Then while keeping the fingers on the tendon, the limb is flexed until the tendon feels tight. The tapping on the tendon is done with a pendulous motion. If the limb is tense, the reflex will not be elicited. Keep tapping the tendon rhythmically and the animal will relax with time. The strength of the reflex is proportional to the force applied to the tendon. The withdrawal (flexor) reflexes are difficult to examine objectively as in most of our patients, purposeful movements are still present. Consequently the animal can inhibit 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 its limb. The front limb reflexes can be used as a comparison for the hind limbs and vice versa. Dogs, especially old dogs, often have weak flexors reflexes because of orthopedic problems (osteoarthritis, cruciate rupture with secondary arthritis, hip dysplasia, etc.). What is important is completeness of the flexion. The examiner observes for flexion of each of the limb articulations. The perineal reflex is assessed by gently touching the perineal region of the animal under its tail. Avoid touching the tail as it removes the surprise effect. The animal may then inhibit the reflex. The reflex is evaluated on each side, separately. The expected response is a downward contraction of the tail, more reliable than the anal sphincter contraction. Performed in this manner, the afferent limb of the reflex is the sensory part of the pudendal nerve while the efferent limb consists in the caudal nerves. To evaluate the motor part of the pudendal nerve, a rectal exam observing for the anal sphincter strength is preferable. The cutaneous trunci reflex is frequently absent in the cat. This absence is meaningless unless the reflex is present partially or unilaterally. In the dog, the reflex is consistent but it is sometimes necessary to apply stimulation multiple times to elicit this reflex. The afferent limb of this reflex is made of the regional spinal nerves, which carry the impulse to the 8th cervical spinal segment by way of the spinal cord white matter. The impulses created by pinching the skin on one side ascend the spinal cord bilaterally. The efferent limb is the lateral thoracic nerve, which causes the skin to flinch. The reflex is elicited by gently pinching the skin, from the level of the wing of the ilium to T2, approximately one inch on either side of the dorsal processes. The reflex is evaluated on both sides. If the reflex is present at the level of the wing of the ilium, there is no need to assess its presence any further along the animal's back, because the afferent limb must be intact all the way to allow the contraction of the cutaneous trunci muscles by the lateral thoracic nerve (which originates from the C8 and T1 spinal segments but mainly from C8). Pain perception Response to pain is evaluated at the end of the examination so as to keep the patient's cooperation. With experience, one realizes that it does not always need to be examined as many clues are gleaned during the examination to inform the clinician as to the presence of pain perception. In reason of the nature of our patients, the expressions "deep" and "superficial" pain should probably not be used in veterinary medicine. There is decreased, increased or absence of pain perception. Neck/Back pain The animal posture is observed during the gait evaluation. Dogs with neck pain move as a block, avoiding turning their head/neck as they change direction. They look up with their eyes only, avoiding lifting their head. Neck manipulation is unnecessary but if the veterinarian has the habit to do it, it is preferable to do it using a treat instead of forcefully moving the head/neck up, down, left and right. Enticing the dog to move his head/neck in all directions with a cookie avoid iatrognic accidents such as decompensation of atlanto-axial luxation. Dogs with back pain often have a hunched back. The back is palpated for presence of back pain. The palpation is performed from the upper thoracic to the lumbosacral region. CONCLUSION 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 point" ensuring that all parameters have been evaluated. More importantly, it is an invaluable document in the follow up and monitoring of the neurological patient. You can order the "The Canine And Feline Neurological Examination CD Rom" at www.neuroexamination.com. Lesion Localization: Cranial Nerves and Brainstem For clinical purposes, the brain is divided into three parts: Brainstem Cerebellum Thalamocortex The cerebellum although part of the brainstem, will be covered separately due to its different function. The present conference is a review of the principles governing lesion localization as it relates to the brainstem and the cranial nerves that originate from it, i.e., cranial nerves III to XII. BRAINSTEM: MIDBRAIN, PONS and MEDULLA Rostral to caudal, the brainstem consists of the midbrain, pons and medulla (rostral and caudal medulla). The clinically important anatomical structures of the brainstem are: The reticular formation The cranial nerves III to XII The medial longitudinal fasciculus (MLF) The (ascending) sensory pathways and (descending) motor pathways (or upper motor neurons) The respiratory centres Reticular formation The bulk of the brainstem is made of the reticular formation, the so-called ascending reticular activating system (ARAS). The ARAS is a network of fibers responsible for the animal's arousal, wakefulness. The most important and most consistent feature that localizes the lesion within the brainstem is the presence of an abnormal arousal state: somnolence, stupor or coma. Somnolence may not be obvious at the time of examination due to excitement or nervousness of the animal. The owner may report a lack of playfulness, tiredness or increase in sleeping time. It is essential to question the owner about changes in the mental status of the animal. Somnolence is the most consistent abnormality with brainstem disease and in most cases is not present at the time of examination. The assessment of the mental status is taken from the case history not from what is seen in the examination room. The term Asomnolence@ is a preferred terminology for the decrease in the arousal state observed with brainstem diseases. The term Adepression@ has a cortical significance. Cranial nerves The nuclei of the cranial nerves III to XII are located within the reticular formation of the brainstem. The cranial nerve deficits are of extreme localization value. They define the location of the lesion within the brainstem. It is crucial to remember the location of the nuclei. The cranial nerve deficits are to the brainstem what the spinal reflexes are to the spinal cord. In spinal cord diseases, a decreased or absent spinal reflex localizes the lesion to the spinal cord segment(s) where the cell bodies of the reflex being examined originate. A cranial nerve deficit localizes the lesion in the part of the brainstem where the cell bodies of this cranial nerve originate. Medial longitudinal fasciculus (MLF) The MLF is made of two fasciculi. The fasciculi are situated in the core of the brainstem, close to the midline and adjacent to the ventricular system. They run along the entire brainstem, from the midbrain, through the pons, to the medulla. The MLF connects the vestibular nuclei to the nuclei of the cranial nerves responsible for the eye movements, i.e., CN III (oculomotor), IV (trochlear) and VI (abducent). The CN III, IV, VI and VIII and the MLF are the structures involved in the physiological nystagmus (also called vestibular nystagmus, normal nystagmus or oculocephalic reflex). The MLF, as with the reticular formation, are not as valuable for lesion localization as they extend along the entire brainstem. It only indicates brainstem lesion. Their usefulness is particularly demonstrated in the comatose patient, helping to distinguish between signs of cerebral versus brainstem involvement. The presence of a physiological nystagmus indicates relatively good brainstem function because the nuclei of nerves III, IV, VI and VIII and the MLF must be intact for this reflex to occur. Ascending sensory and descending motor pathways (UMN) These pathways are the continuation of the spinal cord ascending sensory pathways, and the midbrain descending motor pathways (upper motor neurons). Brainstem lesions may lead to gait abnormalities. However it is less consistent than the presence of somnolence. Although there are three anatomical sites within the brainstem where the pathways cross over, the only crossing that is clinically relevant is the one in the rostral part of the midbrain. A lesion within and below the rostral midbrain causes ipsilateral proprioceptive deficits (ataxia +/- knuckling) and upper motor neuron (UMN) weakness whereas a lesion above, gives contralateral deficits. Decerebrate rigidity is a posture occasionally observed in patients that have sustained severe head injury or other diseases involving the rostral midbrain. With severe lesion, the inhibition of the brain on the antigravity muscles is lost leading to opisthotonus of the head and neck, and extensor rigidity of all four limbs. The posture is transient occurring when the comatose patient is stimulated. It carries a poor prognosis. The upper motor neurons are damaged and as a result, their inhibitory effect on the lower motor neurons is lost. A seizure is an example of a natural event causing a transient decerebration. Respiratory centres These centers are resistant to disease processes. The most common pattern of abnormal breathing is the "central neurogenic hyperventilation" observed with midbrain lesions. Medullary lesions cause irregular (or regularly irregular) respiratory patterns. CASE EXAMPLES An eight year-old male-neutered dog is presented with lethargy, somnolence and a head tilt to the left of three days in duration: The somnolence localizes the lesion intracranially, to the brainstem. The presence of a head tilt to the left implies involvement of the left vestibular system. The vestibular nerve has its nuclei in the rostral medulla. Consequently, the lesion is in the left rostral medulla. A four year-old female-neutered cat is presented with somnolence, right facial paralysis (right nerve VII) and a right dilated non-responsive pupil. The menace gesture is present bilaterally except that on the right, the animal does not blink but abducts his eyeball causing the third eyelid to protrude. When the penlight is shone into the right eye, the right pupil remains dilated while there is constriction of the left pupil. With the light shone in the left eye, the left pupil constricts but the right pupil remains dilated (right oculomotor nerve - CN III): This patient has an intracranial disease involving the brainstem since he is somnolent. There are at least two lesions: (1) the right rostral medulla to explain the right facial nerve deficit and (2) the right midbrain to explain the right oculomotor nerve deficit. This cat likely has an inflammatory disease since the lesions are multifocal. Most inflammatory diseases lead to clinical signs attributable to ONE lesion. This is interesting because at necropsy, these diseases are multifocal, diffuse or disseminated. There must be significant destruction or dysfunction to create neurological deficits. The frequent involvement of the vestibular system warrants separate coverage. The Vestibular System ANATOMICAL STRUCTURES: Receptors in the semi-circular canals. Vestibular nerves. Four pairs of nuclei situated on each side of the rostral medulla; these nuclei send information to the: Cerebellum (flocculo-nodular lobes) Reticular formation MLF Vestibulospinal tracts (UMN). CLINICAL SIGNS ASSOCIATED WITH VESTIBULAR DISORDERS The presence of a HEAD TILT is the salient feature of vestibular diseases and should be present if a vestibular disease is diagnosed. The head tilt, in most cases, persists for the animal's life although there is improvement in the severity. PERIPHERAL VESTIBULAR DISEASE: NO SOMNOLENCE The vestibular syndrome is peripheral if the mental status of the animal is normal. Peripheral disease means inner ear disease. ACUTE: Head tilt toward the side of the lesion. Nystagmus. Initially the nystagmus is resting. As the animal improves, the resting nystagmus subsides and is replaced by a positional nystagmus. In either case, the nystagmus can be horizontal or rotatory. The nystagmus however never is vertical nor changes in direction. The fast phase is opposite to the side of the lesion. Variable degrees of vestibular ataxia (disequilibrium): more pronounced in the early phase with improvement as the nystagmus disappears. The ataxia parallels the nystagmus. CHRONIC (5+ days): Head tilt toward the side of the lesion. +/- Nystagmus. If present usually positional. The nystagmus in peripheral vestibular disease often disappears due to compensation by the contralateral vestibular system. +/- Vestibular ataxia. Peripheral vestibular disease implies dysfunction of the receptors located in the inner ear. Evaluation of the hearing with electrodiagnostic (Brain Auditory Evoked Response) testing is a way to assess for wider involvement of the disease. Look also for ipsilateral Horner's syndrome, facial nerve paresis/paralysis and, dry eye or decreased tear production since the parasympathetic innervation of the lacrymal glands runs with the facial nerve. The sympathetic chain and the facial nerve are closely associated with the middle ear. Concomitant presence of facial nerve paralysis and/or Horner's syndrome indicates a more extensive disease (inner and middle ear involved) and a thorough diagnostic work up should be advocated. The neurological structures of the middle ear are the sympathetic chain to the eye and the facial nerve. These are axons. In the inner ear, the neurological structures are the receptors for hearing and equilibrium. Receptors are more sensitive to dysfunction / destruction than axons. This anatomical difference explains the expression "middle-inner" ear disease. When a head tilt is present, one cannot clinically rule out middle ear involvement. The axons must have significant compression or damage for abnormalities to be seen. CENTRAL VESTIBULAR DISEASE: SOMNOLENCE Depression from involvement of the reticular formation. Head tilt on the side of the lesion. +/- Nystagmus. Any direction and may change in direction. +/- Ipsilateral proprioceptive deficits (from involvement of the ascending proprioceptive pathways). +/- Ipsilateral UMN weakness (from involvement of the descending motor pathways). +/- Ipsilateral cranial nerves V, VI and VII deficits. These nerves enter the rostral medulla in close vicinity to the vestibulo-cochlear nerve. The cerebellum and pons are close to these structures. The expression "cerebello-ponto-medullary angle lesion" originates from the close anatomical association of these structures and their simultaneous involvement. CEREBELLUM The cerebellum coordinates the fine tuning of movements. In cerebellar diseases, there is no weakness as the UMNs are intact, AND there are no proprioceptive deficits because the conscious proprioceptive pathways, the fasciculus gracilis and fasciculus cuneatus, are not affected and carry the information from the proprioceptive receptors to the cerebral cortex. CLINICAL SIGNS ASSOCIATED WITH CEREBELLAR DISORDERS By order of frequency, there may be: +/- Head or whole body tremors. The tremors usually worsen with movements: intentional tremors. +/- Cerebellar ataxia: incoordination with strength preserved and without proprioceptive deficits (no knuckling). Dysmetria, most often as hypermetria. +/- Absence of menace response ipsilaterally if the disease is unilateral. This menace gesture evaluates the integrity of the visual pathways, the cognitive function, the facial nerve and also the cerebellum. And less commonly: +/- Vestibular signs if one of the flocculonodular lobes is involved. And rarely: With severe lesion, there may be decerebellate rigidity: extensor rigidity of the neck and front limbs while the hind limbs are in flexion. The PARADOXICAL VESTIBULAR SYNDROME occurs with involvement of the cerebellum and/or the cerebellar peduncles. This syndrome has a rare occurrence. It is diagnosed on a group of neurological abnormalities that include cerebellar signs ipsilateral to the cerebellar lesion and the concomitant presence of a head tilt opposite to the lesion. Granulomatous meningo-encephalitis (GME) and tumors have both been reported with the syndrome.

© 2004 - Joane Parent DMV, MVetSc, ACVIM Neurology - All rights reserved