NEUROLOGIC HISTORY AND MECHANISMS OF DISEASE

by R.M. Clemmons, DVM, PhD
Associate Professor of Neurology and Neurosurgery

The Neurologic History:

Part of the minimum data base for evaluation of any proposed neurologic patient is the neurologic history. Not only can this help describe the type of condition and possible causes of the problem, it can also help confirm that the problem is a neurologic disorder. It can be one of the most important parts of the initial examination, leading to the formation of the appropriate differential diagnosis. The owners description may lead to determining the exact nature of the problem, how long it has been present and whether the problem has been progressive.

Signalment:

The signalment includes the species, breed, age, sex and color. While many conditions affect all animals, certain diseases are unique to some species and even to certain breeds of that species. Wobbler's disease is most common in the horse and dog. Moreover, in dogs, it is most often recognized in young Great Danes and older Doberman Pinchers. One would not think of feline leukemia, if treating a dog.

The age of the animal can also be important. Younger animals are more prone to congenital problems, infections and toxicities. Older animals are more likely to have degenerative, metabolic, infectious and neoplastic diseases.

The sex and color of the patient can alter the differential list as well. Hypocalcemia is more common in females around the time parturition. Mammary neoplasia is more common in females, while prostatic disease is most common in male dogs. Blue-eyed, white cats are often congenitally deaf.

Specific History:

The diet, exercise, living conditions (outdoor or indoor), past illnesses, vaccination records, and medications can all be important in developing the differential diagnosis. If the diet is improper, nutritional or secondary metabolic diseases may develop. Animals who lack exercise may hasten the development of degenerative diseases. Having access to other animals and potential trauma from living outside may increase the risk of infectious or traumatic disease. Seizures secondary to canine distemper generally occur after the patient has recovered from the original infection. Lack of preventative medication (such as heartworm prevention) may lead to neurologic symptoms secondary to developing the disease. On the other hand, certain medications may allow manifestation of a previously sub-clinical problem. For example, certain heartworm preventatives can lower the seizure threshold. Treatment with aminoglycoside antibiotics can lead to disorders of cranial nerve VIII. Occurrence of the disease process following pesticide application or the availability of such pesticides may help determine the nature of intoxication.

Mechanisms of Disease:

The underlying causes for neurologic diseases are similar to those causes for disease within the body in general. The basic mechanisms of neurologic disease are congenital, inflammatory, metabolic, toxic, nutritional, traumatic, vascular, degenerative, neoplastic and idiopathic. (See Table 1) Congenital diseases will, for the most part occur in young animals or in older animals who de-compensate for the condition. The most common metabolic diseases in small animals are hypoglycemia and hepatoencephalopathy. The most common nutritional disease is thiamine deficiency. Toxicities in small animals are usually secondary to organophosphate intoxication, lead poisoning or ethylene glycol ingestion. Knowing the basic mechanisms of disease can help identify the cause of the patient's neurologic disease.

Table 1. Mechanism of Neurologic Disease and Some Common Examples.

Congenital Disorders	Hydrocephalus, True Epilepsy, Cerebellar Hypoplasia, Congenital Deafness, Vertebral Column or Neural Tube Defects, Lysosomal Storage Diseases

Inflammatory (Infectious) Disorders	Viral Infection (Canine distemper, Feline infectious peritonitis, Feline leukemia, Panleukopenia, Rabies), Bacterial Infection (meningitis, discospondylitis, Lyme's disease), Fungal Infection (cryptococcosis, aspergillosis), Protozoal Infection (toxoplasmosis, neosporidiosis), Rickettsial Infection (Rocky Mountain spotted fever, ehrlichiosis), Granulomatous Meningoencephalitis, Polyradiculoneuritis, Myasthenia gravis, Polymyositis

Metabolic Disorders	Hypoglycemia, Hepatoencephalopathy, Electrolyte Disturbances (hyper or hypocalcemia), Hypoxia, Hypothyroidism, Osmolality Disturbance, Acid-Base Disturbance

Toxic Disorders	Organophosphates, Lead, Ethylene glycol, Chlorinated hydrocarbons, Aminoglycoside antibiotics

Nutritional Disorders	Thiamine deficiency, Vitamin E deficiency

Traumatic Disorders	Head injury, Spinal Cord injury, Traumatic Disc rupture, Peripheral Nerve injury

Vascular Disorders	Fibrocartelaginous infarction, Septicemia, Vasculitis

Degenerative Disorders	Degenerative myelopathy, Intervertebral disc disease, Cerebellar degeneration

Neoplasia	Gliomas, Astrocytomas, Oligodendrogliomas, Meningiomas, Neurofibromas, Metastatic neoplasia

Idiopathic Disorders	Cranial nerve syndromes, Self-mutilation syndrome, Acquired epilepsy

Diseases may be symmetrical or asymmetrical. While metabolic, nutritional and toxic disorders are almost always symmetrical, inflammatory, traumatic, vascular and neoplastic diseases are almost always asymmetrical. This can help rule/out certain diseases from the differential. In addition, traumatic and vascular diseases are more commonly acute and non-progressive; whereas inflammatory, degenerative and neoplastic diseases are either acute or chronic, progressive diseases. (See Table 2.)

Table 2. Onset and Progression of Disease Mechanisms

Acute, Non-progressive
	1. Traumatic Disorders 2. Vascular Disorders
Acute, Progressive and Symmetrical
	1. Metabolic Disorders 2. Nutritional Disorders 3. Toxic Disorders
Acute, Progressive and Asymmetrical
	1. Inflammatory (Infectious) Disorders 2. Neoplasia
Chronic, Progressive and Asymmetrical
	1. Inflammatory (Infectious) Disorders 2. Degenerative Disorders 3. Neoplasia

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Last updated 27 August 2002