Mechanisms of Neurologic Disease

PURPOSE

The purpose of determining the mechanism of neurologic disease is to come closer to a patient’s diagnosis by determining the cause of the patient’s neurologic dysfunction. By determining how the dysfunction is occurring, the second part of the neurologic diagnostic process is complete (after localization; see Chapter 2, Localization of Neurologic Disease), giving a more complete picture of the neurologic diagnosis.

WHEN TO DETERMINE THE MECHANISM OF NEUROLOGIC DISEASE

An attempt to determine the mechanism of neurologic disease needs to be made in every patient who presents with a symptom due to nervous system dysfunction. Determination of mechanism is a thought process that should occur during and after every neurologic history, and during and after every neurologic examination.

NEUROANATOMY OF DISEASE MECHANISM

Determination of mechanism is based less directly on neuroanatomy than is localization (see Chapter 2, Localization of Neurologic Disease). Mechanism is intertwined in localization (and, therefore, neuroanatomy), however, because your choice of mechanisms is limited to those processes that are likely to affect that region of the nervous system.

EQUIPMENT NEEDED TO DETERMINE THE MECHANISM OF NEUROLOGIC DISEASE

None (other than the equipment used for the neurologic examination—the determination of mechanism is a thought process).

HOW TO DETERMINE THE MECHANISM OF NEUROLOGIC DISEASE

In neurologic diagnosis, it is most helpful to think in terms of general mechanisms of neurologic dysfunction and to try to decide which of these mechanisms is likely to be causative before proceeding with further diagnostic studies. Table 3-1 lists the major categories of neurologic disease mechanism. These mechanisms are generic and broad; for example, the compressive mechanism would include diverse disease processes, such as a tumor compressing or infiltrating brain tissue, a thoracic disc compressing the spinal cord, or a subdural hematoma causing mass effect on the brain. The same pathologic process can even produce clinical symptoms through several different mechanisms; for example, an intracerebral aneurysm may produce disease when it bleeds or when it compresses important structures (e.g., a posterior communicating artery aneurysm causing a compressive oculomotor nerve palsy).

TABLE 3-1 General Mechanisms of Neurologic Disease^a

Mechanism	Comments
Compressive	Includes any processes that produce dysfunction by compression of nervous system structures [e.g., tumors (benign or malignant), intervertebral discs compressing nerve roots or spinal cord, aneurysm compressing a cranial nerve]
Degenerative	Includes any process that causes progressive dysfunction due to nervous system degeneration; this mechanism is especially causative in many dementing illnesses (e.g., Alzheimer’s disease and other degenerative dementias), many movement disorders (e.g., Parkinson’s disease, Huntington’s disease), and some neuromuscular diseases (e.g., amyotrophic lateral sclerosis)
Demyelinative	May involve central nervous system myelin (e.g., multiple sclerosis) or peripheral nervous system myelin (e.g., demyelinative peripheral neuropathies, such as Guillain-Barré syndrome); demyelinative disorders often have accompanying inflammation—think of demyelinative as a subset of inflammatory etiologies
Epileptic	Produces dysfunction by abnormal electrical activity of cerebral hemispheric cortex
Hemorrhagic	Produces dysfunction by bleeding into the brain or other tissues (e.g., intraparenchymal brain hemorrhage, intraventricular hemorrhage, subarachnoid hemorrhage)
Infectious	Dysfunction occurring due to a microorganism (e.g., bacterial, viral, parasitic) invading nervous system structures; probably best thought of as a subset of inflammatory etiologies
Inflammatory	Dysfunction occurring as a result of any inflammatory process involving the central or peripheral nervous system (e.g., autoimmune, granulomatous)
Ischemic	Dysfunction due to insufficient blood supply in the central nervous system (e.g., brain, brainstem, cerebellar, or spinal cord ischemia or infarction) or the peripheral nervous system (e.g., infarction of a peripheral nerve)
Migrainous	Mechanism of brain dysfunction that is thought to be due to spreading waves of depression of cortical activity; can lead to migrainous visual, motor, sensory, or aphasic symptoms, even in the absence of headache
Metabolic (including toxic)	Mechanism of diffuse brain or brainstem dysfunction due to effect of endogenous metabolic abnormalities (e.g., hyponatremia, hepatic or uremic dysfunction) or exogenous toxins (e.g., medications)
Traumatic	Central or peripheral nervous system dysfunction due to any kind of traumatic disruption of these structures
^a In addition to the mechanisms in this list, some congenital processes may produce dysfunction due to the absence, malformation, or other developmental abnormality of nervous system structure or function since birth (whether on a microscopic or macroscopic level); this mechanism is most common with pediatric neurologic diagnoses.