A thunderclap headache is a headache that is severe and has a sudden onset. It is defined as a severe headache that takes seconds to minutes to reach maximum intensity. Although approximately 75% are attributed to "primary" headaches—headache disorder, non-specific headache, idiopathic thunderclap headache, or uncertain headache disorder—the remainder are secondary to other causes, which can include some extremely dangerous acute conditions, as well as infections and other conditions. Usually, further investigations are performed to identify the underlying cause. Signs and symptoms A headache is called "thunderclap headache" if it is severe in character and reaches maximum severity within seconds to minutes of onset. In many cases, there are no other abnormalities, but the various causes of thunderclap headaches may lead to a number of neurological symptoms. Causes Approximately 75% are attributed to "primary" headaches: headache disorder, non-specific headache, idiopathic thunderclap headache or uncertain headache disorder. The remainder are secondary to a number of conditions, including: Subarachnoid hemorrhage (10–25% of all cases of thunderclap headache) Cerebral venous sinus thrombosis Cervical artery dissection Hypertensive emergency (severely raised blood pressure) Spontaneous intracranial hypotension (unexplained low cerebrospinal fluid pressure) Stroke (headache occurs in about 25% of strokes but usually not thunderclap character) Retroclival hematoma (hematoma behind the clivus in the skull, usually due to physical trauma but sometimes spontaneous) Pituitary apoplexy (infarction or hemorrhage of the pituitary gland) Colloid cyst of the third ventricle Meningitis, sinusitis Reversible cerebral vasoconstriction syndrome (previously Call-Fleming syndrome, several subtypes) Primary cough headache, primary exertional headache, and primary sexual headache The most important of the secondary causes are subarachnoid hemorrhage, cerebral venous sinus thrombosis, and dissection of an artery in the neck. In subarachnoid hemorrhage, there may be syncope (transient loss of consciousness), seizures, meningism (neck pain and stiffness), visual symptoms, and vomiting. 50–70% of people with subarachnoid hemorrhage have an isolated headache without decreased level of consciousness. The headache typically persists for several days. Cerebral venous sinus thrombosis, thrombosis of the veins of the brain, usually causes a headache that reflects raised intracranial pressure and is therefore made worse by anything that makes the pressure rise further, such as coughing. In 2–10% of cases, the headache is of thunderclap character. In most cases there are other neurological abnormalities, such as seizures and weakness of part of the body, but in 15–30% the headache is the only abnormality. Carotid artery dissection and vertebral artery dissection (together cervical artery