Cerebral Blood Flow & CSF 2023/24 (Year 1 BDS/DTH) PDF

Year 1 BDS and DTH Life Sciences 2023/24 Cerebral Blood Flow & CSF Safiya Robinson Slides compliments Dr Yen Lin Learning Objectives To identify key features of the anatomy of cerebral circulation To recognise the physiological significance of the CSF and the ‘blood brain barrier’ To iden...

Year 1 BDS and DTH Life Sciences 2023/24 Cerebral Blood Flow & CSF Safiya Robinson Slides compliments Dr Yen Lin Learning Objectives To identify key features of the anatomy of cerebral circulation To recognise the physiological significance of the CSF and the ‘blood brain barrier’ To identify communications between the facial and cerebral circulations and the functional significance of these; include a consideration of the cavernous sinus Meninges Refer to membranous coverings of brain and spinal cord 3 layers: Dura mater, arachnoid mater and pia mater Have 2 major functions: Provide supportive framework for cerebral and cranial vasculature Acting with CSF to protect CNS from mechanical damage Often involved in cerebral pathology, as common site of infection (meningitis) and intracranial bleeds Meninges Dura Mater Outermost layer of meninges and is located directly underneath bones of skull and vertebral column Is thick, tough and inextensible Consists of 2 layered sheets of connective tissue: 1. Periosteal layer – lines inner surface of bones of cranium 2. Meningeal layer – located deep to periosteal layer. Is continuous with dura mater of spinal cord Dural venous sinuses are located between 2 layers of dura mater Responsible for venous drainage of cranium and empty into internal jugular veins Receives its own vascular supply – primarily from middle meningeal artery and vein Innervated by CNV SDL - Dura Mater Gray’s Clinical Relevance: Haematoma Collection of blood As cranial cavity is effectively closed box, can cause rapid increase in intra- cranial pressure Death if untreated 2 types involving dura mater: 1. Extradural – arterial blood collects between skulls and periosteal layer of dura. Causative vessel is usually middle meningeal artery, tearing as consequence of brain trauma 2. Subdural – venous blood collects between dura and arachnoid mater. Results from damage to cerebral veins as they empty into dural venous sinuses Arachnoid Mater Middle layer of meninges, lying directly underneath dura mater Consists of layers of connective tissue, is avascular, and does not receive any innervation Underneath arachnoid is a space known as sub-arachnoid space Contains CSF, which acts to cushion brain Small projections of arachnoid mater into dura (arachnoid granulations) allow CSF to re-enter circulation via dural venous sinuses Pia Mater Located underneath sub-arachnoid space Very thin, and tightly adhered to surface of brain and spinal cord Only covering to follow contours of brain (gyri and fissures) Highly vascularised, with blood vessels perforating through membrane to supply underlying neural tissue Clinical Relevance: Meningitis Refers to inflammation of meninges Usually caused by pathogens, but can be drug induced Bacteria are most common infective cause Most common organisms are Neisseria meningitidis and Streptococcus pneumoniae Immune response to infection causes cerebral oedema, consequently raising intra-cranial pressure Has two main effects: Part of brain can be forced out of cranial cavity – known as cranial herniation In combination with systemic hypotension, raised intracranial pressure reduces cerebral perfusion Both complications can rapidly result in death Dural Venous Sinuses Lie between periosteal and meningeal layers of dura mater Collecting pools of blood which drain CNS, face and scalp Ultimately drain into internal jugular vein Do not have valves 11 in total Gray’s Dural Venous Sinuses Straight, superior and inferior sagittal sinuses found in falx cerebri of dura mater Converge at confluence of sinuses (overlying internal occipital protuberance) Straight sinus is continuation of great cerebral vein and inferior sagittal sinus From confluence, transverse sinus continues bilaterally and curves into sigmoid sinus to meet opening of internal jugular vein Cavernous sinus drains ophthalmic veins and can be found on either side of sella turcica Blood returns to internal jugular vein via superior or inferior petrosal sinuses Clinical Relevance: Cerebral Venous Sinus Thrombosis Presence of thrombus within one of the dural venous sinuses Occludes venous return through sinuses, and causes accumulation of deoxygenated blood within brain parenchyma Can lead to venous infarction Complicated by accumulation of CSF, which can no longer drain through thrombosed venous sinus Common clinical features are headache, nausea and vomiting, and neurological defects Definitive diagnosis is usually made by CT or MRI scan with contrast, which demonstrates obstruction of venous sinuses Treatment is with anticoagulation  Cavernous Sinus Located within middle cranial fossa, on either side of sella turcica of sphenoid bone (pituitary fossa) Enclosed by endosteal and meningeal layers of dura mater Only site in body where ICA passes completely through venous structure ICAs contribute to heat exchange between warm arterial blood and propulsion of venous blood for cooler venous circulation Receives venous drainage from: Facial veins (no valves) – superior ophthalmic vein forms anastomosis with facial vein. Potential route where infection can spread from extracranial to intracranial site Ophthalmic veins – enter cavernous sinus via superior orbital fissure Superficial middle cerebral vein – contributes to venous drainage of cerebrum Sphenoparietal sinus – empties into anterior aspect of cavernous sinus Central vein of retina – drains into superior ophthalmic vein, or directly into cavernous sinus Pterygoid plexus – located within infratemporal fossa Empty into superior and inferior petrosal sinuses, and ultimately into internal jugular vein Left and right cavernous sinuses are connected in midline by anterior and posterior intercavernous sinuses Travel through sella turcica of sphenoid bone Anatomical Location and Borders Anterior – superior orbital fissure Posterior – petrous part of temporal bone Medial – body of sphenoid Lateral – meningeal layer of dura mater running from roof to floor of middle cranial fossa Roof – meningeal layer of dura mater that attaches to anterior and middle clinoid processes of sphenoid bone Floor – endosteal layer of dura mater that overlies base of greater wing of sphenoid bone Contents Several important structures pass through cavernous sinus to enter orbit Travels through cavernous Travels through lateral wall sinus of cavernous sinus Abducens nerve (CN VI) Oculomotor nerve (CN III) Carotid plexus (post-ganglionic Trochlear nerve (CN IV) sympathetic nerve fibres) Ophthalmic (V1) and maxillary Internal carotid artery (V2) branches of trigeminal (cavernous portion) nerve Clinical Relevance: Cavernous Sinus Thrombosis Formation of clot within cavernous sinus Most common cause is infection, typically spreads from extracranial location eg orbit, paranasal sinuses, ‘danger zone’ of face Infection is able to spread due to anastomosis between facial vein and superior ophthalmic veins Common clinical features include headache, unilateral periorbital oedema, proptosis (eye bulging), photophobia and cranial nerve palsies Abducens nerve most commonly affected Treatment typically with abx therapy Where cause is infection, thrombosis of cavernous sinus can rapidly progress to meningitis What is Cerebrospinal Fluid? Ultrafiltrate of plasma that surrounds brain and spinal cord Secreted by choroid plexus which is found in roof of ventricles Derived from blood plasma (but lower in protein) Total volume = 140mls but 500mls secreted each day from choroid plexus in ventricles Excess CSF is drained into venous sinuses via arachnoid granulations Brain floats in CSF – reduces its effective weight from 1.5kg to 50g Has many functions: 1. Protection – acts as cushion for brain, limiting neural damage in cranial injuries 2. Buoyancy – by being immersed in CSF, net weight of brain is reduced to approx. 25g. Prevents excessive pressure on base of brain Contents of CSF CSF Blood pH 7.33 7.41 Osmolarity 295 mOsm/L 295 mOsm/L Glucose (fasting) 2.5 – 4.5 mmol/L 3.0 – 5.0 mmol/L Protein 200 – 400 mg/L 60 – 80 g/L Sodium 144 – 152 mmol/L 135 – 145 mmol/L Potassium 2.0 – 3.0 mmol/L 3.8 – 5.0 mmol/L Chloride 123 -128 mmol/L 95 – 105 mmol/L Calcium 1.1 – 1.3 mmol/L 2.2 – 2.6 mmol/L Urea 2.0 – 7.0 mmol/L 2.5 – 6.5 mmol/L Ventricular System Structures that produce CSF, and transport it around cranial cavity Lined by ependymal cells, which form structure called choroid plexus – CSF is produced here Embryologically, derived from lumen of neural tube 4 ventricles in total: right and left lateral ventricles, third ventricle and fourth ventricle Ventricular System Lateral ventricles Left and right lateral ventricles are located within respective hemispheres of cerebrum Have ‘horns’ which project into frontal, occipital and temporal lobes Volume increases with age Third ventricle Lateral ventricles connected to third ventricle by foramen of Monro Situated in between right and left thalamus Anterior surface contains 2 protrusions: Supra-optic recess – located above optic chiasm Infundibular recess – located above optic stalk Fourth ventricle Receives CSF from third ventricle via cerebral aqueduct Lies within brainstem, at junction between pons and medulla oblongata Drains into 2 places: Central spinal canal – bathes spinal cord Subarachnoid cisterns – bathes brain, between arachnoid and pia mater. CSF is then reabsorbed back into circulation CSF Pulsation and Reabsorption CSF produced by choroid plexus, located in lining of ventricles Consists of capillaries and loose connective tissue, surrounded by cuboidal epithelial cells Plasma filtered from blood by epithelial cells to produce CSF Drainage of CSF occurs in subarachnoid cisterns Small projections of arachnoid mater (arachnoid granulations) protrude into dura mater Allow fluid to drain into dural venous sinuses CSF Pulsation and Reabsorption CSF Production CSF produced by choroid plexus which covers 2 lateral ventricles and roof of 3rd and 4th ventricles Around 500ml of CSF is produced each day, with around 150ml being present in body at any given time CSF is produced continuously which keeps fluid in circulation around CNS CSF will move from lateral ventricle to 3rd and then 4th ventricle From 4th ventricle, CSF moves out into subarachnoid space and/ or central canal of spinal cord through 2 lateral foramina of Luschka and medial foramen of Magendie CSF Clearance CSF drains into superior sagittal venous through arachnoid villi, small protrusions of arachnoid mater into venous sinus Physiologically, pressure of CSF within subarachnoid space is greater than that within venous sinus therefore will drain into venous sinuses Achieved via creation of giant CSF-creating vacuoles in arachnoid cells  The Blood: Brain Barrier Squamous epithelial cells form endothelial wall of capillaries; luminal surface of these cells comes into contact with circulating blood and its constituents Abluminal (outer) surface is in contact with circumferentially continuous basement membrane Endothelial cells are anchored to each other by zonula occludens (tight junctions) as well as zonulae adherens Zonula occludens – provide structural support to endothelial wall, circumscribe cells and provide seal with all adjacent cells Zonula adherens – connects adjacent cells Therefore endothelium functions as impermeable barrier between capillary lumen and brain tissue Pericytes also involved in formation of blood-brain barrier Encircle endothelial cells of capillaries and are able to contract in order to regulate capillary blood flow Also promote formation of tight junctions and inhibit production of chemicals that promote vascular permeability Astrocytes are highly branched cells with small bodies found both in white and grey matter Their podocytes encircle nerve fibres and neuronal somas but also surround abluminal surface of capillaries At this point, their processes are known as perivascular endfeet https://www.rit.edu/spotlights/blood-brain-barrier Arterial Supply 2 paired arteries which are responsible for blood supply to brain; vertebral and internal carotid arteries Arise in neck, and ascend to cranium Within cranial vault, terminal branches of these arteries form an anastomotic circle – Circle of Willis From this circle, branches arise which supply majority of cerebrum Other parts of CNS (pons and spinal cord) are supplied by smaller branches from vertebral arteries Internal Carotid Arteries Originate at bifurcation of left and right common carotid arteries, at level of 4th cervical vertebrae (C4) Move superiorly within carotid sheath, and enter brain via carotid canal of temporal bone Do not supply any branches to face or neck Once in cranial cavity, pass anteriorly through cavernous sinus Distal to cavernous sinus, each ICA gives rise to: Ophthalmic artery – supplies structures of orbit Posterior communicating artery – acts as anastomotic ‘connecting vessel’ in Circle of Willis Anterior choroidal artery – supplies structures in brain important for motor control and vison Anterior cerebral artery – supplies part of cerebrum ICA then continues as middle cerebral artery, which supplies lateral portions of cerebrum Vertebral Arteries Right and left vertebral arteries arise from subclavian arteries, medial to anterior scalene muscle Ascend posterior aspect of neck, through holes in transverse process of cervical vertebra – foramen transversarium Enter cranial cavity via foramen magnum Within cranial vault, some branches are given off: Meningeal branch – supplies falx cerebelli, sheet of dura mater Anterior and posterior spinal arteries – supplies spinal cord, spanning its entire length Posterior inferior cerebellar artery – supplies cerebellum After this, both vertebral arteries converge to form basilar artery Several branches originate here, and go onto supply cerebellum and pons Terminates by bifurcating into posterior cerebral arteries Circle of Willis Circle of Willis Terminal branches of vertebral and carotid arteries all anastomose to form circular blood vessel – Circle of Willis 3 main constituents: 1. Anterior cerebral arteries – terminal branches of internal carotid arteries 2. Internal carotid arteries – located immediately proximal to origin of middle cerebral arteries 3. Posterior cerebral arteries – terminal branches of basilar artery To complete circle, 2 ‘connecting vessels’ are also present: 1. Anterior communicating artery – connects 2 anterior cerebral arteries 2. Posterior communicating artery – branch of internal carotid, connects ICA to posterior cerebral artery Circle of Willis - In Situ Anterior cranial fossa Middle cranial fossa Posterior cranial fossa  Cerebral Arteries Lateral view of brain Mid-sagittal view of brain Clinical Relevance: Stroke Brain is particularly sensitive to oxygen starvation Stroke is acute development of neurological deficit, due to disturbance in blood supply of brain 4 main causes of cerebrovascular accident: 1. Thrombosis – obstruction of blood vessel by locally forming clot 2. Embolism – obstruction of blood vessel by embolus formed elsewhere 3. Hypoperfusion – lack of blood supply to brain, due to systemically low blood pressure 4. Haemorrhage – accumulation of blood within cranial cavity Most common cause is embolism In many patients, atherosclerotic embolus arises from vessels of neck Recommended Reading