What it is
External ventricular drain (EVD): A catheter is placed through the skull and brain into the lateral ventricle, connected to a closed external drainage system. This allows continuous monitoring of intracranial pressure (ICP) and — critically — the ability to drain small amounts of CSF to reduce pressure when it rises. EVDs are placed in the operating room or at the bedside in the ICU, typically at Kocher's point just in front of the coronal suture.
ICP monitor (parenchymal bolt): A small pressure transducer is placed a short distance into the brain tissue, measuring ICP continuously without drainage capability. Simpler to place and maintain than an EVD, and used when CSF drainage is not needed but ICP monitoring is.
Why ICP monitoring matters
After severe traumatic brain injury or subarachnoid hemorrhage, the brain can swell or accumulate blood and fluid, raising intracranial pressure. Sustained elevated ICP above 20–22 mmHg compromises cerebral perfusion — the brain stops receiving adequate blood flow — and causes secondary injury on top of the initial event. Monitoring ICP allows the ICU team to make targeted decisions about interventions: head positioning, osmotic therapy, sedation, CSF drainage, or escalation to surgery.
What patients and families should know
These devices are placed in the neuro-ICU as part of acute neurological monitoring. They are temporary, removed once the acute phase of injury has passed and monitoring is no longer needed. Placement carries a small risk of hemorrhage along the catheter tract and a risk of infection that increases over time — which is why EVDs are managed with strict sterile protocols and are not intended for indefinite placement.