The Arachnoid

Beneath the toughness of the dura lies a layer called the arachnoid mater. Transparent and delicate, it drapes the brain like the finest of veils. One of my favourite writers, Maria Popova, once wrote that the brain "is a cathedral built of gossamer threads", and nowhere is that truer than in the arachnoid. Under the microscope, it looks less like a membrane and more like spun sugar, or a spider's web catching morning dew. It is beautiful, but it is also treacherous. When blood seeps into it, the cathedral darkens.

- By Dr MAZDA TUREL

One evening, a woman in her fifties was rushed in with the sudden, thunderclap headache that makes every doctor move faster. "It felt like something exploded in my head," she whispered on meeting me. She wasn't exaggerating. A CT scan confirmed a subarachnoid haemorrhage-blood flooding into the delicate space beneath the arachnoid membrane. The culprit was a ruptured aneurysm at the junction of the anterior communicating artery, a tiny outpouching of vessel wall that had finally given way.

The thing about subarachnoid haemorrhage is its drama. Patients go from reading the newspaper to collapsing on the bathroom floor in minutes. Families describe it as "she was fine, and then she wasn't". The arachnoid space, meant to cradle cerebrospinal fluid in quiet suspension, suddenly becomes a pool of blood-irritating, inflaming, threatening. It is one of the few neurosurgical emergencies that unites every department-the ER, radiology, the ICU, and the operating theatre-in a single collective gasp.