Ataxia in felines – determining lesion location via neurological assessment

This so-called neuroanatomical diagnosis is achieved based on the neurological examination. Unfortunately, the often uncooperative nature of the feline patient makes this essential step a real challenge. Having a more targeted approach to the neurological examination in these feline patients based on the presenting problem is therefore more realistic. Narrowing down to the part(s) of the nervous system that may be affected is essential, as, from a diagnostic point of view, the differential diagnosis is entirely dependent on the neuroanatomical diagnosis (certain diseases may only affect certain parts of the nervous system).

Furthermore, running a limited number of investigations to narrow down the differential list to a specific part of the nervous system means less cost for the owner and less time spent for the clinician reaching a diagnosis. With these aims in mind, the approach to a cat presented with ataxia as its main complaint should focus on identifying which of the three types of ataxia is present by using the neurological examination.

Ataxia is defined as an uncoordinated gait and can arise from a lesion affecting three distinct anatomic sites in the nervous system: a sensory peripheral nerve or a spinal cord lesion (general proprioceptive ataxia); a vestibular lesion (vestibular ataxia); or a cerebellar lesion (cerebellar ataxia; ^{Figure 1}). Ataxia can be further divided into hypometria (shorter protraction phase of gait) or hypermetria (longer protraction phase of gait).

Understanding the origin of the ataxia

General proprioceptive (GP) ataxia reflects the lack of information reaching the central nervous system (CNS) responsible for the awareness of movement and position of the neck, trunk and limbs in space.

As a consequence, there may be a delay in the onset of protraction of the limb, which may cause a longer stride than normal. The cat may walk on the dorsal part of its foot or may drag its digits. These signs often overlap with those caused by upper motor neuron (UMN) paresis. GP ataxia results from lesions affecting the ipsilateral GP pathways in the spinal cord (fasciculus gracilis and fasciculus cuneatus in the dorsal funiculus for GP in the pelvic limbs and thoracic limbs respectively), ipsilateral caudal medulla oblongata, contralateral medial lemniscus in the pons, mesencephalon and thalamus, and contralateral cerebral cortex (mostly parietal lobe).

Although lesions within the thalamus and cerebral cortex will cause GP ataxia in humans, the ataxia generated from lesions within these structures in domestic species is usually too subtle to be detected on gait evaluation. The pathways of the GP sensory system are anatomically adjacent to most of the UMN pathways necessary for gait generation. The change in the gait, therefore, generally reflects a combined dysfunction of both UMN paresis and GP ataxia with delayed onset of protraction of the limb and lengthened stride. From a lesion localisation point of view, UMN paresis and GP ataxia visible in the gait can occur as a consequence of a lesion affecting the brainstem or spinal cord. Compared to UMN paresis, disorders of the lower motor neurons (LMN) only cause paresis and not ataxia.

Vestibular ataxia occurs with lesions affecting either the peripheral or central vestibular apparatus. In addition to ataxia, animals will often have concurrent neurological signs that reflect a vestibular disorder, such as head tilt (unilateral vestibular disorder) or head sway (bilateral vestibular disorder), pathological nystagmus, or positional strabismus. Cats with vestibular ataxia often have a broad-based gait (especially in the pelvic limbs) with leaning towards the side of decreased vestibular tone. Some cats may have substantial swaying when walking and will occasionally fall; recumbent animals may be seen to roll. Weakness or paresis is only seen with central vestibular disease and is not a feature of peripheral vestibular disease.

Cerebellar ataxia can be seen in cats that have lesions within the cerebellar cortex. Other signs of cerebellar disease, such as intention tremors, are often present. Cerebellar ataxia is characterised by hypermetria and dysmetria. Hypermetria associated with cerebellar ataxia consists of over-flexion during limb protraction and is therefore distinct from the over-reaching, longstrided gait noted in animals with combined GP ataxia-UMN paresis. Dysmetria is a component of cerebellar ataxia and is manifested by a loss of synchronous limb movements.

Vestibular or cerebellar ataxias are accompanied by other signs of dysfunction of the vestibular apparatus or cerebellum respectively.

Neurological assessment

It is essential to try to characterise which subclassification(s) of ataxia is/are contributing to the gait pattern. The presence of ataxia should suggest a lesion of the spinal cord, brainstem, cerebellum, or peripheral vestibular apparatus as discussed previously; multifocal disease with involvement of at least two of these regions should also be a consideration. Associated neurological signs are used to localise the lesion to one of these parts of the nervous system (see ^{Panel 1}). Correct anatomic diagnosis is crucial in establishing a differential list, as some causes of ataxia are specific to certain regions of the nervous system. Additionally, the choice of ancillary diagnostic tests is guided by lesion localisation and the differential list.

The initial part of the examination is devoted to observing the cat’s posture and gait. For a cat that is reluctant, but able, to move about in the room, enticement with a toy or laser pointer can be particularly effective. This hands-off approach should focus in detecting the following neurological clues to neuroanatomic diagnosis.

• Head tilt: often indicates a unilateral vestibular disorder (peripheral or central). The head is usually tilted toward the same side as the lesion. Lesions affecting the cerebellar portion of the vestibular apparatus (flocculonodular lobe or cerebellar peduncle) can cause a central vestibular syndrome with a paradoxical head tilt.

• Leaning or falling to one side: indicates a unilateral vestibular disorder (peripheral or central).

• Wide excursion of the head from side to side: indicates a bilateral vestibular disorder (peripheral or central).

• Intention tremor of the head: indicates a cerebellar disorder.

• Symmetrical hypermetria on all four limbs or on one side and in the absence of paresis: indicates a cerebellar disorder (or at least lesion affecting the spino-cerebellar pathways in the spinal cord in the absence of other signs of cerebellar disorder).

• Concurrent UMN paresis in limbs with no effect on the eyes or head posture: indicates brainstem or spinal cord disorder.

At a minimum, the handson part of the examination should focus on evaluating the cat’s postural reactions, segmental spinal reflexes and selected cranial nerve functions, such as the menace response, pupil size and symmetry, and detecting presence of pathological nystagmus.

Postural reaction testing aims to detect subtle deficits that were not obvious on gait evaluation. These reactions reveal the cat’s awareness of the precise position and movements of parts of its body, as well as the cat’s ability to generate movements in the part tested. The best reactions to test in cats are the hopping response, wheelbarrowing and tactile placing, as paw position testing (or “knuckling” response) can be very difficult to assess in this species. Postural reactions can be abnormal in the presence of central vestibular lesion, brainstem or spinal cord lesion. They are usually normal with peripheral vestibular lesion or cerebellar lesion (although a delayed then exaggerated response may be seen with the latter lesion localisation).

With gait evaluation, postural reaction testing helps to narrow down the lesion localisation as being cranial to T3 spinal cord segments (all four limbs affected or both thoracic and pelvic limb affected on the same side) or caudal to T3 spinal cord segments (both pelvic limb or only one pelvic affected). Segmental spinal reflex evaluation helps narrow down further the lesion localisation by testing the integrity of the C6-T2 and L4-S3 intumescences, as well as respective segmental sensory and motor nerves that form the peripheral nerve, and the muscles innervated.

Lesions at the level of these intumescences result in LMN signs in the muscles innervated (such as the loss of segmental spinal reflexes as well as reduced muscle tone and size). Segmental spinal reflexes are best performed with the cat in dorsal recumbency between the thighs of the examiner. Withdrawal reflex and the patellar reflex are the most reliable in cats. Other spinal reflexes (triceps, biceps, extensor carpal radialis and gastrocnemius) are more difficult to perform and to interpret.

The menace response is elicited by making a threatening gesture at the eye tested while the other eye is blindfolded. The expected response is a closure of the eyelid. It is absent in very young cats (less than 10 to 12 weeks). This response tests the retina, optic nerve (cranial nerve 2 = CN2), contralateral optic tract and contralateral forebrain, ipsilateral cerebellum and facial nerve (CN7).

The visual placing response requires intact visual and motor pathways and can be useful in assessing visual function in a cat where the menace response is ambiguous. It is tested by carrying a cat towards a tabletop. On approaching the surface, the cat will reach out to support itself on the table before the paw touches it. In the context of an ataxic cat, the menace response may be abnormal with cerebellar lesion (absent on the same side as the lesion with intact visual placing) or with multifocal CNS disease process (inflammatory, infectious or metastatic disease).

Evaluation of pupillary size and equality in ambient light as well as in darkness is also an important part of the evaluation of an ataxic cat. Normal pupils should be symmetrically shaped and equal to each other in size. Horner’s syndrome (manifesting as miosis, enophthalmia and protrusion of third eyelid) can be associated with peripheral vestibular lesion, especially in cats with otitis media/ interna, nasopharyngeal polyps and middle-ear tumour. The presence of spontaneous or positional nystagmus indicates vestibular disorder. Vertical or nystagmus that changes direction with different positions of the head indicates a central vestibular disorder.

Localising the neuroanatomic lesion

The aforementioned neurological assessment should help test the integrity of the various components of the nervous system that may be involved in an ataxic cat (such as GP, vestibular and cerebellar system) and detect any functional deficit(s) present. Normal findings are as important as abnormal ones in localising a lesion. Neurological abnormalities detected on examination should be added to the list of abnormal findings collected from the history. Attempts should be made to explain all the abnormal findings by a single lesion within one of the following specific regions of the nervous system: peripheral or central vestibular system, brainstem, spinal cord or cerebellum.

Lesions within these regions result in predictable and specific neurological signs (see flowchart). Note that in localising a lesion, it is not necessary that all the clinical signs referable to one location or syndrome are present. If a single lesion cannot explain all the abnormal findings identified, the lesion localisation is considered as being multifocal or diffuse.

Differential diagnostics

Differential diagnoses should be established based on the neuroanatomic origin of ataxia (^{Table 1}). The differential diagnosis list can be developed by taking into account the patient signalment, historical data (mode of onset and pattern of development of the condition) and neurological findings (neuro-anatomic diagnosis as peripheral or central vestibular, cerebellar or general proprioceptive ataxia). Disease processes that can affect the nervous system are traditionally classified according to the “VITAMIN D” mnemonic: vascular/ischaemic, infectious, trauma, autoimmune/inflammatory, metabolic/systemic, inherited/congenital, neoplastic, drug/toxic. Each category has a typical signalment, onset and progression that helps to narrow down the differentials.

With a clear knowledge of the region of the nervous system involved, and a differential list reduced to no more than three or four disease processes, consideration should be given only to those diagnostic tests that will help to narrow down the list further. These tests should ideally be run in succession from least invasive through to more invasive.