1. Field of the Invention
This invention relates generally to measuring and monitoring intracranial pressure in humans or other vertebrates. In particular, this invention is an apparatus and method which can accurately determine over a period of time, without invasion of the body, whether a subject is experiencing elevated pressure in the cranial vault for the purpose of diagnosing instances of subdural hematoma or other life-threatening conditions.
2. Description of the Related Art
Monitoring of intracranial pressure is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies, or other conditions, that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery.
Intracranial pressure is regularly measured and monitored by means of a pressure sensor inserted through the skull into the brain. Usually a hole is drilled in the skull, and a catheter with a pressure sensor is inserted into the brain fluid. To obtain a pressure volume index, the change in intracranial pressure is monitored after a known bolus of saline solution is inserted at a known rate. This known procedure, while simple and accurate, is not suitable for long term monitoring, because an open wound must be maintained in the skull for the catheter with the pressure sensor. Antibiotics are only partially effective in treating cranial infections, so the pressure sensor can only be left in situ for two weeks or less.
Long term monitoring of intracranial pressure, without the need for maintaining a open wound in the skull, is possible if a pressure sensor with a transmitter is implanted in the brain. The intracranial pressure is thereafter monitored by means of a receiver located outside the skull. Such a solution is, however, unattractive because of risks involved in implanting anything in the brain, and because of the problems of providing power to a transmitter implant. One such remote pressure sensor is described in U.S. Pat. No. 4,124,023 to Fleischmann et al. However, this device uses nuclear material as an energy source, making it poorly suited for implantation into a human brain.
Other methods, claiming to be non-invasive, are based on the measurement of some quantity that depends on intracranial pressure, but which does not have a fixed relationship to intracranial pressure.
One such method is described in U.S. Pat. No. 4,204,547 to Allocca. Allocca occludes the blood flow in a jugular vein for a few seconds, and measures the resulting rate of change of blood flow within the jugular vein upstream of the occlusion as an indicator of the intracranial pressure.
Another method, as proposed in U.S. Pat. No. 4,564,022 to Rosenfeld et al., directs a sensory stimulus towards the patient, e.g. a flash of light into the eyes, and measures the latency of a negative-going wave of the electrical brain activity as an indicator of intracranial pressure.
These known indirect methods may be used, under very restricted conditions, as possible indicators of variations of the intracranial pressure in a patient. However, values for the intracranial pressure cannot be monitored over an extended period of time without using invasive monitoring methods. Such monitoring is possible by inserting a pressure transducer into the brain of the patient being monitored. This is, however, a traumatic and undesirable procedure.
Recent research suggests that ultrasound can be used to monitor the intracranial pressure in a patient on a continuing basis by using a single ultrasonic transducer aimed at the patient's head to emit a pulse of acoustic energy, part of which propagates through the cranial vault and is reflected by the bone structure on the other side. Thus measurement of the transit time through the intervening matter contained in the cranial vault can be accurately measured by measuring the time between the generation of the ultrasonic pulse to the return of the reflected wavefront.
An apparatus which can accomplish the measurement described is disclosed as the CONSTANT FREQUENCY PULSED PHASE-LOCKED LOOP MEASURING DEVICE in U.S. Pat. No. 5,214,955 (the '955 Patent) to Yost et al., which is herein incorporated by reference. One of the shortcomings of application of the Yost '955 system to the long term monitoring of intracranial pressure is that of the "soft" interface between the face of the transducer and the skull of the target patient. The flesh covering the skull is literally soft, and the transducer's interface with the patient's surrounding skin may vary. Near-field reflections from this interface and the underlying interface between the scalp and the skull can cause erroneous results in the measurement of intracranial pressure unless extreme measures are taken to ensure consistent placement of the measurement transducer. Even if adequate transducer placement precautions are taken, certain other conditions such as the edema which frequently accompanies severe head injuries can exacerbate the problems of accurate placement of the measurement transducer.