Session #2001

Abstract

Neurological disorders in reptiles are reported commonly, but evaluation of a reptile patient with suspected neurological disease can be challenging, especially if the clinician is unfamiliar with the normal behaviour of that species. Clinical signs are rarely pathognomonic and many conditions are only definitively diagnosed at post-mortem. The aim of this masterclass is to discuss the approach to neurological disorders in pet reptiles, concentrating on the neurological examination and in-practice diagnostics. Information gained from a standard neurological examination will vary between species. Different species will therefore be compared to show how we may adapt our recognized techniques to gain the most reliable information without undue stress to the patient.

Approach to the Reptile With Neurological Signs

A variety of neurological disorders have been reported in reptiles, but assessment can be affected by species-specific behaviour, body temperature, individual temperament, and stress levels. Each case needs a logical approach to identify the exact presenting problems, localise the lesion, and start to formulate differential diagnoses. Thorough history taking, a full physical examination, and more specific neurological assessment will be required, although this may need to be staged and adapted to the individual.

Anatomical Differences

Although the general structure of the neurological system in a reptile is similar to that of a mammal, there are a few key anatomical differences which may have clinical relevance:

• Most species lack an external ear or, if present, this is poorly developed. There is only one middle ear bone (the columella) which transmits vibrations, and the tympanic cavity is reduced in some species such as snakes. These species are generally limited to hearing only low-frequency vibrations, whereas species with a more developed tympanic cavity and columella should be able to hear a wider range of sounds.
• Visual ability varies between species depending on the degree of diurnal/nocturnal activity and eye position. The reptile iris is composed of skeletal muscle which allows them to voluntarily control pupil dilation and constriction. A parietal eye is well-developed in some species such as iguanas. Other species, such as some snakes, may rely less on vision if nocturnal but can sense infrared radiation through labial pits.
• Unlike mammals, reptiles do not have a true subarachnoid space, so CSF collection, if attempted, is from the subdural space.
• The spinal cord extends to the tip of the tail with no cauda equina. Locomotor centres are found within the spinal cord, allowing a degree of functional autonomy from the brain, so reptiles with a spinal cord injury may have a better prognosis for regaining ambulation than mammals.

The Initial Consultation

The initial consultation usually starts with history taking, although any reptile which is cold on initial presentation should be gradually warmed to an appropriate temperature on arrival. A thorough history is vital including the exact nature and progression of the neurological signs, any other medical concerns, and any predisposing husbandry or diet factors. Husbandry questionnaires can be a helpful way of obtaining general background information. It is particularly important to establish whether there could have been any history of possible trauma or toxin exposure and whether there have been any new animals in the collection or other animals affected. Neurological signs are not always obvious in the initial consultation, so if the owner has videos of the animal’s abnormal behaviour at home (such as a snake failing to strike prey), this can be helpful. A full physical examination can then be performed either at this stage or following a neurological examination. This will help determine whether signs are truly due to a primary neurological disorder or whether they could be due to an abnormality in another body system (e.g., musculoskeletal).

Neurological Examination

1.  Observe from a distance—mentation, gait, and posture should all be assessed, ideally including swimming ability if an aquatic species. In dangerous species, such as larger or venomous snakes or crocodilians, this may make up the majority of the neurological examination.

2.  Cranial nerves—these can be assessed as in mammals, although responses vary depending on the species. The palpebral reflex and jaw tone are usually reliable, but the palpebral reflex can only be performed in those species with true eyelids. Response to sound is very variable between different groups of reptiles. The menace response is absent in most cases.

3.  Postural reactions—foot placement can be variable, but wheelbarrowing can be performed in many lizards and tortoises. The righting reflex is usually reliable in species where this can be tested.

4.  Spinal reflexes—these are particularly variable depending on species and even individuals, but the presence of a normal reflex will provide useful information to aid localisation, but the lack of a reflex does not necessarily indicate pathology.

The order of assessment can affect results depending on tolerance to handling/stress levels, so it may need to be adapted to the individual. Alternatively, examination may need to be staged to reduce stress levels and achieve more reliable results.

Localisation of Lesion

Based on history and neurological examination, the next stage is localising the problem to the brain (forebrain, brainstem, or cerebellum), spinal cord, or peripheral nervous system. Typical signs with a forebrain lesion include change in mentation, seizures, or sudden onset blindness despite no obvious ocular lesions; brainstem lesions can cause mentation changes (often more pronounced than seen with forebrain lesions), tetraparesis, proprioceptive ataxia, and cranial nerve deficits; whereas a cerebellar lesion is typically associated with intention tremors. Spinal lesions can result in a variety of presentations from paresis of the tail to complete quadriparesis, depending on the level affected and severity of the lesion.

Diagnostics

Diagnostic approach will depend on whether signs are due to a primary neurological disorder or secondary to other disease and the likely location of the lesion. If signs appear secondary to non-neurological pathology, such as renal or hepatic dysfunction, blood work may be helpful. If however, signs are thought to be due to a primary neurological problem and localised to the spinal cord, then imaging would be the logical initial diagnostic. In practice, radiography can be used to help identify many spinal fractures and other deformities. Myelography has been used in some cases but is challenging to perform. For more subtle lesions, advanced imaging such as CT or MRI may be necessary. CSF can be sampled from the atlanto-occipital site under anaesthesia, although there is minimal information on normal values in reptiles. If an animal has been recently obtained or there has been a new addition to the collection, infectious disease screening should be prioritised. Once the lesion has been localised, differential diagnoses can be refined, and a treatment plan can be formulated.

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