Generally speaking, there are three main types of stroke:

1. Ischaemic stroke - a blood vessel gets blocked reducing blood flow to an area of the brain. Ischaemic stroke is the most common type of stroke with 85% of admissions to hospital with stroke being classified as this.

2. Haemorrhagic stroke - a burst blood vessel causes bleeding into the brain. This may be as a result of a weakness in the blood vessel called an aneurysm, conditions such as high blood pressure may make blood vessels more likely to rupture and patients who take anti-coagulation drugs make them higher risk of bleeding on the brain.

3. Transient ischaemic attack (TIA) - Symptoms will only last for a short time.

This is a brain injury that is caused by an outside force. Trauma can damage the brain tissue or cause bleeding in or around the brain that may cause increased pressure in the skull. Even perceived mild injuries can cause significant damage to the brain and may induce a reduced level of consciousness.

Neoplasms are abnormal growth (e.g. a tumour) that may be benign or malignant. Symptoms depend on where the tumour is and how big it is. Some tumours may invade parts of the brain and effect the function of that area. As it grows the neoplasm can take up significant space causing an increase in the intracranial pressure, which may present in altered conscious state.

A seizure is the transient occurrence of signs or symptoms due to abnormal, excessive or synchronous neuronal activity in the brain. Put simply loss of consciousness, convulsions or sensory disturbance due to abnormal electrical activity within the brain.

Seizures may be recurrent in patients with a diagnosis of epilepsy, however isolated seizures can be caused by metabolic disturbances, toxins, structural defects or infection.

For patients having a tonic clonic seizure – note the time. If it lasts longer that 5 minutes or they are having recurrent seizures without recovery in between this is classified as convulsive status epilepticus.

Metabolic encephalopathy is an umbrella term for a wide range of abnormalities in brain chemistry. This can happen when a systemic condition such as renal disease, endocrine disease, liver failure or heart failure cause disturbance in the way the brain works.

Examples of this could be hepatic encephalopathy, hypoglycaemic encephalopathy or uraemic encephalopathy.

A systemic infection may cause a reduced level of consciousness due to hypoperfusion of the brain or a metabolic encephalopathy caused by sepsis. However, the brain itself can become infected, this is called encephalitis. When the covering of the brain or spinal cord becomes inflamed or infected this is called meningitis. These infections can be caused by bacteria or more commonly virus. Patients should improve with treatment of the underlying cause.

Many classes of drugs may affect the central nervous system. Prescription drugs such as benzodiazepines, opiates, anaesthetic agents. Alcohol and recreational drugs may also cause a reduced level of consciousness.

Conversely withdrawal from drugs or alcohol may present with a change in conscious state, all patients admitted to hospital should be asked about alcohol and drug use, and risk of withdrawal assessed.