Traumatic brain injury (TBI) is a frequent cause of death and disability affecting millions of people every year worldwide. It is initiated by mechanical forces that cause sudden head motion. Such motion produces deformation of the brain and surrounding tissues and thus may result in axonal injury, contusion, or hematoma. The trauma launches a cascade of biochemical reactions often leading to ischemia, hypoxia, brain swelling, and edema. TBI may also induce damages of the cranial vasculature, with alterations of the blood vessel that put the neural tissue at risk. TBI can either cause vessel rupture and hemorrhage (bleeding), or a pathophysiological change of the vessel structure which is secondary associated with some kind of dysfunction. Hemorrhage is easy to recognize, commonly categorized according to its location as epidural (between the skull and the dura mater – associated with disruption of the middle meningeal vessels), subdural (between the dura mater and arachnoid membrane – usually concerning the bridging veins), or subarachnoidal (between arachnoid membrane and the pia mater). Intracerebral bleeding may also occur when the membranes surrounding the brain are impaired. In case of contusion, vascular damage and mechanical cortical damage can occurr at the same time. Even if bleeding is not present, function and microstructure of the injured vessels might be impaired. Vessel disruption and hemorrhage alter the cerebral blood flow, increase the intracranial pressure, affect the maintenance of the blood-brain barrier (exchange of nutrients and waste that occurs at the capillary level), and disrupt the CNS homeostasis by exposing the neural tissue to disregulated blood flow.
Understanding cerebrovascular injuries and the mechanisms behind them is crucial for diagnostics and treatment strategies. Monson et al. (in press) describe the current state of knowledge on the mechanics of cerebral vessels during head trauma and how they respond to the applied loads. They provide a summary of the experimental research focused on the loading conditions during the TBI. Experiments with physical models for instance show that there is significant relative motion between the brain and skull during the trauma. In the sagittal plane, this motion tends to be largest at the vertex and smallest at the brain base. Constraints at the base can lead to brain rotation which pushes the parietal cortex into the cranial bones, possibly causing of contusion and subdural hematoma. In general, it appears that rotation is more damaging than translation. Computer models represent another approach of the TBI research and provide accurate predictions of brain deformation for many loading scenarios. These models enable to estimate exterior loading conditions according to the internal deformation of tissues that are directly involved in injury. However, validation of these models is needed, as well as specific models focusing on blood vessels. The authors also provide a summary of what is known about cerebral vessel response to extreme deformations, passive physical properties, structural failure, and subfailure damage and dysfunction. In another article, Saad et al. (in press) describe various kinds of intracranial hemorrhage in biomedical imaging. Both macroscopic hematomas and microhemorrhages are described according to the distinctions based on intracranial compartments, and traumatic vs nontraumatic cases.
Intracranial infections represent serious brain diseases that occur in various forms and often may be hard to recognize in their earlier stages. A fast diagnosis is crucial for an effective treatment. Various technique of CT and MRI imaging have been developed to distinguish the symptoms in the brain and its associated tissues (see for example Hsu 2010). Radiologists recognize several categories of infections according to the origin (e.g. congenital and neonatal), location (intra-axial, extra-axial), or characters. In general, infections of the brain parenchyma, meninges and ventricles can have bacterial, viral, fungal, or parasitic origins. Bacterial infections usually develop from early cerebritis (inflammation of the cerebrum) to formation of abscesses (accumulation of infectious material and microorganisms) within the cranium. Some bacteria have more specific effects. Streptococcus pneumonia and Neisseria meningitides are common cause of bacterial meningitis (inflammation of the meninges and the cerebrospinal fluid). Tuberculomas, abscesses and tuberculous meningitis indicate presence of Mycobacterium tuberculosis (TB). Frequent viral infections are Herpes Simplex Encephalopathy involving Herpes simplex virus 1 (HSV-1), or infections induced by Human Immunodeficiency Virus (HIV) leading to cerebral atrophy and white matter disease. Fungal infections usually generate abscesses filled with fungi. Cryptococcosis and fungal meningitis are frequent fungal infections in some specific geographical regions. Examples of parasites causing intracranial infections are Toxoplasma gondii, or Taenia solium causing cysticercosis, which also leads to acquired epilepsy (see Vaccha et al. 2016). Consequences of intracranial infections could be in some cases lethal, in other cases they can cause a severe damage. For instance, infections of meninges and cerebrospinal fluid leading to meningitis can further evolve into subdural (between the dura mater and the underlying arachnoid layers) and epidural (between the meninges and the bone) abscesses, hydrocephalus (excessive accumulation of fluid in the brain), ventriculitis (inflammation of the ventricles containing and circulating cerebrospinal fluid throughout the brain), and venous thrombosis. Brain infection can be extended into the bone tissue and cause cranial pathologies like osteomyelitis or mastoiditis. Sometimes the infections can even lead to bone fractures. The origin of brain infection is often associated with traumas, when the microorganism spread from the wound into the soft tissues of the endocranium.
Magnetic Resonance Imaging (MRI) methods has a primary use in medicine, especially in diagnosis and image-guided surgery. In neuroscience, attention is mainly focused on the brain, and vessels are not always a target of the imaging procedures. The crucial aspect of using imaging during surgery concerns the correspondence with the real physical structures. This correspondence is affected by a displacement of the brain during surgery called brain shift, which can result in 5 – 10 mm difference from the MRI data. Several technical procedures are used in order to avoid this mismatch. Since intraoperative MRI devices are not always available, using local markers for orientation and navigation could be a plausible alternative. In a recent paper, Grabner and colleagues suggest to use the system of veins on the surface of the cerebral cortex as reference landmarks. These veins are well visible during the surgery, and can potentially improve navigation. The study is focused on developing a non-invasive MRI technique for the visualization of the superficial cortical veins and validation of that method by comparing MR images with high resolution photographs of human cadavers.
Considering Magnetic Resonance Angiography (MRA), the main concern of using this method is the use of a contrast agent, and the possibility of overlooking the superficial cortical veins because of the slow blood flow. Alternatively, the authors suggest to use Susceptibility-Weighted Imaging (SWI), which is a blood-oxygen-level dependent technique of MRI with an ability to image vessels smaller than a voxel. Gradient-echo based T2∗-weighted imaging was performed in this study using a 7 Tesla MRI scanner. Image processing relied on automated vessel segmentation and overlaid on anatomical MRI. The results showed high correlation between segmented veins in MRI and actual venous anatomy of the sample, and therefore surface venograms could serve as alternative navigation system for neurosurgery.
Reliable methods of imaging and segmentation of vessels are valuable also in theoretical fields, where those methods could contribute to the investigation of the function of the endocranial venous system. The importance of the veins is usually estimated according to their size, although functional information on these vessels is still scanty, and methodological research to improve craniovascular studies can be beneficial in both anthropology and medicine.
A new PhD student in the team working on craniovascular anatomy! Stanislava Eisová was in our laboratory few years ago, publishing a paper on parietal bone and vessels in which she investigated correlations between craniovascular morphology, skull size, and bone thickness. She got a Master Degree in Anthropology of Past Populations at the University of West Bohemia, Czech Republic. Now she begins a PhD program on Anthropology and Human Genetics at the Faculty of Science of the Charles University, Prague. Her project will be on craniovascular traits, anthropology, paleoanthropology, and paleopathology. She will investigate craniovascular characters in normal samples, pathological conditions, and fossil specimens. The project is co-directed by Petr Velemínský.
The human brain is the most expensive and costly organ in terms of energetic resources and management. However, the current understanding of its sophisticated thermal control mechanisms remains insufficient. Wang et al., 2016, have reviewed the most recent studies on brain thermoregulation and examined the anatomical and physiological elements associated with selective brain cooling. Modern humans have a brain that is approximately three times larger than a primate with a similar body size, which uses 20%– 25% of the total body energy compared with a maximum of 10% in other primates and 5% in other mammals. The evolution of a large and expensive brain in modern humans effectively influences critical factors such as temperature, and functional limits can affect cerebral complexity and neural processes. Brain thermoregulation depends on many anatomical components and physiological processes, and it is sensitive to various behavioral and pathological factors, which have specific relevance for clinical applications and human evolution. The anatomical structures protecting the brain, such as the human calvaria, the scalp, and the endocranial vascular system, act as a thermal interface, which collectively maintains and shield the brain from heat challenges, and preserves a stable equilibrium between heat production and dissipation. Future advances in biomedical imaging techniques would allow a better understanding of the physiological and anatomical responses related to the cerebral heat management and brain temperature in modern humans.
Gizéh Rangel de Lázaro
This month we have published a review on craniovascular traits and anthropology, freely available to download from the Journal of Anthropological Sciences. The article describes many vascular traits that can be analyzed on skulls, through the traces they leave on the bone surface or within the bone itself. The traces of the middle meningeal vessels, the traces of the venous sinuses, the diploic channels, and the endocranial foramina, can provide information on the vascular networks and, indirectly, on the physiological processes associated with their growth and development. The functional information available from these imprints is partial and incomplete, but it is the only one we have on blood flow when dealing with fossils, archaeological remains, or forensic cases. Methods are an issue, because of the difficulties with small samples, scoring procedure, statistics of ordinal and nominal variables, and with an intrinsic limitation in current anatomy: we still ignore the variations and processes behind many macroanatomical features, even in our own species. Previous articles on this topic deal with middle meningeal artery, vessels and thermoregulation, diploic channels, and parietal bone vascularization. Most of these papers are part of a project funded by the Wenner-Gren Foundation through an International Collaborative Research Grant, entitled “Cranial anatomy, anthropology, and the vascular system”. This beautiful drawing of a sectioned skull is by Eduardo Saiz.
The Rhoton Collection is composed by an outstanding anatomical presentations of the brain created by the renowned surgeon and educator Dr. Albert Rhoton Jr throughout his life. These presentations were made using bright blue and red dyes in the blood vessels, so that surgeons could easily visualize and explore the brain and vascular structures for planning surgical interventions.
[Here a post in Spanish]
Gizéh Rangel de Lázaro
In general, members of the Primate Order possess larger brains for body size than other mammals, with modern humans (Homo sapiens) evolving the largest brains. The Internal Carotid Artery (ICA) provides blood supply to the brain but there are distinct anatomical differences between the primate groups. While vascular and other soft-tissues are very rarely fossilised, evidence of the ICA passage is retained where it was encapsulated in bony tubes or as distinct grooves of the endocranial surface. The ICA evolved in primates from two main pathways in the auditory complex: the promontory artery branched from the cochlear space to supply blood to the brain while the stapedial artery branched from the obturator foramen of the stapes to provide blood to the cerebral meninges and the orbito-facial complex. Primate groups are recognised by ICA anatomy: Within the Strepsirrhini, Lorisiformes and Cheirogaleids both lack the stapedial and promontory arteries with the External Carotid Artery (ECA) supplying blood to the brain instead of the ICA; whereas, non-Cheirogaleids possess the ICA and stapedial artery but often retain a much smaller promontory artery, while the anthropoids (apes and monkeys) lack the stapedial artery entirely retaining only the promontory artery.
The evolution of these distinct differences was examined by comparing living and fossil primates and reconstructing the hypothetical phylogenetic pathways. The size differences between the stapedial and promontory arteries were compared to endocranial volume (ECV) to investigate the influence on brain size. Only the size of the promontory artery had a consistent correlation with brain size. There was no reported correlation between brain size and size of the stapedial artery, with the stapedial only correlating with size of the promontory artery. This suggests that throughout primate evolution, the trend for body and brain size increases caused the stapedial artery to become restricted as head size increased but size of the obturator foramen did not scale equally with the head. Most early primates evolved a reduction in the size of the stapedial artery and quickly accommodated an increase in the promontory artery allowing even greater blood flow to the brain and driving the encephalisation process and the eventual loss of the stapedial artery in anthropoids.
The fossil record offers several possible approaches to study the evolution of the human brain. Besides cerebral size and shape, we can make inferences about cognitive functions and metabolic processes. Analyses of the craniovascular system are required to better understand both aspects. A recent article in the Royal Society Open Science journal adds new evidence into this issue comparing cerebral blood flow rate and endocranial volume in fossil hominids. The metabolic rate of the human brain is tightly related to the cerebral blood flow, which is mainly supplied by internal carotid arteries (ICAs). The authors measured the dimensions of the carotid foramen, the external opening of the carotid canal, in 35 fossil skulls, and calculate the size of the internal carotid arteries lumen. Then, they calculated the blood flow based on the shear stress, arterial lumen radius and blood viscosity (using supporting data from human and rats models). Their results show that the ICAs blood flow rate increases disproportionately in hominids, when scaled against brain volume. The authors then speculate about metabolic rate and its association with greater synaptic activity, cognitive functions, and life-history evolution. The paleoneurological information considered in the article is not much updated, and the sample includes many casts, which reliability is not comparable with original specimens. Also, inferences on cognition or life-history sound probably too much speculative when dealing with a simple carotid canal. Nonetheless, this paper supplies a good perspective in vascular biology, with a clear application in paleoanthropology. The possibility of calculating the cerebral blood flow in fossil specimens is interesting and opens new research opportunities.