Neurological cases can be challenging for the clinician. We must be aware, that before we decide for more advanced diagnostic procedures, the most important thing in evaluating dogs and cats with neurologic signs is gathering information on patient’s signalment and history and making a complete neurological examination. This is essential to establish an accurate anatomic diagnosis.
Knowledge of breed predilections can be very helpful when considering differential diagnoses. There are some congenital and inherited disorders, that have been seen in only one or few breeds. Also certain diseases are more likely with specific age groups (congenital disorders, neoplasia). For getting the accurate and complete history it is important to let the client describe the problem as detailed as possible and not get mislead by his interpretation. Owners can notice subtle changes in pet’s behavior, that may not be apparent to the examiner. With careful questioning we can find out about duration of the problem (acute, chronic, episodic), progression of the state, identify possible triggering factors, find out about previous therapies … Onset and progression of clinical signs are of primary importance and can already give us some information about possible etiologies.
The neurological exam should be preceded by a thorough general physical examination. This allows us to detect some abnormalities in other body systems, that might affect the nervous system, mimic a neurological disorder or could influence the prognosis.
Neurological examination is set up of two parts: observation and hands-on examination. Observation of the animal starts right at the beginning, ideally already while taking history. Before handling the patient, it is wise to let him walk freely around the room or let owner to walk him over a non-slip surface. If the animal is unable to walk by itself, body support should be provided in order to be able to detect possible subtle voluntary movement.
During observation we should assess animal’s mental status and behavior (depression, altered states of consciousness, aggression, compulsive walking, head pressing …), posture and body position at rest (head tilt, spinal curvature, wide-based stance, decerebellate or decerebrate rigidity, abnormal limb postures …), evaluate the gait (ataxia, paresis, circling, lameness) and identify abnormal involuntary movements (tremors, seizures, myoclonus …).
Observation is followed by hands-on examination, which covers cranial nerve assessment, postural reaction testing, spinal reflexes and sensory evaluation (pain perception). It is important to start with procedures least likely to upset the patient. Uncomfortable or painful procedures (palpating painful areas) are left until the end, because it might be difficult to complete the examination and evaluate degree of pain once pain is elicited.
First we test the motor and sensory function of the cranial nerves (CNN) by observation and eliciting reflexes and responses. Cranial nerve dysfunction may result from a disorder affecting single or multiple nerves, or a cluster of abnormalities can be seen with diseases affecting the middle and inner ear or brainstem. Analyzing CNN dysfunction should always be done in conjunction with assessment of proprioception and mentation to determine whether it goes about brain or peripheral nerve disease.
The complex series of responses that maintain an animal in an upright position are called postural reactions. Testing those reactions is used to determine whether animals are aware of the precise position and movements of their body, especially the limbs. Proprioceptors, that are located in the joints, tendons and muscles, collect the sensory information and transmit it to the cerebral cortex, where it is consciously perceived (conscious proprioception). A careful and systematic evaluation may reveal subtle deficits and asymmetries that may not be observed during routine gait examination. Postural reactions are also useful in distinguishing between orthopaedic and neurologic disorders. Clinical evaluation of the postural reactions includes proprioceptive positioning (knuckling), hopping, hemi-walking and wheelbarrowing. In animals with significant weakness it is important to support the body weight during the testing. The responses are complex in their pathways and entire nervous system is needed to be able to perform postural reactions. Testing them do not provide specific information about lesion localization but suggest the presence of neurologic dysfunction somewhere along the neurologic pathway.
Spinal reflexes assess the integrity of the sensory (afferent) and motor (efferent) components of the reflex arc. Its evaluation is a way to classify the neurological disorder as LMN or UMN. A LMN is an efferent neuron, the last in the chain of neurons which connects CNS and effector organ (it synapses with an effector organ) and produce muscular contraction. If it is damaged, it causes reduced or absent reflexes and muscle tone, flaccid paresis and/or paralysis and early and severe muscle atrophy. UMN is also an efferent neuron, which originates in the brain and synapses indirectly with an LMN to influence its activity. It is responsible for the initiation and maintenance of normal movements and tone of the extensor muscles. The lesions of UMN usually result in loss of motor function and loss of inhibitory function that UMN system has on LMN. That is why clinically we can see normal to increased reflexes and extensor muscle tone, spastic paresis and/or paralysis and chronic mild to moderate muscle atrophy.
Different spinal cord segments are mediating each spinal reflex, so differentiating between LMN and UMN signs on the reflexes enables the examiner to localize the lesion to specific spinal cord segment or peripheral nerves. Functionally the spinal cord can be divided into four regions: cranial cervical (C1-C5), cervicothoracic (C6-T2), thoracolumbar (T3-L3) and lumbosacral (L4-S3). The limb reflexes that are most useful to test in dogs and cats include withdrawal (flexor) and patellar reflex. Others that are less often evaluated are extensor carpi radialis reflex, biceps brachii and triceps reflexes, cranial tibial and gastrocnemius reflexes and perineal reflex.
Apart from conscious proprioception, the evaluation of animal’s ability to feel a noxious stimulus (nociception) can be helpful to localize the lesion. The purpose of testing pain perception is to detect areas of sensory loss. It is important to bear in mind, that withdrawal of the limb indicates only an intact reflex, but the behavioural response (turning the head or vocalizing) shows that the sensory spinal cord tracts to the brain are intact. When assessing the pain, we start with gentle stimuluses and escalate them gradually as needed. The pathways responsible for deep pain sensation are located deep in the spinal cord and are more resistant to damage than other pathways, including ones for proprioception, motor function and superficial pain. Therefore, testing the deep pain sensation is necessary only in absence of superficial pain. In patients with severe spinal cord injuries the absence of deep pain is important as it is a poor prognostic factor.
The neurological examination tests the integrity of the various components of the nervous system and can detect functional deficits. Normal results are as important as abnormal ones in establishing the anatomical diagnosis. Understanding of nervous system structure and function is required to correctly interpret neurologic examination findings and than localize lesions, which is the basis of clinical neurology. We can divide nervous system into three clinically important neuroanatomic regions: brain, spinal cord and peripheral nerves. The aim is to find the focus of the pathology and explain all the abnormal findings with a single lesion. It is not necessary that all of the clinical signs possible with one location are present. If a single lesion cannot explain all the listed abnormal findings, we should consider multifocal or diffuse disease. Once a neurologic lesion has been localized, we can generate a list of likely differential diagnoses. Based on that, we can choose the right diagnostic tests to confirm or exclude each of them.
- Platt S. R., Olby N. J.: BSAVA Manual of Canine and Feline Neurology, 3rd Ed.
- Nelson R. W., Couto C. G.: Small animal internal medicine, 4th Ed.
- Dewey C. W., da Costa R. C.: Practical Guide to Canine and Feline Neurology, 3rd Ed.