The classic symptom of<unk>  is thunderclap headache (a headache described as "like being kicked in the head", or the "worst ever", developing over seconds to minutes). This headache often pulsates towards the occiput (the back of the head). About one-third of people have no symptoms apart from the characteristic headache, and about one in ten people who seek medical care with this symptom are later diagnosed with a<unk> . Vomiting may be present, and 1 in 14 have seizures. Confusion, decreased level of consciousness or coma may be present, as may neck stiffness and other signs of meningism. Neck stiffness usually presents six hours after initial onset of<unk> . Isolated dilation of a pupil and loss of the pupillary light reflex may reflect brain herniation as a result of rising intracranial pressure (pressure inside the skull). Intraocular<unk>  (bleeding into the eyeball) may occur in response to the raised pressure: subhyaloid<unk>  (bleeding under the hyaloid membrane, which envelops the vitreous body of the eye) and vitreous<unk>  may be visible on fundoscopy. This is known as Terson syndrome (occurring in 3–13 percent of cases) and is more common in more severe<unk> . Oculomotor nerve abnormalities (affected eye looking downward and outward and inability to lift the eyelid on the same side) or palsy (loss of movement) may indicate bleeding from the posterior communicating artery. Seizures are more common if the<unk>  is from an aneurysm; it is otherwise difficult to predict the site and origin of the<unk>  from the symptoms.<unk>  in a person known to have seizures is often diagnostic of a cerebral arteriovenous malformation. The combination of intracerebral<unk>  and raised intracranial pressure (if present) leads to a "sympathetic surge", i.e. over-activation of the sympathetic system. This is thought to occur through two mechanisms, a direct effect on the medulla that leads to activation of the descending sympathetic nervous system and a local release of inflammatory mediators that circulate to the peripheral circulation where they activate the sympathetic system. As a consequence of the sympathetic surge there is a sudden increase in blood pressure; mediated by increased contractility of the ventricle and increased vasoconstriction leading to increased systemic vascular resistance. The consequences of this sympathetic surge can be sudden, severe, and are frequently life-threatening. The high plasma concentrations of adrenaline also may cause cardiac arrhythmias (irregularities in the heart rate and rhythm), electrocardiographic changes (in 27 percent of cases) and cardiac arrest (in 3 percent of cases) may occur rapidly after the onset of<unk> . A further consequence of this process is neurogenic pulmonary edema where a process of increased pressure within the pulmonary circulation causes leaking of fluid from the pulmonary capillaries into the air spaces, the alveoli, of the lung.<unk>  may also occur in people who have had a head injury. Symptoms may include headache, decreased level of consciousness and hemiparesis (weakness of one side of the body).<unk>  is a frequent occurrence in traumatic brain injury, and carries a poor prognosis if it is associated with deterioration in the level of consciousness. While thunderclap headache is the characteristic symptom of<unk> , less than 10% of those with concerning symptoms have<unk>  on investigations. A number of other causes may need to be considered.
Subarachnoid hemorrhage
Migraine
Subdural hematoma
Meningitis
Cerebral venous sinus thrombosis
Migraine 
ROSAH syndrome
Epidural hematoma