The subarachnoid bolt is a device used to measure intracranial pressure (ICP) directly and continuously in patients with acute brain injuries. This monitoring involves placing a small, hollow screw into the skull to access the space surrounding the brain. Its primary function is to provide real-time pressure readings, which guide treatment in neurocritical care settings. Monitoring ICP offers immediate data on conditions inside the rigid skull, allowing for rapid intervention when pressure rises to dangerous levels. Tracking pressure dynamics can prevent secondary brain injury and improve outcomes for severely ill patients.
Why Intracranial Pressure Must Be Monitored
The skull is a rigid, fixed space containing brain tissue, blood, and cerebrospinal fluid (CSF). This relationship is described by the Monro-Kellie doctrine, which states that the total volume inside the cranium must remain constant. If one component’s volume increases, such as from swelling or bleeding, the volume of the other two must decrease to maintain stable pressure. CSF and venous blood volume are the primary buffers that compensate for small increases in volume.
When these compensatory mechanisms are exhausted, even a small additional increase in volume causes the intracranial pressure to rise exponentially. Elevated ICP is dangerous because it reduces the cerebral perfusion pressure (CPP), the pressure gradient that drives blood flow to the brain. If CPP drops too low, the brain does not receive enough oxygen and nutrients, leading to tissue damage. Sustained high ICP can also result in brain herniation, where tissue is squeezed across rigid structures, leading to severe neurological damage or death.
Immediate ICP monitoring is necessary for conditions involving sudden, severe volume increases, such as traumatic brain injury (TBI), cerebral edema, or subarachnoid hemorrhage. For unconscious patients, clinical observation alone is often insufficient to detect these life-threatening pressure changes. Continuous monitoring allows medical teams to detect high pressure early and administer treatments like draining CSF or adjusting ventilation settings. The normal ICP range is generally considered to be between 5 and 15 mm Hg.
How the Subarachnoid Bolt is Placed and Functions
The subarachnoid bolt is a pressure-sensing device that offers a straightforward method for obtaining immediate ICP readings. The device consists of a hollow screw inserted through a small hole drilled into the skull, typically performed with a twist drill at the patient’s bedside. The insertion site is carefully chosen, often on the non-dominant side of the brain, to avoid major veins and sensitive motor areas.
The bolt is screwed into place until its tip sits just past the inner layer of the skull, positioned in the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This placement allows the device to sense the pressure of the CSF and the adjacent brain tissue. Once the bolt is secure, a tiny pressure-sensing catheter or microtransducer is passed through the hollow center of the bolt. This sensor is then secured with a locking nut and connected to an external transducer and monitor.
Unlike the intraventricular catheter (IVC), which is considered the most accurate method and can also drain CSF, the subarachnoid bolt does not enter the brain’s ventricles. It provides ICP data only, but its placement is often easier and faster, especially when the ventricles are compressed and difficult to access due to severe brain swelling. The bolt system uses fiberoptic or strain gauge technology in the microtransducer to continuously measure pressure waves, providing real-time data to the clinical team.
Managing Risks and Monitoring During Use
Maintaining a sterile field during and after insertion is essential to manage the risk of infection, such as meningitis. Although the infection rate is low, strict adherence to sterile dressing changes is necessary for the duration of monitoring. Hemorrhage is another risk along the sensor’s insertion track, though clinically significant bleeding is uncommon.
Ongoing monitoring requires careful management of the pressure-sensing system to ensure accuracy. The external transducer must be correctly zeroed and leveled to a specific anatomical landmark, often the tragus of the ear, to ensure the reading reflects the actual pressure inside the head. Device malfunction is a common challenge, as the sensor may drift in its zero point over time, leading to inaccurate readings. The medical team must compare the bolt’s readings with the patient’s neurological status and watch for signs of worsening condition.
The subarachnoid bolt is typically used for short-term monitoring and is removed once the patient’s condition stabilizes and the risk of elevated ICP subsides. Removal is a straightforward procedure involving the unscrewing of the bolt and sensor, followed by stitching the incision closed.