Diseases of the Nervous System

Chapter 8 Diseases of the Nervous System

The nervous system can be divided into two primary divisions: the central nervous system (CNS), composed of the brain and the spinal cord, and the peripheral nervous system (PNS), composed of the cranial nerves and the peripheral nerves that connect the outside sensory world to the brain.

The functional cell of both systems is the neuron, whose job is to transmit electrical impulses to and from the brain. Pathology anywhere within the transmission system results in interruption of messages and clinical neurologic symptoms. The individual symptoms vary depending on the location of the lesion. For the purpose of this chapter, diseases are divided into those of the brain, spinal cord, and PNS.



In small-animal medicine, traumatic brain injuries are encountered frequently. The injuries generally have an acute clinical onset resulting from a traumatic experience (e.g., being hit by a car, having the head closed in a door, or falling). Injury to the brain from trauma can result from direct injury to the nervous tissues (primary event) or from secondary events, which intensify or worsen the neurologic damage and produce systemic derangements. Primary events may produce disruption of fiber tracts, which cannot be repaired, or reparable cell damage, which is reversible. Secondary events such as increased intracranial pressure (ICP), edema, hypoxia, and seizures occur as a result of the primary trauma. Increased ICP is caused by both edema and hemorrhage in or around the brain. Because the brain is encased in a nonflexible shell of bone (the skull), herniation of nervous tissue (primarily the brainstem) through the foramen magnum results. Treatment of head trauma involves preventing or decreasing the secondary effects of trauma.


Table 8-1 Clinical Rating Scale for Evaluation of Craniocerebral Trauma

Criteria Score*
Motor activity
Normal gait, normal reflexes 6
Hemiparesis, tetraparesis, or decorticate activity 5
Recumbent with intermittent extensor rigidity 4
Recumbent with constant extensor rigidity 3
Recumbent with intermittent extensor rigidity/opisthotonos 2
Recumbent, hypotonic with depressed-absent spinal reflexes 1
Brainstem reflexes
Normal PLR and OVRs 6
Slow PLR and normal to reduced OVRs 5
Bilateral/unresponsive miosis and normal-to-reduced OVRs 4
Pinpoint pupils and reduced-to-absent OVRs 3
Unilateral/unresponsive mydriasis and reduced-to-absent OVRs 2
Bilateral/unresponsive mydriasis and reduced-to-absent OVRs 1
Level of consciousness
Occasionally alert and responsive 6
Depressed/delirious, but capable of response to stimulus 5
Obtunded/stupor, but responds to visual stimuli 4
Obtunded/stupor, but responds to auditory stimuli 3
Obtunded/stupor, but responds to noxious stimuli 2
Comatose and unresponsive to noxious stimuli 1

* Prognosis: grave for 3-8 total score; poor to guarded for 9-14 total score; and good for 15-18 total score. PLR, pupillary light reflex; OVR, oculovestibular reflex.

From Fenner WR: Diseases of the brain. In Ettinger SJ, Feldman EC, editors: Textbook of veterinary internal medicine, ed 5, Philadelphia, 2000, WB Saunders, by permission.


An enlarging tumor within the brain produces tissue compression and/or replaces healthy neuronal tissue causing clinical signs that are progressive. Primary brain tumors are typically singular, but metastatic tumors or secondary brain tumors may be solitary or multiple in occurrence. Most tumors are metastatic by the time the animal is examined. The disease is typically seen in older animals.

Idiopathic Epilepsy

Idiopathic epilepsy is a syndrome characterized by repeated episodes of seizures for which there is no demonstrated cause. The diagnosis is one of exclusion.

Idiopathic epilepsy is predominantly a disease in German shepherds, miniature and toy poodles, Saint Bernards, cocker spaniels, beagles, Irish setters, golden retrievers, and some mixed breeds. Seizures usually begin between 1 and 3 years of age. Affected animals may exhibit a short aura during which the animal may act abnormally. They may hide, seek companionship, vocalize, or exhibit other abnormal behaviors. Seizures are usually generalized in nature, lasting anywhere from 1 to 2 minutes. After the seizure, the animal is usually disoriented and occasionally blind. Seizures may occur singly or in clusters and may reoccur at fairly regular intervals. In some animals, inciting events such as excitement or estrus have been shown to precipitate seizure activity. Although the cause of idiopathic epilepsy is unknown, a hereditary basis has been suggested.

Status Epilepticus

Animals prone to seizures may exhibit status epilepticus, a medical emergency. Continual seizures for a prolonged period (>5-10 minutes) can lead to irreversible coma and death if not treated aggressively. Owners should be advised to seek emergency assistance if this situation develops.


Just like the brain, the spinal cord is protected by a bony housing, the vertebral column. The spinal cord is located within the spinal canal, dorsal to the vertebral bodies. Between each of the vertebral bodies is a cushion known as the intervertebral disk. These disks are composed of an outer fibrous layer, the annulus fibrosus, and an inner gel-like nucleus, the nucleus pulposus. Their presence allows a larger range of motion in the vertebral column and prevents the vertebral bodies from rubbing against each other (Fig. 8-1).

Intervertebral Disk Disease

By far, one of the most common disorders involving the spinal cord of small animals is intervertebral disk disease. Disk protrusions can occur in all breeds of dog and occasionally in cats. It has been reported that 75% to 100% of all disks in chondrodystrophic breeds have undergone degenerative changes by 1 year of age. Disk protrusion or extrusion occurs most commonly in the cervical, caudal thoracic, and lumbar spine. Two types of herniations have been reported. Type 1 (common in younger dogs) involves acute rupture of the annulus fibrosus and extrusion of the nucleus pulposus up into the spinal canal. In type 2 herniation (common in older [>5 years] large-breed dogs) the extrusion occurs over a longer period, producing less acute and less severe clinical signs. The severity of spinal cord injury depends on the speed at which the disk material is deposited into the spinal canal, the degree of compression, and the duration of compression. Clinical signs may be related to the location of the lesion (Fig. 8-2).

Spinal Cord Trauma

Acute spinal cord injuries of the dog and cat usually result from motor vehicle accidents, gunshot wounds, or fights. The spinal cord trauma is sudden in onset and may be related to the velocity of cord compression, the degree of compression, and the duration of the compressive force. Signs of injury are typically nonprogressive, although they may worsen over the first 48 hours before stabilization. Injury may occur at a single or multiple levels within the spinal cord.

Blunt trauma to the spinal cord causes tissue injury through both “direct” and “indirect” mechanisms. Direct effects are due to primary disruption of neural pathways within the cord. Indirect effects are less well understood and include edema, hemorrhage, ischemia, lactic acidosis, inflammation, and neuronophagia by WBCs. It appears that mechanical deformation of any type can trigger these secondary events within the spinal cord. Autodissolution of the cord may be seen as early as 24 hours after injury.

Aug 31, 2016 | Posted by in GENERAL | Comments Off on Diseases of the Nervous System
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