Disorders of the brain

Learning outcomes
After studying this section you should be able to:
■ list three causes of raised intracranial pressure (ICP)
■ relate the effects of raised ICP to the functions of the brain and changes in vital signs 
■ outline how the brain is damaged during different types of head injury
■ describe four complications of head injury 
■ explain the effects of cerebral hypoxia and stroke 
■ outline the causes and effects of dementia
■ relate the pathology of Parkinson's disease to its effects on body function.
Increased intracranial pressure This is a serious complication of many conditions that affect the brain. The cranium forms a rigid cavity enclosing: the brain, the cerebral blood vessels and cerebrospinal fluid (CSF). An increase in volume of any one of these will lead to raised intracranial pressure (ICP). Sometimes its effects are more serious than the condi-tion causing it, e.g. by disrupting the blood supply or distorting the shape of the brain, especially if the ICP rises rapidly. A slow rise in ICP allows time for compen-satory adjustment to be made, i.e. a slight reduction in the volume of circulating blood and of CSF. The slower the rise in ICP, the more effective is the compensation. 

Rising ICP is accompanied by bradycardia and hyper tension. As it reaches its limit a further small increase in pressure is followed by a sudden and usually serious' reduction in the cerebral blood flow as autoregulatiou fails. The result is hypoxia and a rise in carbon dioxide4 levels, causing arteriolar dilation, which humeri increases ICP. This leads to progressive loss of function.; ing neurones, which exacerbates bradycardia and hyper-tension. Further cerebral hypoxia causes vasomotor: paralysis and death. The causes of increased ICP are described on the fol-lowing pages and include:
• cerebral oedema 
• hydrocephalus, the accumulation of excess CSF 
• expanding lesions inside the 'skull, also known as space-occupying lesions - haemorrhage, haematoma (traumatic or spontaneous) - tumours (primary or secondary). Expanding lesions may occur in the brain or in the: meninges and they can damage the brain in various ways
Effects of increased ICP Displacement of the brain Lesions causing displacement are usually one sided but` may affect both sides. Such lesions may cause: 
• herniation (displacement of part of the brain from its usual compartment) of the cerebral hemisphere between the corpus callosurn and the free border of the falx cerebri on the same side 
• herniation of the midbrain between the pons and the free border of the tentorium cerebelli on the same side 
• compression of the subarachnoid space and flattening-of the cerebral convolutions 

distortion of the shape of the ventricles and their ducts herniation of the cerebellum through the foramen magnum protrusion of the medulla oblongata through the foramen magnum ( coning'). 
Obstruction of the flow of cerebrospinal fluid e ventricles or their ducts may be pushed out of position or a duct obstructed. The effects depend on the position of the lesion, e.g. compression of the aqueduct of the midbrain causes dilation of the lateral ventricles and the third ventricle, further increasing the ICP.