Leung ordered a computed tomography (CT) scan. This showed that blood had seeped into her cranial cavity and dangerously raised the pressure inside her head. This had caused her blackout, severe headache and other symptoms.

In more severe cases, excessive pressure in the brain could kill brain tissue, resulting in stroke-like symptoms of weakness in parts of the body and neurological damage. It could also cause instant death.

Lau was lucky. A ruptured aneurysm was the likeliest cause of the bleeding.

Doctors are not clear why aneurysms develop, although a popular theory is that a weakness in the wall of the blood vessel allows a 'balloon' to form on the side of the vessel as the pressure of rushing blood pushes against the vessel wall. The balloon swells under the continued pressure, filling with blood and growing larger until it pops. When it does, internal bleeding occurs. The bleeding usually stops by itself as the body's natural clotting function kicks in.

Aneurysms can be found throughout the body, but they don't often cause problems because they are likely to burst before they can grow large enough to press on important tissue. But in the brain, which is contained by the skull's rigid structure, the leaking blood - with nowhere to go - creates excessive pressure in the cranial cavity, faster than the body can naturally reabsorb the blood.

Leung now needed to pinpoint the location of the suspected aneurysm and bleeding. Lau was given a digital subtraction angiography (DSA), in which a special iodine-containing dye called contrast was injected via a catheter near the base of her brain so that her blood vessels would show up clearly in an X-ray. Although imperative to her diagnosis and treatment, the DSA carried a risk of stroke, as the dye could irritate her blood.

As the insertion of the catheter injures the blood vessel, it could also cause a stroke-causing clot.

Lau's DSA showed that the aneurysm resided deep in the middle of her brain. If doctors left the aneurysm alone, Lau ran the risk of suffering another ruptured aneurysm. So despite the risks involved in tampering with a blood vessel inside her brain, treatment was necessary.

Two decades ago, patients would have had to endure a craniotomy, where the head was opened to expose the brain and the aneurysm. But technological advances allowed Leung to use a much less invasive method called coiling to seal off the aneurysm.

First, Lau was injected with contrast so that Leung could use an X-ray to guide him during the procedure.

Then, a catheter was inserted into Lau's femoral artery near her groin, as it was large enough for the catheter and far away enough from the brain to avoid risking a stroke with a blood clot.

The catheter was then guided all the way up the artery into her brain.

'It is like a highway,' says Leung of the path of the artery. 'There are a lot of exits, but if we stay on the main road, it will lead right to the brain.'

In this catheter is a tiny wire made of an amazing 'memory metal'. When it is travelling up the artery, the wire remains straight. But when it reaches and is inserted into the cavity of the aneurysm, it 'remembers' its original shape and curls into a perfect sphere to completely fill and seal off the blood vessel balloon.

Lau's treatment was a success. But the attention now turned to her younger identical twin sister, Amy. The medical community is aware that twins often exhibit a tendency to develop similar diseases. So there was a chance that Amy might also have an aneurysm.

In the past, doctors could use only DSA to screen accurately for brain aneurysm. But the risks that were involved made screening in asymptomatic patients very controversial.

However, in the past five years, a safer screening process - called a magnetic resonance angiogram (MRA) - has emerged.

In MRA, the brain is scanned after the contrast is injected at a more remote location, such as the arm. This method lowers the risk of a procedure-induced stroke. But the quality of the scan is still good enough to pick up the diluted contrast and provide a fairly clear look at the blood vessels.

Given Lau's close shave with death, Amy agreed to an MRA screening. It turned out that she, too, had an aneurysm - in the mirror position of her sister's. Identical or mirror-position aneurysms in twins are so rare that only 15 cases have been documented in medical literature.

With this rather unsettling information in hand, Amy is now much more aware of the health risk she bears. Her sister's operation has informed her of the symptoms to watch out for if things go badly.