Patent No. 6589189 Non-invasive method and apparatus for monitoring intracranial pressure

 

Patent No. 6589189

Non-invasive method and apparatus for monitoring intracranial pressure (Meyerson, et al., Jul 8, 2003)

Abstract

An intracranial pressure (ICP) monitoring system and method for using such is disclosed. The system stimulates and interprets predictable external effects of elevated ICP. In one embodiment, the system non-invasively and continuously monitors ICP by stimulating and interpreting predictable changes measured in the otoacoustic emission (OAE) signal of the patient. The system may alternately non-invasively and continuously monitor ICP by stimulating, measuring, and interpreting other responses which rely on the transmission of vibrations through the middle ear cochlear interface, such as tympanograms (TGRAMs), ocular-acoustic reflex, auditory brainstem response (ABR), or cochlear microphonics.

Notes:

This application claims the benefit of U.S. provisional application No. 60/175,099, Jan. 7, 2000.

 

FIELD OF THE INVENTION

The present invention relates generally to intracranial pressure (ICP) monitoring. Specifically, the invention relates to a method and apparatus of non-invasively, without requiring a breach of the skin, skull, or dura, monitoring ICP. More specifically, the invention provides a method and apparatus for stimulating and interpreting predictable external effects of elevated ICP such as changes in cochlear impedance coupling to monitor ICP. In one embodiment, the system non-invasively and continuously monitors ICP by stimulating and interpreting predictable changes measured in the otoacoustic emission (OAE) signal of the patient.

BACKGROUND OF THE INVENTION

Intracranial pressure is closely related to cerebral perfusion (blood flow in the brain). Elevated ICP reduces cerebral perfusion pressure (CPP), and if uncontrolled, results in vomiting, headaches, blurred vision, or loss of consciousness, escalating to permanent brain damage, and eventually a fatal hemorrhage at the base of the skull. Increased ICP is a medical/surgical emergency. Particular instances where it is desirable to monitor ICP are in traumatic brain injury (TBI) victims, stroke victims, hydrocephalus patients, and patients undergoing intracranial procedures, "shaken baby" syndrome, kidney dialysis, or artificial liver support. Current methods of monitoring ICP are typically invasive, expensive, and/or difficult to perform outside of a hospital setting.

Traumatic Brain Injury

An estimated 1.75 million TBI's occur annually (extrapolated from 1,540,000 TBI's in 1991) in the United States. The U.S. Department of Education, National Institute on Disability and Rehabilitation Research in conjunction with 17 TBI research hospitals around the U.S. have established a set of indicators for classification of TBI: 1) Documented loss of consciousness for an unspecified time; 2) Amnesia for the event. No recall of the actual trauma; 3) A Glasgow Coma Scale (GCS) score of less than 15 during the first 24 hours.

Of these indicators, amnesia assessment is a preferred indicator of TBI severity. Amnesia of one day is considered moderate, while one month of amnesia indicates severe TBI. Although amnesia is a good indicator of TBI severity and a reasonable predictor of long term outcomes, this slow evaluation method provides no help in emergency response to patient diagnosis or treatment.

The GCS is a TBI severity assessment system using subjective observations in three basic categories: eye opening (E), best motor response (M), and verbal response (V). A patient's GCS score is the sum of the patients E, B, and V scores. This sum ranges from 3 to 15 with 3 indicating severe TBS and 15 indicating no or very mild TBS. The non-invasive nature of CT scans make them a very common procedure for TBI patients whose GCS score is mild or moderate, but the data is slow and expensive. The patient must be brought to the equipment and, in many cases, the patient cannot be immediately moved. The cost is compounded because CTs do not provide direct assessment of ICP (two or more scans are required to assess trends) and in 9-13% of patients, the CT will be normal even with elevated ICP. Due to the invasiveness of current ICP monitoring procedures, the general practice is to not start invasive ICP monitoring unless the patient's GCS score is less than or equal to 8, at which time the drawbacks of the procedure are outweighed by the severity of patient condition. This means 90% of hospitalized TBI patients are assessed only with GCS and possibly a CT scan. Significant rehabilitation problems have resulted in patients with mild or moderate GCS scores, highlighting the need for non-invasive ICP monitoring techniques. GCS assessment and CT scans are helpful, but clearly point out the time-sensitive need for more direct data.

Stroke

First time strokes can unpredictably lead to brain swelling. Strokes are divided into two main categories, (1) hemorrhagic (the bursting of a cerebral blood vessel), and (2) the more common ischemic (the blockage of a cerebral blood vessel). Correct diagnosis of the stroke type is critical because the clot-dissolving drug t-PA (and analogs), used to treat ischemic strokes, is contraindicated for hemorrhagic strokes. Furthermore, the diagnosis of stroke type is time critical because starting t-PA treatment more than 3 hours after stroke could result in a higher rate of bleeding into the brain. Approximately 80% of all strokes each year are ischemic. ICP in this type of stroke initially remains low, but elevates as the loss of blood traumatizes the brain. ICP will also elevate when the clot is removed and blood flow is restored. Hemorrhagic strokes involve the direct complication of elevated ICP.

Hydrocephalus

Ventroperneal shunts are implanted to treat hydrocephalus. A CT scan cannot be used for patients with hydrocephalus because the ventricles of the brain commonly remain swollen even with normal ICP, and the risk of invasive systems cannot be justified. Diagnosis of shunt system problems are currently based on symptoms reported by the patient or caregiver and are thus subjective. OAE is stable over a period of months and an ICP baseline could be stored for these patients and compared with measurements during future visits. Current ICP measurement technology does not provide adequate means for treating patients with hydrocephalus because of the possible inaccurate readings and the risk inherent in invasive measurement procedures.

Intracranial Procedures

There is a current medical need for ICP monitoring for patient recovering from elective intracranial surgery. A retrospective study found elevated ICP postoperatively in 17% of patients who underwent supratentorial or infratentorial surgery. Of these, over one fourth experienced clinical symptoms latent or concurrent to ICP elevation. Medical personnel need to be able to identify these patients and administer therapy before any clinical symptoms are detected. It is interesting to note that during the study, which used invasive methods to measure ICP, the infection rate was 1.2%, highlighting the risk of invasive ICP monitoring.

Laparoscopic and Abdominal Insulflation

Laparoscopic procedures are often performed requiring abdominal insulflation concomitant with Trendelenburg (head down tilt) position. The combination of anesthetic, body position, and insulflation can substantially elevate ICP. Due to the prohibitive additional cost and risk, routine ICP monitoring during these procedures is not done. However, there is growing concern about elevated ICP during these procedures.

Liver and Kidney Support

There is a current medical need to assess ICP variation in patients who are in the latter stages of liver failure and require external liver support (i.e. artificial liver). As the liver fails, toxins build up in the body and this build up generally causes elevation of ICP. One measure of liver function (or therapy function) is to monitor ICP. As toxins build, ICP increases, thus allowing the physician (and possibly the patient) to anticipate when the next therapy session should commence. While on the artificial liver machine, toxins are removed, and ICP should fall, providing an indication of therapy function. A similar situation exists for patients being treated for kidney failure, either by hemodialysis or peritoneal dialysis.

Others

Additional causes for an increase in ICP include the following: meningitis, encephalitis, intracranial abscess, hemorrhage, shunt blockage, tumors, Reye's Syndrome, "shaken baby" syndrome, and benign intracranial hypertension.

Normal intracranial pressure (ICP) for adults is between 5 mm/Hg and 15 mm/Hg. When ICP level is considered abnormal is controversial, however, it becomes a concern as it rises higher than 20 mm/Hg.

ICP is closely related to cerebral perfusion (blood flow in the brain). To a first approximation, the cerebral perfusion pressure (CPP) is the difference between an individual's arterial blood pressure (ABP) and intracranial pressure (ICP). Thus, approximately, CPP=ABP-ICP. If one assumes ABP to be constant, then an increase in ICP results in less blood flow to the brain. Because of this relationship, and the difficulty of measuring CPP directly, ABP and ICP are often measured to assess CPP. In a healthy individual, automatic regulation mechanisms in the body keep ABP, ICP, and thus CPP within a normal range. These automatic regulation systems are often non-functional in brain trauma, stroke, hydrocephalic patients, and patients with liver or kidney failure, so that monitoring and management of ICP becomes a critical aspect of medical care. In addition, during surgeries such as abdominal laparoscopy, cardiac bypass, and following any type of cranial surgery, continuous, non-invasive monitoring of ICP, if it were economically and technically feasible, would be beneficial. Elevated ICP reduces CPP, and if uncontrolled, results in vomiting, headaches, blurred vision, or loss of consciousness, escalating to permanent brain damage, and eventually a fatal hemorrhage at the base of the skull.

Current ICP monitoring techniques are generally grouped as either invasive and non-invasive. The invasive group is further divided into soft tissue, for example lumbar puncture, and bone drilling procedures, for example subarachnoid screws or plugs, subdural catheters, and ventriculostomy catheters.

Lumbar Puncture

In a lumbar puncture or spinal tap, a clinician delicately passes a fine needle through the lower region of the back into the fluid of the spinal cord. Once the spinal spaces have been penetrated, ICP can be estimated by attaching a pressure sensor. The communication between the fluid in the spinal column and the cranium allows the physician to ascertain the pressure in the cranium. Though invasive, a lumbar puncture is sometimes preferred because it is a soft tissue procedure rather than a cranial procedure. Generally, a non-neuro clinician will not feel comfortable performing a cranial procedure, but will perform a lumbar puncture. This procedure does allow transient manipulation or sampling of the intracranial fluid system, but is often painful and many times results in after affects, and always raises patient apprehension. It is a short term procedure and is generally not considered for long term ICP monitoring.

Cranial ICP Assessment Methods

There are five common current invasive methods of measuring ICP which breach the skull: ventriculostomy, intraparenchymal fiberoptic catheter, epidural transducer, subdural catheter, and subdural bolt. These have varying degrees of invasiveness. A subarachnoid screw involves inserting the screw in a hole which has been drilled through the skull bone, but does not breach the dura. Such systems can be threaded like a screw, or just a "friction fit" plug. A subdural catheter involves inserting the catheter a hole in the skull and dura, and squeezing the catheter between the dura and the brain itself. Ventriculostomy catheters are inserted through a hole drilled in the skull and dura, and are blindly forced through the gray matter such that the tip of the catheter is positioned in one of the cranial ventricles.

Of these methods, only a ventriculostomy can also be used to deliver therapy, which is usually draining fluid from the ventricles. The epidural approach has the lowest complication rate, but all suffer gradual loss of accuracy. The failure mechanism is stiffening of the dura and/or localized hematomas at the monitoring site. This known degradation starts immediately after implant and will make the transducer unreliable anywhere from 1 to 3 weeks post implant.

This invasive group, although medically accepted and routinely used, suffers several drawbacks. The transducer has to be calibrated in some fashion before insertion. The placement of the system requires a highly trained individual; in almost all clinical settings, this procedure is limited to physicians, and in most cases further limited to a specialist such as a neurosurgeon. This generally limits these procedures to larger medical facilities. Furthermore, there is a relatively short term (32-72 hours) reliability and stability of the system, either because of leaks or plugging of the transducer, or inadvertently being disturbed, or even being pulled out. This concern generally limits these procedures to a more intense monitoring setting such as an ICU. There are also associated risks of transducer placement such as brain or spinal cord damage and infection. Even though these risks are low, these concerns generally limit the group of non-invasive ICP monitoring techniques to the hospital setting and prevents standard use of the techniques in clinic or nursing home settings.

In the non-invasive group, the accepted, commercially available method of monitoring ICP consists of taking a CT or other image of the head, interpreting the image and observing changes in various features. This method requires a high level of skill to read and assess the images and requires that the patient be brought to the imaging equipment. In many cases, a scan is delayed or put off simply because the patient is not stable enough to be moved. Even after the patient is stable, the various tubes and equipment connections to the patient have to be accounted for during the trip to the CT, and many times additional personnel are required, with a respective increase in cost. In addition, the scans themselves are single measurements--"snap-shots" in time, of which at least two are required to assess subtle-changes and variations. A `series` of scans could approximate continuous monitoring, but is not economically practical.

Other non-invasive ICP monitoring techniques have been developed. A non-invasive ICP monitoring system is taught in U.S. Pat. No. 4,841,986 to Marchbanks. This system is based on fine volume measurements of the external auditory canal during elicitation of the human stapedial reflex. The concept is that at normal ICP, the stapedial reflex will pull on the stapes, resulting in distortion of the tympanic membrane (ear drum). This will define a volume for that ICP level. As ICP increases, the stapes will be pushed away from the cochlea, and the stapedial reflex will pull on the stapes differently, resulting in a different distortion of the tympanic membrane, which can be measured as a delta volume. This system requires a rather loud sound to be output in the patients ear. Severe ambient noise and sealing constraints are inherent in the technology which lead to a cumbersome and time consuming setup. The system has a non-linear response, acting much like a threshold function.

A compliance measuring system taught by Paulat in EP 0933061A1 measures changes in ICP. The system uses micro volume measurements in the auditory canal similar to the Marchbanks system. The system detects the fine volume changes as the time varying ICP waves are communicated to the cochlea via the cochlear aqueduct, through the ossicles to the tympanic membrane. This device is AC coupled so it cannot monitor ICP changes over time (i.e. mean ICP values). It has been proposed that frequencies greater than 10 Hz could not be communicated via the cochlear aqueduct. Stated another way, respiration and heart rate frequencies may not be transmitted through the cochlear aqueduct.

In Bridger in U.S. Pat. No. 5,919,144, a non-invasive system is disclosed based on real-time analysis of acoustic interaction with the brain and changes in tissue acoustic properties as ICP changes.

Other non-invasive techniques include: electro-magnetic techniques taught by Ko in U.S. Pat. Nos. 4,690,149 and 4,819,648, by Alperin in U.S. Pat. No. 5,993,398, and Paulat in U.S. Pat. No. 6,146,336; ultra sonic or vibratory techniques such as U.S. Pat. No. 3,872,858 to Hudson et al., U.S. Pat. No. 4,043,321 to Soldner et al., U.S. Pat. Nos. 4,971,061 and 4,984,567 to Kageyama et al., U.S. Pat. Nos. 5,074,310 and 5,117,835 to Mick, U.S. Pat. Nos. 5,388,583 and 5,951,477 to Ragauskas et al., U.S. Pat. No. 5,411,028 to Bonnefous, U.S. Pat. No. 5,617,873 to Yost et al., U.S. Pat. No. 5,840,018 to Michaeli, U.S. Pat. No. 6,086,533 to Madsen, and U.S. Pat. No. 6,117,089 to Sinha; jugular vein occlusion taught by Allocca in U.S. Pat. No. 4,204,547; ocular latency in U.S. Pat. No. 4,564,022 to Rosenfeld et al.

Another system stated to be "non-invasive" is described in U.S. Pat. No. 4,141,348 to Hittman and companion U.S. Pat. No. 4,186,751 to Fleischmann. This nuclear powered pressure sensor was not grouped with the other non-invasive systems because it is designed to be implanted totally under the scalp of the patient.

Each of the currently used and medically accepted methods of ICP assessment are deficient in some way, and all require a high skill level to administer. Because of the deficits in current measurement methodologies, there is a need for a non-invasive, easily administered, long-term, continuous assessment of ICP.