Vascular Pulsations Research Articles

A pilot clinical study to estimate intracranial pressure utilising cerebral photoplethysmograms in traumatic brain injury patients

PurposeIn this research, a non-invasive intracranial pressure (nICP) optical sensor was developed and evaluated in a clinical pilot study. The technology relied on infrared light to probe brain tissue, using photodetectors to capture backscattered light modulated by vascular pulsations within the brain’s vascular tissue. The underlying hypothesis was that changes in extramural arterial pressure could affect the morphology of recorded optical signals (photoplethysmograms, or PPGs), and analysing these signals with a custom algorithm could enable the non-invasive calculation of intracranial pressure (nICP).MethodsThis pilot study was the first to evaluate the nICP probe alongside invasive ICP monitoring as a gold standard. nICP monitoring occurred in 40 patients undergoing invasive ICP monitoring, with data randomly split for machine learning. Quality PPG signals were extracted and analysed for time-based features. The study employed Bland–Altman analysis and ROC curve calculations to assess nICP accuracy compared to invasive ICP data.ResultsSuccessful acquisition of cerebral PPG signals from traumatic brain injury (TBI) patients allowed for the development of a bagging tree model to estimate nICP non-invasively. The nICP estimation exhibited 95% limits of agreement of 3.8 mmHg with minimal bias and a correlation of 0.8254 with invasive ICP monitoring. ROC curve analysis showed strong diagnostic capability with 80% sensitivity and 89% specificity.ConclusionThe clinical evaluation of this innovative optical nICP sensor revealed its ability to estimate ICP non-invasively with acceptable and clinically useful accuracy. This breakthrough opens the door to further technological refinement and larger-scale clinical studies in the future.Trial registrationNCT05632302, 11th November 2022, retrospectively registered.

Blood pressure pulsations modulate central neuronal activity via mechanosensitive ion channels.

The transmission of the heartbeat through the cerebral vascular system causes intracranial pressure pulsations. We discovered that arterial pressure pulsations can directly modulate central neuronal activity. In a semi-intact rat brain preparation, vascular pressure pulsations elicited correlated local field oscillations in the olfactory bulb mitral cell layer. These oscillations did not require synaptic transmission but reflected baroreceptive transduction in mitral cells. This transduction was mediated by a fast excitatory mechanosensitive ion channel and modulated neuronal spiking activity. In awake animals, the heartbeat entrained the activity of a subset of olfactory bulb neurons within ~20 milliseconds. Thus, we propose that this fast, intrinsic interoceptive mechanism can modulate perception-for example, during arousal-within the olfactory bulb and possibly across various other brain areas.

Internal iliac artery aneurysm masquerading as a sciatic nerve schwannoma: illustrative case.

Schwannomas are common peripheral nerve sheath tumors. Imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) can help to distinguish schwannomas from other types of lesions. However, there have been several reported cases describing the misdiagnosis of aneurysms as schwannomas. A 70-year-old male with ongoing pain despite spinal fusion surgery underwent MRI. A lesion was noted along the left sciatic nerve, which was believed to be a sciatic nerve schwannoma. During the surgery for planned neurolysis and tumor resection, the lesion was noted to be pulsatile. Electromyography mapping and intraoperative ultrasound confirmed vascular pulsations and turbulent flow within the aneurysm, so the surgery was aborted. A formal CT angiogram revealed the lesion to be an internal iliac artery (IIA) branch aneurysm. The patient underwent coil embolization with complete obliteration of the aneurysm. The authors report the first case of an IIA aneurysm misdiagnosed as a sciatic nerve schwannoma. Surgeons should be aware of this potential misdiagnosis and potentially use other imaging modalities to confirm the lesion before proceeding with surgery.

M-Mode ultrasonography in ocular emergencies

M-Mode, or time-motion display, allows a single beam to emit from the ultrasound transducer along a defined track in conjunction with a recorder that captures all motions that occurs along the path. This mode allows high temporal resolution, thus affording the examiner an excellent view of subtle motions. Clinically, this mode is ideal for capturing vessel diameter changes, movement of cardiac valves, and detecting fetal heartbeats. The use of the M-mode or time-dependent intensity modulated ultrasound technique for ophthalmologic investigations are described here. This technique provides the investigator with a means for monitoring structural changes in the eye during physiologic or pharmacologic experimental conditions, or a combination of both, and is particularly useful in studying optically inaccessible structures. The technique has been used to study accommodation changes in axial length and lens thickness as well as the rate of such changes and to study vascular pulsations and choroidal thickness changes at the rear wall of the eye.

Vital Functions Contribute to the Spread of Extracellular Fluids in the Brain: Comparison Between Life and Death.

Vascular pulsations, contractions of vascular smooth muscle cells and breathing have been reported to foster movement and clearance of interstitial and cerebrospinal fluids from the brain. The aim of this study was to estimate the contribution of these vital functions. We compared the spread of an injected hydrophilic tracer (Fluoro-Emerald, a 10 kDa fluorescein-coupled dextran amine) in the brains of live anesthetized and sacrificed rats at 30 and 90 min after injection. To determine the overall pattern of distribution of tracers, we created 3D-reconstructions of the horizontal transections of the whole brain. Immunofluorescence staining with laminin and collagen IV was performed to determine the pattern of distribution of tracer in relation to the cerebrovascular basement membranes. We found that diffusion was widely restricted to the periventricular region in sacrificed rats with no spread to the contralateral hemisphere, while the bulk flow occurred along the vasculature and reached the surface and the contralateral hemisphere as soon as 30 min after injection in live anesthetized animals. The tracer appeared to be localized along the vascular basement membranes and along fiber tracts as reported previously. Thus, our data indicate that vital functions are essential for the remote movement of extracellular fluids within the cerebral parenchyma.

РАСПРОСТРАНЕНИЕ ПУЛЬСОВОЙ ВОЛНЫ ПО МАЛЫМ СОСУДАМ: РЕЗУЛЬТАТЫ ИЗМЕРЕНИЙ И ПОДХОДЫ К МОДЕЛИРОВАНИЮ

The objective of the research work was to develop devices and algorithm for synchronous recording of pulse waves and ECG for measuring the delay time of pulse waves in the branches of various arteries relative to the R wave on an ECG, and to carry out computer simulation of the pulse wave propagation process to determine the dependence of the pulse wave propagation velocity on branching and other hemodynamic and morphological parameters of blood vessels. Material and methods. The study was conducted in 74 healthy subjects aged 18-23 years. The propagation time of the pulse wave by the arterial branches of the vessels of the common carotid, internal, external carotid and radial arteries was measured. The time was calculated by the delay of the beginning of the pulse wave relative to the tip of the R wave on the ECG. Vascular pulsations were recorded using mechanical sensitive and photosensitive sensors, which signals were amplified, digitized, recorded and analyzed using original computer soft wares. Computer simulation of the propagation of pulse waves along the wall of an “equivalent” vessel corresponding to the branching of several arterial vessels was carried out. Results. The velocity of propagation of a pulse wave along the branches of small arterial vessels was lower than its value for larger main arteries. The simulation results confirmed that the propagation velocity of a pulse wave can significantly slow down its movement along branched arterial vessels, which differ in the mechanical properties of the main arteries. Conclusion. The data obtained indicate that the developed devices and measurement algorithms make it possible to register pulse waves of various small arteries and obtain reproducible indices of the delay time of the pulse wave relative to the R wave on the ECG. The time and velocity of the pulse wave propagation depends on the length of the studied vessels, the mechanical properties of the walls of the vessels, which follows from the comparison of the obtained data with the morphological features of the structure of vascular networks. Simulation results for an “equivalent” vessel show that one of the possible causes of a lower pulse wave propagation velocity in small vessels is lower mechanical properties of the branches of small vessels compared with those of larger arteries. However, the identification of the nature of these dependencies and their connection with stiffness of the walls of small vessels requires further study.

The contribution of cerebral vascular pulsations to perivascular drainage in Alzheimer's disease and aging

The contribution of cerebral vascular pulsations to perivascular drainage in Alzheimer's disease and aging

Posthemorrhagic hydrocephalus development after germinal matrix hemorrhage: Established mechanisms and proposed pathways

In addition to being the leading cause of morbidity and mortality in premature infants, germinal matrix hemorrhage (GMH) is also the leading cause of acquired infantile hydrocephalus. The pathophysiology of posthemorrhagic hydrocephalus (PHH) development after GMH is complex and vaguely understood, although evidence suggests fibrosis and gliosis in the periventricular and subarachnoid spaces disrupts normal cerebrospinal fluid (CSF) dynamics. Theories explaining general hydrocephalus etiology have substantially evolved from the original bulk flow theory developed by Dr. Dandy over a century ago. Current clinical and experimental evidence supports a new hydrodynamic theory for hydrocephalus development involving redistribution of vascular pulsations and disruption of Starling forces in the brain microcirculation. In this review, we discuss CSF flow dynamics, history and development of theoretical hydrocephalus pathophysiology, and GMH epidemiology and etiology as it relates to PHH development. We highlight known mechanisms and propose new avenues that will further elucidate GMH pathophysiology, specifically related to hydrocephalus.

Normal cerebral vascular pulsations in humans: changes with age and implications for microvascular disease.

Cerebral syndromes in older humans, secondary stroke in younger persons following trauma, and sickle cell anaemia in children, are linked by unexplained microvascular damage and high cerebral pressure or flow pulsations. The aim of this study was to characterize age-related pressure and flow waveforms patterns entering the brain, to explain these in terms of disturbed physiological function, and to consider clinical implications. Blood flow velocity waves were measured in four cerebral vascular territories by transcranial Doppler of 1020 apparently normal patients (497 men, 21-78 years). Central pressure waveforms were generated from radial artery applanation tonometry with SphygmoCor. Relationships were described in time and frequency domains. Flow waveforms entering the brain showed similar pattern to central aortic pressure waveforms, and similar changes with age. Augmentation index of flow and of pressure had high correlation at different ages, and in men and women (r = 0.58, P < 0.01). Calculated cerebral vascular impedance was similar in both sexes, and at different ages, with low modulus and phase, indicating a dilated, passive cerebral vascular bed. This vascular bed is subject to pressure and flow fluctuations generated directly by the heart and boosted by strong wave reflections from the lower body. Cerebral microvascular damage in older patients is attributable to high pulsatile pressure tearing the delicate media, causing haemorrhage, and high pulsatile flow dislodging endothelial cells, causing thrombosis and microinfarcts. High pulsations in older patients are caused by early wave reflection from the lower body. Reduction of or delay in wave reflection is a logical strategy for aortic stiffening in older humans.

Retinal vascular and structural changes are associated with amyloid burden in the elderly: ophthalmic biomarkers of preclinical Alzheimer\u2019s disease

BackgroundRetinal imaging may serve as an alternative approach to monitor brain pathology in Alzheimer’s disease (AD). In this study, we investigated the association between retinal vascular and structural changes and cerebral amyloid-β (Aβ) plaque load in an elderly cohort.MethodsWe studied a total of 101 participants, including 73 elderly subjects (79 ± 5 years, 22 male) with no clinical diagnosis of AD but reporting some subjective memory change and an additional 28 subjects (70 ± 9 years, 16 male) with clinically established AD. Following a complete dilated ocular examination, the amplitude of retinal vascular pulsations and dynamic response, retinal nerve fibre layer thickness and retinal ganglion cell layer (RGCL) thickness were determined in all patients. Systemic blood pressure and carotid-to-femoral pulse wave velocity were measured. The elderly cohort also underwent magnetic resonance imaging and 18F-florbetaben (FBB)-positron emission tomographic amyloid imaging to measure neocortical Aβ standardised uptake value ratio (SUVR), and this was used to characterise a ‘preclinical’ group (SUVR >1.4).ResultsThe mean FBB neocortical SUVR was 1.35 ± 0.3. The amplitude of retinal venous pulsations correlated negatively with the neocortical Aβ scores (p < 0.001), whereas the amplitude of retinal arterial pulsations correlated positively with neocortical Aβ scores (p < 0.01). RGCL thickness was significantly lower in the clinical AD group (p < 0.05).ConclusionsThe correlation between retinal vascular changes and Aβ plaque load supports the possibility of a vascular component to AD. Dynamic retinal vascular parameters may provide an additional inexpensive tool to aid in the preclinical assessment of AD.

Dysphagia Aortica: A Case of Progressive Dysphagia and Regurgitation in an Elderly Female

Purpose: A 79-year-old female presented with progressive dysphagia to solids and liquids. Her medical history is significant for ovarian cancer treated with chemotherapy and thoracic/abdominal aortic aneurysms. She was evaluated for medical evaluation prior to elective endovascular aneurysm repair (EVAR). Over the past three years, she had worsening dysphagia to solids, then liquids. Within minutes of eating she had regurgitation. She denied weight loss. Physical exam was unremarkable. EGD revealed a narrowing of the esophagus that was able to be traversed. CT showed a narrowed esophagus compressed by the aortic aneurysm (Figure). Extrinsic compression of the esophagus by aortic aneurysms has been rarely reported. The term dysphagia aortica has been used to describe dysphagia occurring secondary to external compression of the esophagus by a thoracic aortic aneurysm, anomaly or tortuous component. The incidence of dysphagia aortica is unknown and is more commonly described outside of the U.S. Investigators suggest that the dysphagia is a late stage symptom of aneurysmal expansion and is a harbinger for aorto-esophageal fistula. The dysphagia usually develops subacutely, over a period of months, and progresses from solid to liquid dysphagia often concomitantly with chest pain and backache. The aortic anomaly is typically identified by CT or by observing obstruction in barium swallow studies. EGD is frequently conducted, but is often unrevealing. Some may observe a narrowing of the esophagus, others are normal. Manometry findings may reveal oscillations consistent with vascular pulsations or increased lower esophageal sphincter pressure. The most commonly offered treatment option is surgical correction of the aorta and success has been reported. However, many forego vascular surgery due to the perceived risks. Conservative measures such as gastrostomy tubes and diet modification can manage dysphagia and regurgitation symptoms. In our patient, surgically correction of the aneurysm with EVAR was offered; however, the patient was hesitant and wanted to reconsider the decision as an outpatient.Figure: [585] The red arrow depicts the narrowed esophagus compressed by the aortic aneurysm.

Vascular Pulsations on Impedance Audiometry as a Sign of a Third-Mobile Window Lesion

Description of a new audiometric sign of a third-mobile window lesion. Subject with known superior canal dehiscence. Diagnostic. Results of impedance audiometry testing. Vascular pulsations seen on impedance audiometry on dehiscent side. Vascular pulsations seen on impedance audiometry may indicate a third-mobile window lesion in the inner ear.

Idiopathic normal pressure hydrocephalus: Thoughts on etiology and pathophysiology

Idiopathic normal pressure hydrocephalus (NPH) remains a mysterious entity, with several gaps in our understanding, despite intensive research. The current theory of its etiology is diminished vascular compliance with redistribution of vascular pulsations. The manifestations of NPH can be explained by the compression of parenchyma and vessels and the accumulation of toxic metabolites in the cerebrospinal fluid (CSF) and in the brain parenchyma. The author believes that NPH is caused by repetitive disruption of the normal propagation of ventricular systole, due to chronic derangements of cerebral arterial hemodynamics. The result is functional obstruction of the inter- and/or intra-ventricular CSF flow, leading to active and progressive ventriculomegaly. The author suggests that transependymal migration of CSF disturbs the osmotic balance of the periventricular zones, contributing to the cellular and axonal damage and dysfunction in NPH. The hypothesis can be tested with computational fluid dynamics and animal models. Based on established knowledge and on this hypothesis, the author proposes a multistep therapeutic scheme for NPH. First the patient's CSF is traded for a hypotonic CSF-like solution. The "clean" CSF, via transependymal migration, will sweep away some of the debris from the parenchyma and it will rehydrate cells and axons in the periventricular areas. Subsequently, vasodilatory drugs can be infused in the ventricles in order to reach compressed vessels, again via the transependymal route. Ventricular drainage can then proceed as per standard practice. The final step is the use of oral antihypertensive medications to decrease the arterial systolic blood pressure and simultaneously increase the arterial diastolic blood flow. The goal is to enhance protection of the periventricular tissues from pressure and osmotic loads. The antihypertensive regimen may be useful in two more groups of patients with NPH: those who are not surgical candidates and those in an early or subclinical phase of the disease.

Migraine headache is not associated with cerebral or meningeal vasodilatation--a 3T magnetic resonance angiography study

Sir, with great interest, we read the article by Schoonman et al. (2008), who claim that ‘in contrast to widespread belief, migraine attacks are not associated with vasodilatation of cerebral or meningeal blood vessels’ and, therefore, ‘future anti-migraine agents may not require vasoconstrictor action’. Whereas the methodology used in this investigation is elegant, we disagree with their claim for the following reasons. First, it may be noted that Schoonman et al. (2008) did observe vasodilatation during nitroglycerin (NTG; 0.5 µg/kg/min over 20 min) infusion in all blood vessels investigated (middle meningeal, internal and external carotid, middle cerebral, basilar and posterior cerebral arteries), but not during the migraine-like headache provoked 1.5–5.5 h after cessation of NTG infusion. However, these initial vasodilator changes can trigger a chain of events leading to headache. Cranial vasodilatation leads to enhanced blood volume following each cardiac stroke with consequent augmentation of vascular pulsations, which may …

O.012 A physical model of occlusive hydrocephalus

Cerebrospinal fluid (CSF) pulsations result from change of blood volume in the closed craniospinal cavity. We used cine phase contrast MR analysis to determine whether spinal CSF pulsations result from spinal vascular pulsations or intracranial subarachnoid pulsations, whether intracranial CSF pulsations result from intracranial large arteries pulsations or cerebrovascular bed changes. We performed a quantified physiological mapping of CSF velocity waveforms along the craniospinal axis. Thirty-six volunteers participated in the study. MR acquisitions were obtained at the intracranial level, the upper, midcervical, cervicothoracic, mid thoracic, and/or the thoracolumbar levels. The temporal velocity information were plotted as wave form and key temporal parameters were determined and analyzed; intervals from the R wave to the onset of CSF systole, to CSF systolic peak, to the end of systole, as well as duration of systole. Three kinds of dynamic channels could be differentiated along the spinal axis, the lateral, medioventral and mediodorsal channels. Lateral spinal CSF pulse waves show significant craniocaudal propagation. No such significant progression was detected through the medial channels along the spine. Through the medial channels, a cephalic progression was observed from the upper cervical level to the intracranial level. At the craniocervical junction, mediodorsal CSF systole appeared the earliest one whereas in the anterior intracranial basal cistern, CSF systole appeared delayed. In conclusion, spinal CSF pulsations seem to result mainly from intracranial pulsations in the lateral channels, whereas local vascular pulsations could modify CSF pulse wave mainly in the medial channels. At the craniocervical junction, our results suggest that blood volume change in the richly vascularised cerebellar tonsils is the main initiating factor of CSF systole; and that spinal vascular pulsations could be considered as an additional early and variable CSF pump.

Urethral instability.

Urethral instability is still evolving as a clinical entity. Using pressure variation of 15 cm water or more at the point of maximum urethral pressure (MUP), urethral pressure profilometry on patients referred for urodynamic assessment for lower urinary tract symptoms revealed urethral instability in 6.4% of 608 patients. The close association between urethral and detrusor instability was noted. Urethral instability appears to be a cause of frequency and urgency of micturition, and its presence increase the risk of urinary incontinence.

TRANSCRANIAL MAGNETIC STIMULATION FOR TINNITUS

Publisher Summary The repetitive transcranial magnetic stimulation (rTMS) is a promising tool for investigating the pathophysiology of tinnitus. Tinnitus refers to a condition in which a patient has a hearing percept that can take the form of ringing, buzzing, roaring, or hissing in the absence of an external sound. Tinnitus can be classified as being either objective or subjective. In the objective form, which is rare, a real sound is generated by an internal biological source, reaching the ear through conduction in body tissues. The source can be vascular turbulence, pulsations, or spasm of the muscles in the middle ear, Eustachian tube, or soft palate. Unlike subjective tinnitus, an observer, using a stethoscope, can often hear objective tinnitus. Subjective tinnitus refers to a phantom auditory sensation for which no objective sound can be identified and only the person who has the tinnitus can hear it. Some patients perceive the phantom sound as coming from inside the ear; others report that the phantom sound is located inside the head, while a few perceive the phantom sound as coming from outside the head.

Ring melanoma of the ciliary body: report on twenty-three patients.

Ring melanoma of the ciliary body is a rare variant of uveal melanoma that has a poor prognosis. Only isolated cases have been reported in the literature. A retrospective review of the clinical features, management, histopathology, and prognosis of 23 consecutive patients with ring melanoma of the ciliary body was performed. Of 8,800 patients with uveal melanoma treated on the Oncology Service at Wills Eye Hospital over a 25-year period, only 23 patients (0.3%) were found to have ring melanoma of the ciliary body. The mean patient age was 59 years (median, 63 years; range, 8-81 years). The only visual symptom was blurred vision in 17 patients (74%), and the other six patients (26%) were asymptomatic. At the initial presentation elsewhere, melanoma was recognized in 13 cases (57%). In the remaining 10 cases (43%), the initial clinical diagnosis was glaucoma in three (13%), iris nevus in three (13%), cataract in two (9%), ciliary body detachment in one (4%), and iridocyclitis in one (4%). The patients were observed elsewhere for a mean of 3 months before the diagnosis of melanoma was suspected and referral to Oncology Service was made. On examination, the tumor involved a mean of eight clock hours of the ciliary body. Seven patients (30%) had complete circumferential (360 degrees) involvement of the ciliary body, and 16 (70%) had incomplete (180-360 degrees) ring involvement. The mean tumor thickness was 8 mm. Associated findings included neovascularization of the iris in one case (4%), shallow anterior chamber in 11 (48%), anterior chamber inflammation in five (22%), cataract in nine (39%), lens indentation in eight (35%), and lens subluxation in three (13%). A prominent episcleral (sentinel) vessel (17 cases, 74%), multilobulation of the mass (19 cases, 83%), blockage of light on transillumination (23 cases, 100%), and ultrasonographic hollowness with intrinsic pulsations (23 cases, 100%) were important features differentiating this tumor from simulating conditions. The tumor was managed with enucleation in all 23 patients. Histopathologic studies revealed epithelioid cell-type melanoma in two patients (9%), mixed cell-type in 17 (74%), and spindle cell-type in four (17%). Extraocular extension was present in eight patients (35%). Metastasis developed in 12 of 23 patients (52%) after a mean follow-up of 55 months. Ring melanoma of the ciliary body is a rare tumor that can remain hidden from ophthalmoscopic examination. Certain features, such as prominent episcleral sentinel vessels, multilobulation of the mass, light blockage on transillumination, and ultrasonographic hollowness with intrinsic vascular pulsations, suggest the diagnosis. The life prognosis is poor.

Vascular pulsations stimulating nitric oxide release during cyclic exercise may benefit health: a molecular approach (review).

It is widely assumed that all exercise, regardless of the degree of difficulty or strenuousness, is good (no pain-no gain). In this speculative review of the literature and our research findings we highlight the fact that strenuous exercise taken to the extreme initiates an immune and vascular proinflammatory situation. However, mild cyclic exercise appears to produce health benefits for an individual. In part, this is due to vascular cyclic pulsations, occurring in mild exercise, stimulating constitutive nitric oxide synthase derived nitric oxide release. This in turn down-regulates vascular endothelial cells and immunocytes, as well as their interaction and inhibits the disassociation of NF-kappaB, preventing the production of proinflammatory cytokines. The nitric oxide so generated may even scavenge excess free radicals, preventing tissue damage. Prolonged strenuous exercise appears to limit these positive phenomena because of the maintained and prolonged high blood pressure that reduces the cyclic pulsations, limiting nitric oxide production. We further note that pathological conditions, i.e., Parkinson's disorder, may benefit from mild exercise, i.e., cyclic nitric oxide production, since the inactivity associated with this disease may lead to compromised nitric oxide production, initiating a progressive deterioration of tissues, including peripheral adrenergic neurons, due to a lack of adequate basal nitric oxide levels required to maintain the vascular microenvironment in a mild state of inhibition. We conclude that mild exercise represents an alternate and economical therapy to preserve health and/or diminish the rate of decline of the normal physiological processes that may even be associated with aging.

The mechanical state of intracranial tissues in elderly subjects studied by imaging CSF and brain pulsations

The biomechanical properties of intracranial tissues influence the mechanical coupling of brain and CSF oscillations to the driving vascular pulsations. Dynamic phase contrast MRI was used to measure the transfer functions that characterize these couplings in normal elderly subjects and patients with Alzheimer’s disease. The transfer functions of both groups were significantly different from the previously reported transfer functions of normal young subjects. The data show that vascular pulsations tend to cause greater spinal cord movements and smaller CSF oscillations in the older subjects than in the younger ones. These results are likely to be due to age-related changes in the mechanical state of intracranial tissues.