Neurology in exotic animals

Introduction
Neurological symptoms are frequently seen in exotic species. However, due to anatomical and behavioral differences with mammalian species, it is more challenging to evaluate and make a correct diagnosis for theneurologic exotic patient. Therefore, a basicknowledge about neuroanatomical differences of exotic species, neurological examination and advanced imaging modalities will help the clinician to specify the localization of the problem, to list differential diagnosis and finally find the definitive diagnosis. Combined with a thorough history and clinical examination, it will help the veterinarian to define the most suitable treatment for the underlying condition.

I. Neuroanatomy
All vertebrates have the same brain division, beside the lamprey which appears to lack a cerebellum. The brain size has increased independently in some vertebrates and is related to behavioral complexity. Birds and mammals have 6–10 times larger brains than reptiles of the same body size.
Most of our exotic patients (fish, reptiles, birds, rodents, rabbits and most marsupials) have a lyssencephalic brain (smooth surface). A gyrencephalic brain with gyri and sulci is seen as an evolution step (observed in some mammals and cetaceans). The convolutions permit an increased cortical surface area to be contained in the same volume of cranial vault. Mammals and birds have 12 pairs of cranial nerves when reptiles only have 11-13 pairs with an extra terminal nerve. Moreover, in some snakes CNXI is reduced or absent. Regarding the vertebrae, there is also a huge difference between the species i.e. birds show a wide variety with the S-shaped cervical column offering more mobility. Furthermore, the fusion of avian vertebrae (notarium in some birds, synsacrum and pygostyle) gives more rigidity and stability for flight or landing at the cost of flexibility.
Birds, reptiles and rabbits lack a cauda equina. The lumbosacral plexus in birds is located near the kidneys in the fossa of the synsacrum. Infection of the kidneys or expending caudo-abdominal masses can lead to a monoparesis. Mammals and birds have 3 meningeal layers: the dura mater, arachnoid mater and pia mater. Cerebrospinal fluid (CSF) can normally be collected in the subarachnoid space. In comparison, reptiles just have 2 meningeal layers: the dura mater and the secondary meninx. For this reason, the CSF is to be collected in the subdural space in those animals.

II. Neurologic examination
As many exotic animals cannot be handled so easily, the hand’s off examination: observation of the behavior, mentation, posture and gait, is of extreme importance in the neurological examination.
Some parts of the hand’s on examination: testing of the postural reactions, cranial nerves, spinal reflexes, spinal palpation and nociception, are not always adaptable because of the anatomical variations and stress while handling (prey vs predator). Reptiles being ectothermic, their reflexes are influenced by the body temperature, therefore reptiles should always be assed in an environment that is within their selected body temperature range. Also, it can be quite challenging to assess mentation in some of those animals. Offering a non-slippery surface, a glove or a perch to assess gait and posture, as well as the postural reactions can be helpful. To evaluate proprioception slowly move a perch or a glove, let step a hooded raptor on the glove or perform a paper movement test can also be useful. To assess reptile’s neurological status the righting reflex- correcting the orientation of the body when the animal is placed on the back- is frequently performed. Regarding the cranial nerves examination, you must remember that birds and some reptiles has striated muscles within the iris, which explains a voluntary control of the iris movement. In this case, objective assessment of the pupillary light reflex (PLR) is difficult in those animals. Furthermore, the consensual PLR is absent in birds and reptiles due to a complete decussation of the optic nerves at the chiasm. It is unsteady in rabbits due to a partial decussation of only 10% of the optic nerves.
The menace response is often absent in birds, reptiles and rabbits. Therefore, to evaluate vision in exotic animals, it is more reliant to perform the cotton ball test. The palpebral reflex cannot be evaluated in snakes due to the anatomical lack of eyelids. The cornea is protected with a single, transparent scale: the spectacle. By observing the patient while offering food or putting a drop of honey on the tip of nose in rabbits most nerves involved in tongue movement, chewing and swallowing can be correctly assessed. The most reliable spinal reflexes are the withdrawal and wing-withdrawal reflex as well as the peri-anal or vent reflex. The patellar reflex can be present but is missing in species lacking a patella like ostriches. In rabbits, the patellar reflex is usually elevated. In dogs it would be interpreted as hyperreflexia and associated with a lesion of the upper motoric neuron, but it is likely to be normal in rabbits. Spinal palpation and nociception are performed as in cats and dogs responding in pain-like behaviors (withdrawal, licking, biting, immobility, and vocalization). In reptiles it is difficult to assess nociception as they tend to show unspecific responses like dys-/anorexia, closed eyes, lameness, decreased activity, discoloration or darkening of skin. It is important to make the difference between feeling the pain and having a reflexive response to a noxious stimulus.

III. Interpretation
The combination of a thorough history, the physical examination and the neurologic examination allows to establish a neuroanatomical localization and to formulate a differential diagnosis list after the VITAMIN D scheme:
- V: vascular
- I: inflammatory/infectious
- T: traumatic/toxic
- A: anomalous
- M: metabolic
- I: idiopathic
- N: neoplastic/nutritional
- D: degenerative
One’s ability to determine a differential list, neuroanatomical localization and the specific diagnostic tests helps to choose complementary diagnostic test to gain a definitive diagnosis.

IV. Neurodiagnostic
Like in dogs and cats diagnostic imaging: radiography, myelography, computer tomography (CT), myelogram ,magnet resonance tomography (MRI) and CSF collection are today more commonly used. The most limiting factors in exotic patient are the size of the patient and some specific anatomical differences. Because of the tendency of the avian spine (notarium, synsacrum) for spinal fractures CT and MRI are much more sensitive diagnostic modalities than radiographs. Furthermore, with the synsacrum and the lack of a cauda equina, the lumbar puncture site should be avoided for myelography, myelogram or CSF collection in birds. Like those latter, reptiles, are also lacking a cauda equina and the lumbar site should also be avoided for puncture or collection. CSF collection should be performed before the myelogram. It is quite difficult in birds regarding the very small cerebellomedullary cistern, but quite easily performed in reptiles and rabbits. It is important to know that sea turtles and crocodilians also possess a spinal venous sinus, which can lead to blood contamination of the punctured CSF.
So far, the major issue regarding CSF examination is that, beside the rabbit and ferret, we lack normal references values for most of our exotic species. In a general point of view, CSF should be clear and colourless, free of erythrocytes and leukocyte numbers and protein level should be compared with that is normally found in the peripheral blood. Ultrasound is usually used in neurology to diagnose peripheral nerve tumors or lesions, hydrocephalus or to aspirate disc material in cases of disco-spondylitis. Newer research shows great results in transcranial ultrasound to detect intracranial masses, lesions or vessels in thin skulled species like small birds or rodents.
Electrodiagnosis: electromyography (EMG), nerve conduction studies, brainstem auditory evoked response (BAER) and electroencephalography (EEG) are uncommonly used in exotic medicine but are really useful tools to characterize and localize neurological disorders and should in the future be part of the routine diagnostic approachin neurological evaluation of exotic animals.

Conclusion
Neurology in exotic animals is certainly challenging, but not impossible. Knowing what is normal, increasing confidence and routine within the neurologic examination of different exotic species, will allow you to make an accurate and definitive diagnosis, and will help you to adjust the therapy for the specific neurological condition and patient.

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