How the Nervous system can be affected by disease
Some knowledge of anatomy is an essential prerequisite for the practice of any clinical discipline, but no where is this more true than in the diagnosis of neurological disorders. The localisation of the areas of the nervous system involved in disease calls for a fairly thorough knowledge of the location of various masses of grey matter, and of the courses of various tracts.
In recent years, considerable advances in neurological diagnosis have become possible by the use of sophisticated imaging techniques like computed tomography (CT), and magnetic resonance imaging (MRI). In interpreting these images a thorough knowledge of the gross anatomy of the head, neck and brain (or of other regions concerned) is invaluable.
Damage to nervous tissue can occur in various ways. Any part of the brain or spinal cord may be damaged by direct injury (trauma). Apart from other obvious causes such injury may occur during child birth. If nervous tissue is deprived of blood even for a short period irreversible damage may result. Localised damage of this kind may occur if one of the arteries supplying the brain is blocked. This may occur by clotting of blood within the vessel (thrombosis). Such an event is more likely in older individuals in whom the arteries have undergone a degenerative change known as arteriosclerosis. A vessel can also be blocked by some extraneous material (e.g., clot, fat, air)reaching it from some other part of the body through the circulation. Such matter is called an embolus. Sometimes an artery may rupture, the blood leaking into brain tissue (haemorrhage) causing considerable damage. A haemorrhage in the brain is often fatal. Bleeding may be caused by rupture of small abnormal dilatations of arteries (aneurysms). Aneurysms may be congenital, or may be produced due to weakening of the arterial wall in the region.
Another cause of brain damage is the presence of any abnormal mass within the cranial cavity. As the cranial cavity cannot expand such a mass inevitably presses on brain tissue. Such a spaceoccupying lesion may be a tumour, a collection of pus, a collection of blood (in the epiduralspace) etc. Apart from producing general signs of increased intracranial tension, local effects are produced depending on the area involved.
Increased intracranial tension, specially when it is rapid in progression, can lead to further brain damage in a number of ways. Swelling of the brain (cerebral oedema) following trauma, or infection, can itself act like a space occupying lesion. Brain tissue is pressed against the wall of the cranial cavity leading to damage. Part of the brain may herniate through a wound in the skull.
As pressure on a region of brain tissue increases it can lead to occlusion of blood vessels and infarctions. When intracranial pressure increases to the level of arterial pressure blood flow ceases and brain death ensues.
Nervous tissue may be affected by infections, both acute and chronic. An infection in the brain is referred to as encephalitis; and that in the spinal cord is called myelitis. Defects in neural tissue may also be caused by maldevelopment (congenital anomalies), by degeneration in old age, and by various metabolic disorders. Finally, alterations in nervous function may occur in the absence of recognisable structural changes. These are called functional disorders.
From the above it will be obvious that in some cases a neurological disorder will be of acute onset (as in trauma, vascular accidents, or acute infections). In such cases the patient may at first show signs indicative of widespread functional deficit quite out of proportion to the actual area involved. Deep unconsciousness (coma) is often present. In course of time, however, considerable recovery may take place, leaving residual defects dependent upon the area involved. In slowly developing diseases on the other hand, considerable structural damage may occur before symptoms become obvious. In the descriptions that follow we will deal only with signs and symptoms referable to the actual area of lesion.