The evaluation of the veterinary patient with neurologic disease can be challenging, but is essential in order to delineate steps towards appropriate diagnostics, treatment and prognostication. The foundations of neurolocalization include obtaining as thorough history, physical examination and neurologic examination. The most important question to be asked is "Does this patient have disease of the nervous system?" and, if so, does it involve the brain, spinal cord or neuromuscular system? If one area cannot be isolated as an origin of all of the clinical signs, then a multifocal localization can be made. The key, however, is to try to localize to one area in the nervous system. Other diseases can mimic neurologic dysfunction, such as cardiac disease and musculoskeletal disorders.

Divisions of the nervous system include the brain, spinal cord and neuromuscular subdivisions. The brain is divided into the forebrain (cerebral cortex) which contains four lobes - the frontal lobe which is responsible for intellect, behavior, motor and sensory skills; the parietal lobe which is responsible for proprioception and nociception; the temporal lobe which is responsible for emotion and hearing and the occipital lobe which is responsible for vision. Within the forebrain is the diencephalon, which contains the hypothalamus. The brain also contains the brainstem, the cerebellum and the central vestibular system. The spinal cord is divided into 4 segments - C1-C5, C6-T2, T3-L3 and L4-S2 - each segment, when dysfunctional, has its own set of neurologic signs associated with it. The neuromuscular system is composed of the nerve root, the peripheral nerve, the neuromuscular junction and the muscle. Signs specific to each portion of the nervous system will be presented to help aid in the neurolocalization process.

Signs of forebrain disease include a change in the content of the patient's consciousness. This can be manifested as changes in behavior, normal habits or routines, intellect and personality. Abnormal sleep-wake patterns and elimination habits may be observed, as well as a failure to recognize the owner. Common signs of forebrain or cerebral disease include seizures, which can be focal, partial or generalized, altered mental status, circling (usually towards the side of the lesion), pacing, obsessive or aimless wandering and head pressing. Papilledema (optic nerve swelling) and irregular respirations may also be observed if there is high intracranial pressure present. The neurologic examination reveals contralateral partial cranial nerve deficits (since the forebrain has supranuclear control over cranial nerve function) and contralateral conscious proprioceptive (CP) deficits, weakness (hemiparesis), visual impairment (with normal pupillary light reflex (PLR) and hemisensory loss. Upper motor neuron (UMN) reflexes may also be apparent contralaterally.

The diencephalon contains the thalamus, epithalamus and hypothalamus. Animals with lesions in the thalamic region may exhibit an altered content of consciousness, behavior and mentation, seizures, aggression and excitability. They may pace, circle or aimlessly wander and head press. They may exhibit unilateral cranial nerve II (CNN II) deficits (lesion at the level of the optic chiasm) such as pupillary dilation, visual loss and absent or diminished PLR). Animals with hypothalamic disease may exhibit abnormal temperature regulation (hyper or hypothermia), abnormal appetite (increased or decreased) and endocrine disturbances such as hyperadrenocorticism, hypothyroidism, diabetes mellitus or diabetes insipidus, since the hypothalamus is the center for release of controlling hormones to the pituitary gland.

The brainstem is divided into the midbrain (mesencephalon), pons (metencephalon) and medulla (myelencephalon). The brainstem is a relay system and contains the ARAS (the ascending reticular activating system) that alerts the forebrain to external stimuli, descending motor pathways (cortico, rubro, vestibulo and reticulo spinal tracts), proprioceptive tracts, pain pathways and the CNN III through XII nuclei. Thus, the efferent and afferent pathways cross at the level of the rostral midbrain. A lesion in the brainstem can result in a level of consciousness change (dullness, stupor or coma) with that animal being appropriate, as well as ipsilateral UMN weakness (paresis), UMN reflexes, proprioceptive deficits as well as complete CNN deficits, which, because of the proximity of the CNN III - XII nuclei to each other, can be multiple. If a lesion is present in the medulla and is severe, ventilatory and cardiac-related changes may occur since the medulla contains the respiratory and cardiac centers.

The cerebellum is responsible for mediating the range, rate and force of a movement. It is inhibitory by nature and, thus, when disease occurs, dysinhibition of motor activity prevails, which is called dysmetria or hypermetria. Dysmetria can affect the limbs (hypermetria or goose-stepping) eyes, pendulous, oscillatory nystagmus, and head (intention tremor). The affected patient may exhibit a truncal ataxia as well. Most animals with cerebellar disease will preserve their mentation, strength and proprioception, but may exhibit central vestibular signs on occasion due to disease of the cerebellar flocculonodular lobe. CNN function is usually normal except for the menace reflex which may be absent with normal vision due to the fact that the efferent branch of the menace pathway (CNN VII) travels through the cerebellum on its way to the brainstem CNN VII nucleus.

The vestibular system can be divided into two separate regions - the central vestibular system which includes the vestibular nuclei, the flocculonodular lobe of the cerebellum, the medial longitudinal fasiculus of the brainstem and the nuclei for CNN III, IV and VI. The peripheral vestibular system is comprised of CNN VIII and its receptors located in the inner ear. The key feature of both types of vestibular disease is a head tilt (if the lesion is lateralizing) although there are exceptions. Signs of central vestibular dysfunction can include a level of consciousness change, loss of balance, falling, rolling (vestibular ataxia), nystagmus (of any type or directionality) that changes with position change, multiple complete CNN deficits, ipsilateral hemiparesis and conscious proprioceptive deficits. These findings depend upon the location of the lesion. Peripheral vestibular signs may include a head tilt towards the side of the lesion. Normal content and level of consciousness as well as normal strength and proprioception, horizontal or rotary nystagmus that does not change with position and CNN VII deficits and Horner's Syndrome in the middle ear is involved. . In certain cases, an "eye drop" may be observed on the same side as the lesion due to CNN VIII dysfunction.

The spinal cord can be divided into 4 different regions: C1-C5, C6-T2 (brachial plexus), T3-L3 and L4-S2 (lumbosacral plexus). Signs of spinal cord dysfunction correspond to the location of the lesion. An animal with a C1-C5 myelopathy may have neck pain alone, or may have bi- or unilateral tetraparesis or plegia (all 4 limbs weak or paralyzed) depending upon the type and extent of injury. In some cases, the hind limbs may be one grade weaker than the front limbs due to the more lateral position of the hind limb motor tracts in the spinal cord. UMN signs (hyperreflexia, increased extensor tone) may be observed in all 4 limbs, and all 4 limbs should have normal withdrawal reflexes. Ipsilateral proprioceptive loss may be observed. Animals with a C1-C5 myelopathy or radiculopathy (nerve root impingement) may have an abnormal head carriage or may exhibit a root signature (lameness in the affected limb due to nerve root irritation). Animals with a C6-T2 myelopathy may have uni or bilateral tetraparesis or plegia and proprioceptive loss. They may exhibit (lower motor neuron (LMN) signs (hyporeflexia, decreased tone, diminished withdrawal) in the front limbs, but have UMN signs to the hind limbs. They may exhibit neck pain, and in some instances (T1-2 lesion), they may have an ipsilateral Horner's due to dysfunction of the sympathetic tract.

An animal with a T3-L3 myelopathy exhibits hind limb proprioceptive loss, that may progress to hind limb weakness or paralysis (paraparesis or paraplegia). The front limbs may have normal tone or have somewhat increased tone due to damage to a population of cells called Border cells that populate the T11-L1 region of the spinal cord. When they are damaged, dysinhibition of increased extensor tone in the front limbs occurs. Spinal hyperpathia may be present if the lesion is extradural and the panniculus reflex usually will be present approximately two vertebral bodies caudal to the lesion. Animals with an L4-S2 myelopathy may exhibit the following signs: Hind limb proprioceptive loss, paresis or paralysis; if the lesion is solely in L4-L6, the patellar reflex (femoral nerve) may be diminished and the cranial tibial reflex (sciatic) may be exaggerated. If the lesion is L7-S2, the cranial tibial reflex may be diminished as well as the withdrawal reflex in one or both limbs. L7-S2 lesions may also cause diminished tail tone, fecal and/or urinary incontinence. Spinal hyperpathia may or may not be present. In cases of spinal cord disease, conscious proprioception is lost first, then voluntary motor, then superficial pain then deep pain. It usually returns in the reverse order.

The neuromuscular system is divided into 4 sections - the nerve root, the peripheral nerve, the neuromuscular junction and the muscle. Signs associated with nerve root dysfunction (radiculopathy) include: spinal hyperpathia, a root sign signature, paresis or paralysis of one or more limbs, generalized weakness or paralysis, muscle atrophy (focal or diffuse) or a voice change. With few exceptions, animals with neuromuscular disease will have normal mentation. Examples of radiculopathy would include a generalized form-polyradiculoneuritis (Coon hound paralysis) or a focal form-brachial plexus avulsion. Neuropathies can be focal or diffuse and associated signs may include paresis or paralysis of innervated structures muscle atrophy, fasiculations, voice change and reduced or absent reflexes. In rare instances, sensory neuropathies can occur - either breed-related or trauma related or can be associated with the autonomic nervous system. Junctionopathies occur where there is abnormal neurotransmission at the neuromuscular junction. Examples include myasthenia gravis, tick paralysis and botulism. Most affected animals have normal mentation, but may exhibit selected cranial deficits (most commonly CNN VII), generalized motor weakness or paralysis which may be episodic myasthenia gravis or continuous (tick paralysis/botulism). Reflexes may be normal (MG) or hyporeflexic (tick paralysis, botulism) and muscle atrophy may or may not be present. Signs of myopathy include either generalized or focal weakness, a stiff, stilted gait, muscle atrophy. Limit joint movement due to muscle contractions, voice change or trismus (inability to open the mouth). Pain on muscle palpation may or may not be present. The important feature of neuromuscular disease is in general, weakness with normal proprioception.

Neurolocalization is the keystone of neurologic diagnosis and treatment. This is accomplished through the attainment of a thorough history, physical and neurologic examinations.