A Refresher on the Neurological Exam

Wendy Archipow, BVSc, MS, DACVS-SA, Specialist in Small Animal Surgery Veterinary Specialist Services, Underwood

Neurological Exam:

The neurological exam is made up of six parts: the distance exam, cranial nerve assessment, postural reactions, spinal reflexes, sensation, and muscle tone assessment. The neurological exam allows us to determine if a neurological abnormality exists, and to localise the problem to the affected part of the nervous system.

The distance exam is performed by watching the animal interact with its environment. The animal’s mentation, body posture, and gait are assessed. Mentation is an animal’s level of consciousness, and its ability to respond to stimulation. An animal with abnormal mentation may be described, in decreasing level of response to stimulation, as depressed, obtunded, stuporous, or comatose. Posture describes the position of the body. Examples of abnormal posture include a head tilt or a head or body turn. An animal may have an abnormal neck, back, or tail position, or may experience tremors. Gait describes the character of the animal’s movement. Effective gait requires both strength and coordination. A loss of strength leads to weakness, whereas a loss of coordination results in ataxia. Ataxia may be subdivided in general proprioceptive ataxia, vestibular ataxia, or cerebellar ataxia. Lameness should be distinguished from neurological dysfunction. An animal with paraparesis is unable to independently ambulate but retains voluntary motor function, whereas an animal with paralysis has lost voluntary motor function.

Cranial nerves are generally assessed in groups as it is difficult to test each one individually. The cranial nerves assessed overlap between the different groups. Intact vision and pupillary light reflexes require cranial nerves II, III, and VII. Normal eyelid position and symmetry require cranial nerves III and V, and the sympathetic nerves to be functional. Eye position and movement depends on cranial nerves III, IV, VI and VIII. Vestibular function is provided by the vestibulocochlear nerve (cranial nerve VIII). Normal facial positioning and sensation requires intact facial and trigeminal nerves (cranial nerves V and VII). Cranial nerves IX, X, XI, and XII provide tongue and laryngeal-pharyngeal function.

Postural reactions maintain the head and body in a normal position. Postural reaction testing is useful to determine if an abnormality is neurologic in origin, as well as to detect more subtle neurological deficits. Tests commonly performed include proprioceptive placing, hopping, hemi-walking, wheelbarrowing, and extensor postural thrust.

Spinal reflexes are an involuntary response to a stimulus. Spinal reflexes do not require conscious input - the reflex arc consists of a sensory receptor, an afferent sensory neuron, spinal cord segment, and an efferent lower motor neuron. Generally only one spinal cord segment is involved. The most reliable reflex in the forelimb is the withdrawal reflex, however other reflexes described are the extensor carpi radialis, biceps brachii, and triceps reflexes. In the hindlimb the most commonly used reflexes are the patellar and withdrawal reflexes, although the cranial tibial and gastrocnemius reflexes may also be useful. Other spinal reflexes commonly tested are the crossed extensor reflex, perineal reflex, and cutaneous trunci reflex.

In contrast to spinal reflexes, sensation is the conscious reaction to a noxious stimulus. Superficial or cutaneous sensation is assessment of the sensory innervation of the skin. This occurs in regions called dermatomes, each of which is supplied by a single sensory nerve. Deep pain sensation, or nociception, is assessed by applying compression to a peripheral bone to stimulate the nerves supplying the periosteum. Deep pain sensation is important for providing prognosis for spinal cord injuries as the spinal cord tracts supplying this sensation are located deep within the spinal cord.

Assessment of muscle tone aids in neurolocalisation as animals with an upper motor neuron lesion will have increased muscle tone, whereas animals with a lower motor neuron lesion will have reduced muscle tone.

Neurolocalisation:

The nervous system is divided into the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system consisting of peripheral nerves.

The brain is divided into the forebrain, cerebellum, and brainstem. Disorders of the forebrain are characterised by changes in mentation, seizures, and head turn, head pressing, or circling. Cerebellar disorders are characterised by intention tremor, hypermetria, and upper motor neuron signs to the limbs. Animals with brainstem lesions also have changes in mentation and upper motor neuron signs to the limbs. Cranial nerve deficits may be seen with brain lesions in any location.

The spinal cord is divided into four sections: the cranial cervical (C1-C5), cervicothoracic (C6- T2), thoracolumbar (T3-L3), and lumbosacral (L4-S1/2) regions. Localisation of lesions within the spinal cord relies on identifying upper versus lower motor neuron signs to the four limbs. Upper motor neuron signs include normal to increased muscle tone and spinal reflexes, and a bladder that is difficult to express. Lower motor neuron signs include decreased muscle tone and spinal reflexes, and a flaccid, easily expressed bladder or incontinence. A spinal cord lesion within the cranial cervical region will result in upper motor neuron signs to all four limbs. A cervicothoracic lesion will cause lower motor neuron signs to the forelimbs and upper motor neuron signs to the hindlimbs. Thoracolumbar lesions result in normal forelimbs and upper motor neuron signs to the hindlimbs. Lumbosacral lesions also have normal forelimbs, but lower motor neuron signs to the hindlimbs and tail.

Peripheral nerve lesions are characterised by lower motor neuron signs. These may affect one or multiple limbs depending on the cause. Other abnormalities such as laryngeal paralysis and dysphagia may also be seen.