Intracranial Pressure Monitor

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A non-invasive continuous optical intracranial pressure monitoring system Technology Overview The project has developed a proven new non-invasive system for continuous monitoring of intracranial pressure (ICP) via a forehead-mounted probe. Although the cranium is a closed rigid structure, interrogation using infrared light provides a potential ‘window’ for monitoring cerebral haemodynamics. The probe contains infrared light sources that can illuminate the deep brain tissue of the frontal lobe, while photodetectors in the probe detect the backscattered light, which is modulated by pulsation of the cerebral arteries. Changes in the pressure surrounding the cerebral arteries affect the morphology of the recorded optical pulse, so analysis of the acquired signal using an appropriate algorithm will enable calculation of non-invasive ICP (nICP) that can be displayed continuously to clinicians. The reported nICP could provide invaluable screening at the triage stage, indicating intracranial hypertension requiring imaging or intervention (such as CSF (Cerebrospinal Fluid) drainage). It could also provide effective guidance for head injury management, notably ICP-targeted treatment regimes. Ultimately this could lead to significant improvements in secondary injury-related mortality, length of hospital stay and reduced post-trauma disability. It could also find application in causes of non-trauma related intracranial hypertension including meningitis, hepatic encephalopathy, hydrocephalus and severe migraine. The prototype currently in development takes the form of a stand-alone monitor connected to a notebook computer which records the signals and calculates nICP in real time. The nICP device has been clinically trialled with more than 20 patients at the Royal London Hospital against an invasive ICP device under the supervision of a consultant neurorsurgeon. Although the nICP instrument has not yet been optimised it has demonstrated performance well above expectations and comparable to the gold standard invasive ICP. A medical device portfolio comprising design details along with clinical evaluation data will be submitted to the MHRA for approval as a CE-marked medical device. Data collected from clinical studies will support further development in partnership with a medical device manufacturer to produce a commercially available nICP monitoring system. What is the problem being addressed? Head injuries are a significant cause of injury and death, with approximately 50,000 cases of severe traumatic brain injury per year in the UK, the majority leading to death or severe disability. Cerebral damage sustained at the time of impact is referred to as primary injury and is irreversible and best treated by prevention (seatbelts, cycle helmets etc). Secondary brain injury occurs after the initial injury and is defined as damage arising from the body’s physiologic response to the primary injury. This may be as a result of bleeding or swelling of brain tissue. As the skull is a closed cavity containing water and other largely incompressible material, even minor swelling can cause significant increase in ICP. Initially, cerebrospinal fluid and venous blood are displaced (as described in the Munro-Kellie doctrine) but once these reservoirs are exhausted, small increases in pressure are transmitted directly to the brain tissue, compromising the arterial blood supply and reducing oxygen and glucose delivery to the brain tissue. This in turn results in further brain swelling which further compromises blood supply. Severe hypoxic brain injury can result, leading to irreversible brain damage. Various strategies exist to arrest or reverse this process so monitoring ICP is a vital tool in the management of severe head injuries. The “gold standard” technique for ICP monitoring is a catheter inserted into the frontal horn lateral ventricle via a right frontal burr hole, connected to a pressure transducer via a fluid-filled catheter. It has the advantage of allowing therapeutic drainage of CSF and administration of drugs however insertion may be difficult if the ventricles are small and even if performed in a sterile environment, infection occurs in 11%. Ventricular catheters measure global ICP and have the additional advantages of allowing periodic external calibration. Most clinicians now use electrical or fibre optic pressure transducers that are inserted into the brain tissue in the right frontal lobe via a twist drill hole (this is smaller than a burr hole). Although these are easier to insert and carry a lower infection risk, they are prone to drift and despite being less invasive than a ventricular catheter, they still carry a small risk of causing significant intracranial bleeding. There has been much research in recent years to find a method for measuring intracranial pressure noninvasively (nICP), including measurement of pressure in the retinal veins, measurement of eardrum displacement, transcranial Doppler ultrasonography and imaging-based solutions. None of these methods have found their way into clinical use as they all require considerable user intervention and are non-continuous. A recent review in Nature Reviews: Neuroscience reported that ICP monitoring is still a standard of care for TBI patients and has been much recent interest in the potential of new non-invasive ICP monitoring (nICP). Methods using tympanic membrane displacement37 and ultrasound “time of flight” techniques have been described, both being a poor surrogate for invasive ICP measurements, but serial intra-patient measurements may be useful to determine temporal changes in ICP. More recently, transcranial Doppler ultrasonography has been used to provide an indirect estimation of cerebral perfusion pressure CPP (Cerebral Perfusion Pressure - the difference between mean arterial blood pressure and ICP) to an accuracy of ±10-15 mmHg, however the systems are expensive and as they require user intervention, do not provide continuous monitoring and can only be used in a high dependency hospital environment. Why is this research important in terms of improving the health of the public and/or patients and the NHS? It is estimated that in the UK, approximately 1 million patients attend hospital with a head injury per year, or which 5% are classed as severe. Of those with severe TBI, over 85% remain disabled after one year and 15% have not returned to work after five years. Traumatic injury kills more people below the age of 45, than any other cause, accounting for 18,000 deaths per year in the UK. Road traffic accidents are the most common cause of head injuries and are especially common in teenagers and young adults. The care received by the patient (in the ‘golden hour’) immediately after injury may profoundly affect the outcome. This resulted in the inception of pre-hospital medicine where care is directed towards ABC (securing the airway, oxygenating the patient and maintaining circulation) with no thought given to head injury until the patient arrives in hospital. Even after arrival in a specialist hospital it may be up to an hour before definitive imaging is performed and surgery or ICP directed therapy is instituted. This paradigm is changing and pre-hospital SOPs (Standard Operating Procedures) now administer drugs to reduce ICP if there is evidence of a head injury but this is blind treatment. Pre hospital or immediate in-hospital ICP monitoring will allow ICP directed therapy from the outset. This device could also facilitate research and clinical monitoring in non-head injury medicine, such as liver failure, migraine, diabetes, anaesthesia, intensive care, renal medicine etc. There is significant evidence to suggest that therapy directed to maintain brain tissue oxygenation as well as ICP/CPP is associated with reduced mortality after severe TBI. Multimodality intracranial monitoring is now widely used during neurointensive care to provide early warning of impending brain ischemia and guide targeted therapy to optimize cerebral perfusion and oxygenation. Despite its limitations, ICP monitoring remains central to the monitoring and management of severe TBI. Conventional approaches to management have concentrated on a reduction in ICP to prevent secondary cerebral ischemia. Treatment is usually initiated if ICP increases >20 mm Hg, although it is likely that the duration of intracranial hypertension and its response to treatment are also important prognostic indicators. Advances in MRI functional imaging have brought a revolution in our understanding of the brain. In particular the mechanism of spinal cord and brain injury is much better understood, however this knowledge has not yet led to significant improvements in injury management, partly due to lack of clinical monitoring data at the bedside. Intensive care and emergency medicine are also in a state of flux with increased emphasis on goal directed therapy and realisation of the importance of early intervention to recovery and survival rates. Aside from trauma, management of other conditions associated with intracranial hypertension such as hydrocephalus, severe migraine and meningitis, could benefit from nICP monitoring. In many cases, especially borderline cases or those in early stages, the risk of invasive ICP monitoring is not justifiable; nevertheless intracranial monitoring could provide invaluable clinical information. A non-invasive ICP monitor would also be an invaluable research tool both for investigation of pathophysiology and for assessment of the effectiveness of treatments for intracranial hypertension. There may also be opportunities for research into ICP responses in high altitude or space medicine.