External causes Head injuries Brain injuries are commonly caused by impacts to the head, such as
coup and contrecoup injuries,
penetrating head injuries,
diffuse axonal injuries and
blast injuries. In these cases the injuries are referred to as
traumatic brain injuries, or TBIs. A coup injury occurs when a moving object strikes the head and causes the brain to hit the side of the
cranium that hit the object. A contrecoup injury occurs when the head strikes a stationary object, causing the brain to rebound and hit the side of the cranium opposite the object. Both coup and contrecoup injuries cause
contusions to the affected area of the brain. Contrecoup injuries are more severe than coup injuries. A
diffuse axonal injury is caused by
shearing forces on the brain that lead to lesions in the brain's white matter tracts. These shearing forces are seen in cases, such as traffic accidents, Penetrating head trauma results from
gunshot wounds,
stabbings and other incidents in which an object breaches the skull and the
dura mater. The object, while creating a hole in its path, produces sonic and pressure waves that create temporary cavitations, which expand and retract, leading to haemorrhages and haematoma. It is the most lethal form of traumatic brain injury, as 70-90% of victims die before reaching the hospital. Blast injuries originate from
explosions and are divided into primary (injuries from the blast wave itself), secondary (wounds from shrapnel), tertiary (nonpenetrating projectile impacts and injuries from falls) and quaternary (other injuries, such as burns). Head injuries sustained close to a blast's epicenter are largely fatal, but those sustained from a distance are often mild or moderate.
Iatrogenic Brain lesions are sometimes intentionally inflicted during
neurosurgery to treat
epilepsy and other neurological disorders. These lesions are induced by excision, by electric shocks (electrolytic lesions) to the exposed brain or by infusion of
excitotoxins to specific areas.
Pathological causes Stroke There are two major types of
strokes, haemorrhagic and ischaemic, both of which cause brain damage. Haemorrhagic strokes occur when a blood vessel in the brain ruptures; those that bleed into the
parenchyma are intracerebral haemorrhages and those that bleed into the
subarachnoid space are subarachnoid haemorrhages. Blood leaks around the brain, increasing the intracranial pressure and compressing the brain tissue. Ischaemic strokes occur when circulation to a brain region is obstructed by a thrombosis or embolism; the brain regions dependent on the obstructed vessel
infarct, depriving brain cells of blood supply and causing them to die.
Genetic disorders Many genetic mutations and hereditary diseases manifest as brain degeneration in childhood, in adolescence or in adulthood. Those that present during childhood are known collectively as
childhood dementia, as they produce a progressive deterioration of cognitive abilities similar to adult-onset
dementia. They are typically
inborn errors of metabolism such as
lysosomal storage disorders, in which enzyme or protein defects cause storage material to accumulate in neurons, resulting in
neurodegeneration; the mechanism by which this causes neurodegeneration is unclear. Examples of disorders of childhood dementia include
Niemann–Pick disease type C,
Sanfilippo syndrome,
Batten disease and
Lafora disease.
Other neurodegenerative diseases There are also
neurodegenerative diseases that are not completely hereditary or genetic in origin, including
Alzheimer's disease,
Parkinson's disease,
multiple sclerosis and
amyotrophic lateral sclerosis. The precise cause of Alzheimer's disease, the commonest type of dementia, is unknown, but autopsies of people who died from complications of it have revealed
amyloid plaques and
neurofibrillary tangles, which lead to
neuroinflammation, loss of
synaptic connections and neurodegeneration. Alzheimer's disease presents with gradual memory loss and decline in cognitive, verbal, motor and executive skills. The period between diagnosis and death varies; though some patients die within five years of diagnosis others live ten years or longer. 70–89% of Parkinson's disease patients experience neurological and psychiatric symptoms like
depression,
anxiety, cognitive impairment and dementia. Multiple sclerosis is an
autoimmune condition that results in focal inflammation and neurodegeneration, leading to the development of plaques in the brain that manifest as visual impairment, numbness, bowel and bladder dysfunction and cognitive impairment. Amyotrophic lateral sclerosis, the most common
motor neuron disease, is characterised by the degeneration of neurons in the
motor cortex,
spinal cord and
brainstem, which are replaced by glial cells. Symptoms vary but include exaggerated reflexes, dysphagia, spasticity, a loss of coordination, muscle atrophy and general weakness. Cognitive impairment presents in 30% to 50% of patients.
Neoplasms Malignant
tumours of the brain can either develop in the brain or
spread to it from cancer of other organs, whereas benign tumours necessarily originate within the brain. Both classes of brain tumours result in brain damage by infiltrating or compressing parts of the brain. The types of primary brain tumours include
astrocytomas,
gliomas,
glioblastomas,
oligoastrocytomas,
ependymomas,
medulloblastomas,
pineocytomas and
pineoblastomas,
meningiomas,
hemangiopericytomas and
craniopharyngiomas. Yet most cases of brain tumours are metastatic, Symptoms of brain tumours include seizures, headaches, loss of appetite, nausea and vomiting, personality and emotional changes, weakness and issues with concentration, vision, hearing and speech.
Viral encephalitis and
viral meningitis, which are caused commonly by the
herpes simplex and
Epstein-Barr viruses, occur when the virus in question triggers an inflammatory response that disrupts the function of neurons and leads to cerebral oedema, haemorrhage and vascular congestion.
Leukocytes and microglial cells may be interfered with as well. Hallucinations, behavioural changes, Parkinsonian symptoms and cognitive decline may result.
Mucormycosis, a fungal infection of the family
Mucoraceae afflicting patients with
diabetes and other
immunocompromising conditions, can infect the nasal cavity or sinuses and thereby make its way to the brain, where it is called rhinocerebral mucormycosis. In advanced cases the fungus infiltrates blood vessels, including the
carotid artery, in or associated with the brain, leading to thrombosis and stroke. Symptoms such as gait disturbances, altered state of consciousness, seizures and dizziness may develop.
Wernicke–Korsakoff syndrome Wernicke–Korsakoff syndrome, caused by a vitamin B1 (
thiamine) deficiency, can result in brain damage. Wernicke-Korsakoff syndrome is typically caused by conditions causing thiamine deficiency, such as chronic heavy alcohol use or by conditions that affect nutritional absorption, including colon cancer, eating disorders and gastric bypass.
Poisoning Exposure to
toxic substances can result in brain injury and stunted neurological development in children.
Mercury, for instance, is a common element in manufacturing processes but is detrimental to human health, particularly neurological health. When mercury deposits in the central nervous system it atrophies the
cerebellum, the
postcentral gyri and the
calcarine sulcus of the brain. Children who were exposed to high doses of
methylmercury, a highly toxic form of mercury, or whose mothers were exposed while pregnant with them, are significantly likelier than the general population to have intellectual disabilities,
cerebral palsy,
developmental delays and
epilepsy.
Lead, a metal ubiquitous in gasoline and household paints in the United States until it was banned from gasoline in 1972 and paint in 1978, causes neurological sequelae in both low-dose chronic exposures and sudden higher-dose exposures. Lead binds to various proteins throughout the human body; this interferes with the
synaptic pruning process that children undergo and leads to cognitive and behavioural impairments. In extreme doses, lead can disrupt calcium receptors, break down
cell membrane components and render the
blood–brain barrier more permeable;
lead encephalopathy results, which manifests as
cerebral edema, seizures, coma and death. ==Diagnosis==