17_Neurology Rich.qxd 8/23/2005 11:22 AM Page 493 CHAPTER 17 Evaluating and Treating the Nervous System SIMON R. PLATT, BVM& S, MRCVS, Dipl ACVIM (Neurology) , Dipl ECVN, RCVS Specialist in Veterinary Neurology The nervous system plays a role in nearly all body processes. Disease syndromes may affect the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system, which includes cranial nerves, spinal cord nerve roots, spinal nerves, peripheral nerve branches and the neuromuscu- lar junction. Evaluation of the nervous system of birds parallels that of mammals, with modifications specific to avian anatomy, behavior and physiology. An understand- ing of basic avian neuroanatomy is necessary to evaluate abnormal function of the nervous system.15,16,62,87 Suspicion of neurologic dysfunction arises from the his- tory and physical examination. The signalment, present- ing chief complaint, time course of clinical signs, and his- tory may suggest the type of disease process or species- specific disorder. A complete neurologic examination is necessary to localize the anatomic distribution, to deter- mine the severity of the disease process, and to assess the prognosis for patient recovery. A minimum database consisting of a complete blood count (CBC), serum or plasma biochemical analysis, and cytologic evaluation of choanal and cloacal swab samples is used to evaluate the contribution of other body systems to the observed clini- cal signs and physical abnormalities. If neuromuscular disease is suspected, electrophysiological tests are indi- cated, including electromyography and nerve conduction velocities, and these are becoming more commonly per- formed in birds. In recent years with the advancement of imaging technology and the improved availability for vet- erinary patients, scintigraphy, computed tomography and magnetic resonance imaging have been valuable diagnos- tic commodities in furthering our understanding of avian CNS disease. Only when a disease location and a patho- logic process have been identified can appropriate treat- ment and prognosis be provided. 17_Neurology Rich.qxd 8/23/2005 11:22 AM Page 494 494 Clinical Avian Medicine - Volume II Anatomy and Physiology premotor neurons of the motor systems of the spinal cord, thus controlling flying and walking. The final affec- Complete reviews of avian neuroanatomy are docu- tors of wing and leg movement are the lower motor mented in other texts.15,62,85,87 For a detailed description of neurons of the brachial and lumbar plexuses, respec- the functional organization of the avian spinal cord, the tively. In-depth review of this area is covered in an avian reader is referred to an avian physiology text.83 The gen- physiology text.15,33 eral organization of the avian spinal cord resembles that of all other vertebrates. There are however, some special- Control of Vocalization and Respiration izations, which birds largely share with the phylogeneti- Birds use their syrinx, an organ at the transition of the cally related class of reptiles. Compared to the mam- trachea and bronchi, to produce sounds. Vocalization malian cord, the most outstanding deviations are the lack depends upon the control of a few small syringeal mus- of a cauda equina and a filum terminale and the occur- cles and the precise regulation of the stream of (expira- rence of a sinus rhomboidalis or lumbosacralis.83 At the tory) air passing through the trachea. This requires a microscopic level there are some significant differences close coordination of the motor centers of the syringeal in the organization of cell groups and pathways. Birds and respiratory muscles. The caudal part of the dorsal differ from most other vertebrates in their locomotion hypoglossal motor nucleus innervates the syringeal mus- due to bipedal walk and flight. It has to be kept in mind cles, whereas the motor cells of the respiratory muscles that specializations in the spinal cord may result from are part of the motor system of the spinal cord. However, adaptations from this peculiar kind of locomotion. several cell groups in the cranial and caudal brainstem have a role in the control of respiration. In songbirds, Control of Eye and Head Movements these centers are all under direct telencephalic (cerebral) A sitting bird is able to survey a large part of its surround- control.120 See Chapter 19, Endocrine Considerations. ings using both head and eye movements. Several types of eye movements can be distinguished such as saccades Control of Coordination (fast flicks elicited by a sudden stimulus) and smooth Birds possess a well-developed cerebellum but, as in pursuit (following a moving target).33 Six muscles are mammals, its precise role in the control of motor activity responsible for all movements of the eye. Branches of the is not well understood. However, there is little doubt oculomotor, trochlear and abducens nerves innervate that the ease and precision of motor performance these extrinsic eye muscles. The two muscles that rotate depend upon an intact cerebellum. The axons of the the eye dorsally are innervated by contralateral centers, Purkinje cells form the output of the cerebellar cortex; with the other muscles innervated by ipsilateral cell the central cerebellar nuclei and some vestibular nuclei groups. These are stimulated by the vestibular centers.37 are the targets of these fibers.33 The Purkinje cells have an inhibitory effect on the activity of the central nuclei. Control of Jaw and Tongue Movements The jaw muscles have a visceral origin; therefore, the nerves innervating these muscles and the corresponding Neurological Examination motor nuclei are considered visceromotor elements.33 A complication in parrots is that they possess a so-called and Lesion Localization kinetic skull. This means that not only the lower jaw but A neurological examination is easily integrated into a also the upper jaw moves to open the beak.33 Movement routine physical examination. The objectives of the neu- of the beak is achieved by four groups of muscles: two rological examination are to confirm if there is a neuro- groups of beak openers and two groups of beak closers. logical abnormality and to specifically localize the abnor- Movements of the tongue are caused by two groups of mality within the nervous system. In conjunction with muscles. The extrinsic muscles are part of the visceral the history, signalment, presenting complaint and the musculature, similar to the jaw muscles, and the intrin- physical examination, the neurological lesion localiza- sic muscles have a somatic origin innervated by the hypo- tion is a piece of a jigsaw, essential to creating a list of glossal nerve. Because the jaws and tongue move in differential diagnoses for the disease. However, caution close harmony during feeding and drinking, a premotor must be used, as some manipulations necessary for the system is needed to coordinate the activity of the tongue neurological examination could exacerbate problems 80 and jaw motor centers.33 such as spinal cord disease. Control of Locomotion OBSERVATION The reticular formation in the brain stem also contains Observation of the bird is essential, as it allows evalua- 17_Neurology Rich.qxd 8/23/2005 11:23 AM Page 495 Chapter 17 | EVALUATING AND TREATING THE NERVOUS SYSTEM 495 tion of the bird’s mentation (level and content of con- Table 17.1 | Cranial Nerves: Function and Applicable Tests sciousness), posture, attitude and gait. Changes in men- Cranial Nerve Nerve Function Applicable Tests tation are revealed by a history of personality change, I. Olfactory Smell None applicable change in awareness of surroundings and inappropriate II. Optic Vision i. Menace response ii. Pupillary light reflex behavioral responses.26 Consciousness is a function of III. Oculomotor Extrinsic and intrin- i. Eyeball position the brainstem (responsible for arousal) and the cerebral sic ocular muscles/ ii. Menace response cortex (responsible for content and regulation).26,40,101,117,119 upper eyelid muscle iii. Pupillary light reflex IV. Trochlear Extrinsic ocular i. Eyeball position muscles PALPATION V. Trigeminal Facial and beak i. Palpebral reflex sensation/beak ii. Jaw palpation The bird’s musculoskeletal system should be palpated movement for asymmetry, masses, tenderness, contour and tone. A VI. Abducens Extrinsic ocular i. Eyeball position muscles and third mass effect, tenderness or contour change requires fur- eyelid muscle ther investigation. The vertebral column should be pal- VII. Facial Muscles of facial None applicable pated for deviations and pain, being cautious not to expression apply too much pressure if there is suspicion of an insta- VIII. Vestibulocochlear Hearing and i. Startle balance ii. Oculocephalic reflexes bility. Unilateral muscle mass loss or atrophy may indi- IX. Glossopharyngeal Muscles of pharynx, i. Gag reflex cate disuse if it is chronic, or a neurogenic loss if it is larynx, crop and acute (within 7 to 10 days).26 syrinx X. Vagus Muscles of larynx, i. Gag reflex pharynx, esophagus CRANIAL NERVES (CN s) and crop XI. Accessory Superficial neck None applicable Several differences exist between the neuroanatomy of muscles avian and mammalian species.15,26 Cranial nerves have VII. Hypoglossal Muscles of tongue, i. Tongue grab/ specific functions and evaluation of these functions can trachea and syrinx inspection help to precisely locate a neurological lesion due to their well-documented anatomy. The general functions and specific tests are summarized in Table