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Neurologic syndromes: localizing a lesion (Proceedings)

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Nov 01, 2009

When a veterinarian is presented with a patient with symptoms of difficulty walking, altered mentation, weakness, collapse or other movement disorders several potential causes must be considered. Disease processes involving the nervous, cardiovascular, endocrine and orthopedic systems may all produce symptoms very similar to one another. In addition, generalized systemic illness of other causes may debilitate patients severely enough that they can not or choose not to walk. Furthermore, in some patients such as geriatric animals the cause of immobility or altered mentation may be caused by disorders of more than one body system. Therefore, the most basic question that must be answered is whether a patient's symptoms are caused by neurologic disease or problems involving other body systems. Signalment, history and physical examination are the initial tools utilized to answer this question. Once neurologic symptoms have been identified it is necessary to localize the patient's symptoms within the nervous system. Once a neurologic lesion is localized a list of differential diagnoses can be formulated and a diagnostic and therapeutic plan developed.

The nervous system is made of component parts that work and interact with one another in a predictable manor. Because of this, abnormalities within any single area of the nervous system can be expected to produce a similar set of symptoms. These groups of symptoms and physical examination findings produce the neurologic syndromes that can be used to localize a patient's symptoms within the nervous system.

The most basic components of the nervous system are the central nervous system, which includes the brain and spinal cord and the peripheral nervous system made up of peripheral nerves, muscle (skeletal, smooth and cardiac), glands and the myoneural junction. The peripheral nervous system controlling motor movement to the body is also sometimes referred to as the neuromuscular system. In clinical veterinary neurology the central nervous system can be further divided into the forebrain, brainstem, and cerebellum within the skull as well as the cervical spinal cord (spinal cord segments C1-C5), the cervical enlargement (segments C6-T2), thoracolumbar spinal cord (segments T3-L3), and the lumbar enlargement and cauda equina (L4-Cd5). When all of a patient's symptoms can not be localized to one area of the nervous system a multifocal syndrome is present.

In order to utilize the syndrome approach to localizing lesions within the nervous system a basic understanding of neurophysiology is necessary. Lower motor neurons (LMN) originate in grey matter of all spinal cord segments and the nuclei of cranial nerves III-VII and IX-XII. The axons of the LMN form the peripheral and cranial nerves and connect the central nervous system to a muscle, gland or organ. The LMNs that innervate the thoracic limbs originate from the cervical enlargement (segments C6-T2) while the LMNs that innervate the pelvic limbs originate from the lumbar enlargement (segments L4-Cd5). Symptoms of LMN disease include reduced or absent reflexes, decreased muscle tone and early and rapid onset of muscle atrophy. Upper motor neurons (UMN) originate in the grey matter of the cerebrum, basal nuclei and brainstem. Axons of the UMN travel through the white matter tracts of the forebrain, brainstem and spinal cord. The UMNs inhibit the LMN and are responsible for initiating motor movement and maintaining tone to the extensor muscles of the limbs. Upper motor neuron symptoms include normal or exaggerated reflexes, increased muscle tone and late onset, milder muscle atrophy (disuse atrophy).

Forebrain (cerebral) syndrome

Patients with forebrain disease often have altered mentation or behavior changes but a normal or near normal gait. Loss of house training, failure to recognize the pet owner, personality changes and confusion within a known environment may be part of the history. Forebrain lesions that are unilateral or asymmetric may cause the pet to walk in circles or turn predominantly in one direction. The circles are usually wide and toward the side of the lesion. Often pets will head press into a corner or get stuck in closets or between furniture at home. Vision may be affected with cerebral disorders and animals may bump into objects when walking, such as door frames and legs to tables or chairs in an examination room. Altered vision from a forebrain disorder along with normal pupillary light and palpebral reflexes is cortical blindness. Despite a normal or near normal gait, postural reactions such as proprioceptive placing and hopping are often markedly altered in limbs contralateral to the lesion. Generalized or focal seizures may also occur in patients with cerebral disease. As symptoms of forebrain disease progress, often from increased intracranial pressure, patients may become stuporous, comatose and papilledema and altered respiration may develop. The diencephalon (thalamus, hypothalamus) is the caudal portion of the forebrain and in addition to the previously mentioned symptoms patients with disorders of the diencephalon may have altered temperature regulation, optic nerve (CN II) deficits and endocrinopathies such as hyperadrenocorticism, diabetes insipidus and hypothyroidism. Pituitary macroademomas are the primary cause of diencephalic symptoms.