A subarachnoid bolt is a medical device used in intensive care for patients with head injuries or conditions that increase pressure inside the head. It is a small, hollow screw surgically inserted into the skull. The function of this device is to directly measure intracranial pressure, or ICP. This pressure measurement provides information to medical teams, allowing for continuous monitoring of the pressure being exerted on the brain and guiding treatment decisions aimed at preventing further injury.
The Purpose of Intracranial Pressure Monitoring
The skull is a rigid space containing the brain, blood, and cerebrospinal fluid (CSF). Intracranial pressure is the pressure exerted by these components within the cranium. According to the Monro-Kellie theory, the volume inside the skull is fixed, meaning if one component increases in volume, another must decrease to maintain stable pressure. A normal ICP reading is below 20 to 25 millimeters of mercury (mmHg).
Elevated ICP can occur from a traumatic brain injury (TBI), stroke, brain tumors, or hydrocephalus. These conditions can cause the brain to swell or lead to an accumulation of blood or CSF, increasing the total volume inside the skull. This rise in pressure can compress brain tissue and reduce cerebral blood flow, depriving the brain of oxygen.
Using a device like a subarachnoid bolt allows medical teams to detect dangerous increases in pressure almost instantly. This early detection allows for prompt interventions, such as administering medication or adjusting ventilator settings. The goal is to lower the pressure and protect the brain from irreversible damage.
The Subarachnoid Bolt Procedure
The insertion of a subarachnoid bolt is a surgical procedure performed in an operating room or at the patient’s bedside in an intensive care unit. The process begins with shaving and cleaning a small area of the scalp. A specific location on the front part of the skull, often Kocher’s point, is marked as the entry point to avoid sensitive areas of the brain.
A neurosurgeon makes a small incision in the scalp and uses a specialized drill to create a burr hole, which is a small opening in the skull. The dura mater, the tough outer membrane covering the brain, is then carefully punctured. The bolt is twisted into the burr hole until its tip sits in the subarachnoid space—the area between the brain and the tissues that cover it.
Once the bolt is securely in place, it is connected to a monitoring system using fluid-filled tubing. This tubing runs from the bolt to an external transducer, which converts the pressure sensed from the CSF into an electrical signal. This signal is then displayed as a continuous waveform and a numerical value on a monitor, providing a real-time measurement of the ICP.
Comparison with Other Monitoring Devices
The subarachnoid bolt is one of several devices used for ICP management. The most common alternative is the external ventricular drain (EVD), a flexible catheter inserted deeper into the brain’s ventricles where CSF circulates. The primary advantage of an EVD is that it can both monitor pressure and drain excess CSF to therapeutically lower high ICP.
In contrast, the subarachnoid bolt is less invasive because it does not penetrate the brain tissue. Its sole function is to monitor pressure; it cannot be used to drain fluid. A bolt may be chosen when placing an EVD is technically difficult, such as when the ventricles are compressed from significant brain swelling.
Another alternative is the intraparenchymal fiber optic catheter. This device is a thin cable with a sensor at its tip that is placed directly into the brain tissue. Like a subarachnoid bolt, it only monitors pressure. The choice between these devices depends on the specific clinical situation, the patient’s condition, and the need for CSF drainage.
Associated Risks and Patient Considerations
The use of a subarachnoid bolt, like any invasive procedure, carries potential risks. A primary concern is infection, as the device creates a direct pathway from outside the body into the space surrounding the brain, which could lead to meningitis. There is also a risk of bleeding or hemorrhage at the insertion site.
The tip of the bolt can become blocked by tissue debris, which may lead to inaccurate pressure readings. Additionally, there is a small possibility of a cerebrospinal fluid leak around the insertion site. For the device to function correctly, the patient must remain relatively still, as excessive movement can affect the readings or dislodge the bolt.