Almost every type of pathology that affects tissues elsewhere in the body may involve the nervous system, but the resulting neurological syndromes may be very different due to its unique structure and function. There are also pathologies involving the electrical excitability of neurons that are specific to the nervous system, with only some similarities to dysfunction of other electrically excitable tissues.
Certain neural tissue responses occur commonly with many types of pathology. Inflammation often occurs, as does increased water content in tissue, which is called edema. Edema may be inside cells (the cells have swollen), called intracellular or cytotoxic edema, or it may be in the interstitial space between cells, called extracellular or vasogenic edema. Pathology of the central nervous system often causes a reaction from astrocytes called gliosis, astrogliosis, astrocytosis, or reactive astrocytosis. With this reaction, astrocytes multiply, move toward the pathology, and get bigger with longer and thicker processes. This forms a type of scar tissue called a glial scar, particularly around cavities where tissue is lost and replaced by fluid. Many disorders may cause loss of myelin out of proportion to loss of axons, which is called demyelination.
Neuropathology could be divided into categories in several ways with varying amounts of gaps and overlaps. One system is: genetic, idiopathic, vascular, epileptic, immune, mechanical, metabolic, infectious, neoplastic, environmental, nutritional, and toxic (with the less-than-ideal acronym GIVE IMMINENT). Specific neurological disorders usually fall into one of these categories, although some may have aspects of several. There are usually many shared features of the disorders in each category of neuropathology, but some are more homogenous than others. Some types of pathology may affect the nervous system only as it is developing or only after development has finished, while some may occur at any time. Those that occur during development are called neurodevelopmental disorders.
Genetic neurological disorders are entirely or mostly caused by gene mutations, such as the disorder called Down syndrome. Many neurological disorders may have a small genetic contribution, or predisposition; these are usually not referred to as genetic disorders. Idiopathic neurological disorders have an unknown cause. Many idiopathic disorders involve the loss of certain groups of neural cells, such as Parkinson’s disease; these are called degenerative or neurodegenerative disorders. Some idiopathic disorders involve episodic or continuous neural dysfunction without the loss of cells, such as migraine headaches.
Vascular neurological disorders are caused by abnormalities of the structure or function of blood vessels, such as the disorder ischemic stroke that may be caused by a blood clot cutting off blood flow to part of the brain. Epileptic neurological disorders involve seizures, which are episodes of a type of abnormal electrical activity in the cerebrum. Most epileptic disorders are idiopathic, genetic, or both, but they are such common and distinctive disorders they are often given their own category.
Immune neurological disorders, most of which are autoimmune, are caused by abnormal immune system activity leading to tissue dysfunction. An example is multiple sclerosis, where the immune system causes demyelination in the central nervous system. Mechanical neurological disorders are caused by physical forces, such as traumatic brain injury. Trauma refers to tissue injury caused by a sudden physical force. The term compression is not used consistently, but usually refers to tissue injury caused by a static or progressive physical force of some speed that is slower than the sudden onset of trauma.
Metabolic neurological disorders are caused by abnormal amounts of substances that are normally present in the body, such as the polyneuropathy that may be caused by chronic high blood sugars from the disorder diabetes mellitus. The term metabolic is also used for rare neurodevelopmental disorders involving abnormal molecular pathways inside cells, many of which are primarily genetic disorders. Infectious neurological disorders are caused by pathogens, such as certain viruses that may invade brain tissue.
Neoplastic neurological disorders are caused by tumors. Neural cells may become neoplastic, but more commonly tumors form elsewhere and then spread to affect the nervous system. Environmental neurological disorders disorders are a diverse group where neural abnormalities develop from things such as heat, cold, radiation, or electricity. Events such as sleep deprivation or a change in setting such as admission to the hospital can lead to diffuse cortical dysfunction in susceptible people.
Nutritional neurological disorders are caused by the deficiency of a dietary component, such as the polyneuropathy that may be caused by vitamin B12 deficiency. Toxic neurological disorders are caused by exposure to a substance that is not normally found in the body, including side effects of medications, such as the polyneuropathy that may be caused by some chemotherapy drugs used for cancer.
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Introduction to Neurology 