Meninges DO SPRING 2024.pdf

Meninges and the Brain NOEL GUISON, MD, PhD DO SPRING 2024 (modified from Michael Wells, PhD) Netter Images are from the Netter Presenter Neuroscience collection, Icon systems OBJECTIVES The meninges are connective tissue membranes that surround the central nervous system and are essential for its n...

Meninges and the Brain NOEL GUISON, MD, PhD DO SPRING 2024 (modified from Michael Wells, PhD) Netter Images are from the Netter Presenter Neuroscience collection, Icon systems OBJECTIVES The meninges are connective tissue membranes that surround the central nervous system and are essential for its normal physiological operation. The learner should be able to Describe the development of the meninges and correlate to some developmental sequelae. Describe the anatomy of the primary meningeal layers in the brain and spinal cord to help understand the functions. Discuss the primary functions of each layer. Compare/contrast the specializations of the meninges in the brain and spinal cord including the dural septa, dural sinuses, and subarachnoid space, their anatomical relationships and functions. Describe the vasculature and innervation of the meninges. Describe some basic pathology associated with the meninges (bleeding, infection, tumors). Overview-Review Meninges consist of three connective tissue “membranes” that surround the central nervous system and are continuous with the connective tissue of peripheral nerves Dura mater Arachnoid mater Subarachnoid space Pia mater Haines Chapt 7 Overview-Review Meninges consist of three connective tissue “membranes” that surround the central nervous system and are continuous with the connective tissue of nerves Dura mater Arachnoid mater Pia mater Functions of meninges forms a “scaffolding” for vasculature and, especially venous drainage for the CNS including formation of a blood brain barrier circulation of cerebrospinal fluid is dependent upon intact meningeal structures suspend the brain in the cranial cavity Both cranial and spinal nerves as they extend beyond the meninges Overview-Review Meninges consist of three connective tissue “membranes” that surround the central nervous system and are continuous with the connective tissue of peripheral nerves Dura mater Arachnoid mater Pia mater Functions of meninges forms a “scaffolding” for vasculature and, especially venous drainage for the CNS including formation of a blood brain barrier circulation of cerebrospinal fluid is dependent upon intact meningeal structures suspend the brain in the cranial cavity Overview-Review Meninges consist of three connective tissue “membranes” that surround the central nervous system and are continuous with the connective tissue of peripheral nerves Dura mater Arachnoid mater Pia mater Functions of meninges forms a “scaffolding” for vasculature and, especially venous drainage for the CNS including formation of a blood brain barrier circulation of cerebrospinal fluid is dependent upon intact meningeal structures suspend the brain in the cranial cavity Grant’s Atlas of Anatomy, 14e, 2017. Fig 7.25. Date 4/5/2021 Development of the Meninges The meninges develop from two primary layers: neural crest mesoderm A primitive single layer surrounds the nervous system between 20-35 days of gestation The primitive layer separate into two layers: ectomeninx (eventually the dura) endomeninx (pia and arachnoid; aka leptomeninges) By the end of the first trimester, the meninges generally have the adult pattern Example of Developmental Defect: Dermal Sinus A congenital dermal sinus is a developmental defect associated with the formation of the meninges often associated with spinal bifida (spina bifida aperta) typically in the lumbar spine area ectoderm fails to completely dissociate from neuroectoderm, leaving an epithelium lined channel to the surface of the skin. This can result in recurrent meningitis. Surgical removal of the defect is usually successful. Dermal Sinus Dura Mater Arachnoid and pia Summary of Meningeal Relationships In adults, the meninges consist of three primary layers from the skull inward: dura mater (pachymeninx) arachnoid mater Leptomeninges pia mater These layers consist of fibroblasts and extracellular collagen in various orientations and densities All these meningeal layers are continuous between the brain and spinal cord Derived from ectomeninx Derived from endomeninx Meningeal Relationships: Dura and Bone The ectomeninx eventually divides into: Periosteal (endosteal) dura (outer layer) Meningeal dura (inner layer) Note a splitting of the periosteal from the meningeal dura creating a dural venous sinus Note the arachnoid granulation within the venous sinus that opens from the subaranoid space that contains CSF Meningeal Relationships: Dura and Bone The ectomeninx eventually divides into: Periosteal (endosteal) dura (outer layer) Meningeal dura (inner layer) In the spinal cord, the ectomeninx is initially continuous with the vertebral periosteum during development, but later separates, leaving an epidural space between the vertebral periosteum and the meningeal dura. Meningeal Relationships: Dura and Bone The ectomeninx eventually divides into: Periosteal (endosteal) dura (outer layer) Meningeal dura (inner layer) In the spinal cord, the ectomeninx is initially continuous with the vertebral periosteum during development, but later separates, leaving an epidural space between the vertebral periosteum and the meningeal dura. Dura to Arachnoid Dura is attached to the arachnoid by a thin layer of dural border cells The arachnoid mater is less fibrous than the adjacent dura It is separated from the pia by the subarachnoid space (contains CSF) This space is continuous around the brain and spinal cord The arachnoid mater attaches to the pia with thin connective tissue strands, the arachnoid trabeculae Meningeal Relationships The pia mater is closely adherent to the surface of brain and spinal cord tissue and surrounds surface vessels on the brain. It generally cannot be identified on gross specimens on the cerebrum, It is thicker on the spinal cord and can be noted as a surface element. Haines Chapt 7 Dura Mater- Details The two primary layers of the dura mater are the Periosteal, and Meningeal dura. These layers are relatively tough and thick, adhering to both the skull and each other. These layers contain large amounts of collagen oriented in coherent fibers with clear orientations. The patterns of these fibers have been conjectured to play a role in cranial mechanics. Dura Mater- Details The dural border cell area between the dura and arachnoid is a third, but thinner and weaker dural layer. This layer forms an area of reduced adherence between the dura and the arachnoid and a potential space. Dura Mater- Border regions Bleeding, typically by a cerebral vein, can dissect the plane of weakness between the dura mater and the arachnoid (sub dural hematoma). When the brain is removed from a cadaver, the arachnoid may split away from the dura at this layer. Dural Septa (Infoldings) The dura has extensions directed away from the skull into the cranial cavity. These dural infoldings (reflections, septa) consist of areas in which the meningeal portion of the dura folds inward to separate the cranial cavity into compartments. While adding some support, these compartments restrict the movement of the brain in the cranial cavity. Injuries such as an expanding lesion or bleeding can force areas of the brain between compartments. FALX CEREBRI Sickle shaped fold Found between the right and left cerebral hemispheres Attached to the internal frontal crest and crista galli anteriorly And the tentorium cerebelli posteriorly TENTORIUM CEREBELLI Crescent shaped fold Forms a roof over the posterior cranial fossa TENTORIUM CEREBELLI Crescent shaped fold Tentorial notch: Inner anterior edge from one anterior clinoid process to the other located anterior to the confluence of the falx cerebri and tentorium cerebelli TENTORIUM CEREBELLI Crescent shaped fold Tentorial notch: Inner anterior edge from one anterior clinoid process to the other passing the anterior confluence of the falx cerebri and tentorium cerebelli Posterior outer fixed edge Posterior clinoid process Superior border of petrous bone with the superior petrosal sinus and Occipital bone with transverse sinus TENTORIUM CEREBELLI TENTORIAL NOTCH Crescent shaped fold Tentorial cerebelli attachment: Anterior clinoid process Superior border of petrous bone and Occipital bone with transverse sinus TENTORIUM CEREBELLI Forms a partial roof for the posterior cranial fossa Supports the occipital lobes and covers the cerebellum Affords passage for the midbrain and upper pons Dural Septa (Infoldings) A smaller septum, the falx cerebelli lies in the midline of the cerebellar hemispheres to varying heights. The smallest septum is the diaphragma sella. It forms the roof of the hypophyseal fossa encircling the infundibulum. Haines Chapt 7 Dural Compartments The compartments formed by the dural septi include the lateral supratentorial compartments separated by the falx cerebri and the infratentorial compartment bordered by the tentorium cerebelli. These rigid dural structures act to limit the expansion of the cranial contents to these compartments. An expanding mass (e.g. tumor or hemorrhage) can push contents between compartments. Haines Chapt 7 Rohen et al. Color Atlas of Anatomy Dural Sinuses Dural sinuses are venous structures formed by two mechanisms: separation of the meningeal and periosteal dura at junctions with the skull. Joining of two layers of meningeal dura at the free edges of dural septa Blood collects in the dural sinuses from connecting veins from the cortical surface and internal structures. This will be covered in more detail in lectures on cerebral vasculature. Haines Chapt 7 Dura-Blood supply The vascular supply for the dura in the cranium originates from several branches of the internal carotid artery. In the spinal cord, supply is from multiple sources located close to the vertebral column. Vessels in the cranium run between the periosteum and the periosteal dura, often in groves on the skull. The middle meningeal artery is a primary example. Skull fractures can disrupt these arteries, producing dissection of the connective tissue layers and an expanding, space occupying hematoma (epidural hematoma). Agur-Essential Clinical AnatomyChapt 8 Dural Innervation Although the brain is insensitive to pain, the dural coverings are pain sensitive. The anterior and middle cranial fossa are innervated by branches of the trigeminal nerve. The dura of the posterior fossa receives sensory branches from C2 and C3 (also C1 when present) and may have some sensation through the vagus nerve. The tentorium is supplied by the tentorial nerve, a branch of the opthalmic nerve (V). Agur-Essential Clinical AnatomyChapt 8 Dural InnervationThe irritation of the nerves in the dura by pathological processes (infections, tumors, bleeding) can produce pain (typically headache) appearing to originate in the affected region according to the afferent nerve innervation. Not really somatotopically localized Agur-Essential Clinical AnatomyChapt 8 Meningeal Headaches Because the dura above the tentorium is innervated by the trigeminal nerve, a headache from the irritation or infection of the dura in this region will be referred to the face. The infratentorial dura is innervaed by the cervical nerves, irritation in this area will be referred to the back of the head. Similarly, tumors in these areas that stretch or irritate the dura may produce headaches that are referred to the same areas. Agur-Essential Clinical AnatomyChapt 8 Arachnoid Mater The arachoid mater is a more delicate membrane that is attached to the dura. It is avascular and not innervated. It is separated from the pia by the subarachnoid space. It has two primary parts: arachnoid barrier layer arachnoid trabecule. The arachnoid barrier layer has cells with tight junctions that form a barrier to the diffusion of CSF from the subarachoid space. Arachnoid trabecule are delicate strands of connective tissue spanning across the subarachnoid space to the pia. These trabeculae actually act to suspend the brain in the subarachnoid space. Haines Chapt 7 Arachnoid Mater Although the arachnoid trabeculae are relatively weak, the weight of the brain in CSF is reduced by about 97% (because the brain is mostly water) from about 1400 g in air to about 50 grams in CSF. In spite of this, the brain can still move in this environment, particularly with more violent forces, because the arachnoid trabeculae are not rigid. Haines Chapt 7 Arachnoid Villi ❖ The arachnoid villi are specializations of the dura arachnoid interface in dural sinuses that allow the drainage of CSF into the venous system. ❖ The operation of the arachnoid villi is essential for CSF circulation and normal intracranial pressure. ❖ Collections of arachnoid villi are called arachnoid granulations. These may calcify in older persons and be called pacchionian bodies. PIA MATER The pia mater forms a relatively delicate covering that is generally closely adherent to the surface of the brain, following its contours. In most areas, there are several layers of pial cells that can be divided into an external epipial layer and an intima pia layer that is close to the glial limitans or glial basement membrane that forms the outermost layer of cerebral cortex. The same is true of spinal cord, although the pia is generally thicker. Agur, AMR et al. GRANT’S ATLAS OF ANATOMY 14e c2017 Wolters Kluwer Fig 7.22A Note how the intima pia is invaginated by blood vessels that appear to enter the brain – perivascular space Haines Chapt 7 Parent A. CARPENTER’S HUMAN NEUROANATOMY 9e c1996 Williams and Wilkins Fig 1.13 PIA MATER The pia mater forms a relatively delicate covering that is generally closely adherent to the surface of the brain, following its contours. In most areas, there are several layers of pial cells that can be divided into an external epipial layer and an intima pia layer that is close to the glial limitans or glial basement membrane that forms the outermost layer of cerebral cortex. The same is true of spinal cord, although the pia is generally thicker. Standring, Susan et al. THE ANATOMICAL BASIS OF CLINICAL PRACTICE 40e c2008 Churchill Livingstone Elsevier Fig 3.13 PIA MATER- Blood vessels Most surface vessels on the brain in the subarachnoid space are covered by one or more layers of pial or leptomeningeal cells (not distinguishable from arachnoid), although some vessels may lie free within the space. This pial layer may follow the vessels into the brain or spinal cord substance, along with a small area of extracellular space (the Virchow-Robin space). The pial covering and spaces extend for varying distances into the brain and may allow for better exchange of the extracellular fluid with CSF. Leukemic cells can enter the brain parenchyma through this space. Meninges of the Spinal Cord The meninges of the spinal cord are primarily the same as in the brain but have some important differences. The periosteum of vertebrae and the meningeal dura are separated by the epidural space. Epidural anesthetics are injected in this space (spinal anesthesia involves injection into the subarachnoid space). The absence of a dural attachment to the vertebrae allows the dura to stretch as the vertebra move. Spinal Cord Meninges- Stretching The stretching of the spinal dura (or resistance to stretch) and its attachment to nerve roots contributes to some neurological signs (Kernig, Brudzinski) that are used to indicate meningitis. Weyhenmeyer&Gallman Mosby Elsevier 2006 Kernig sign. Hip joint is flexed then the knee is extended. Associated pain is Kernig sign due to hamstring stiffness from inflammation affecting lumbar spinal roots. Haines Chapt 7 Spinal Cord Meninges- Stretching The stretching of the spinal dura (or resistance to stretch) and its attachment to nerve roots contributes to some neurological signs (Kernig, Brudzinski) that are used to indicate meningitis. Weyhenmeyer&Gallman Mosby Elsevier 2006 Haines Chapt 7 Brudzinski sign. In a supine patient, passively flex the neck (chin to chest). Leading to involuntary hip flexion. Meningitis results in inflammation and irritation of lumber roots. Spinal Cord Meninges- Filum Terminale The spinal cord dura is anchored to the spinal column indirectly through the exits of the spinal nerves and the filum terminale externum, that connects the end of the dura to the coccyx. Haines Chapt 7 PIA MATER The pia mater of the spinal cord is generally thicker than on the surface of the brain. The pia mater also forms specialized attachments to the arachnoid/dura in the spinal cord: the denticulate ligaments the filum terminale internum. The filum terminale internum attaches to the caudal end of the dural sack that makes up the lumbar cistern. Haines Chapt 7 Spinal Cord Meninges The denticulate ligamentsattach from the spinal cord pia arachnoid to spinal dura at 21 pairs of points between the foramen magnum and the first lumbar spinal nerve. They are described as consisting of pia-arachnoid and project from midway between the dorsal and ventral roots and attach to the dura at points midway between nerve exits. Spinal Cord Meninges The denticulate ligamentsattach from the spinal cord pia arachnoid to spinal dura at 21 pairs of points between the foramen magnum and the first lumbar spinal nerve. They are described as consisting of pia-arachnoid and project from midway between the dorsal and ventral roots and attach to the dura at points midway between nerve exits. Moore K et al. CLINICALL ORIENTED ANATOMY 7E Woldter Kluwers c2014Fig I.31 Spinal Cord Meninges The filum terminale and denticulate ligaments further help stabilize the spinal cord relative to the dura and spinal column. Lumbar Cistern The primary subarachnoid cistern (enlargement of subarachnoid space) associated with the spinal cord meninges is the lumbar cistern. Lumbar cistern contains the extension of the spinal meninges (usually at L1-L2 level) after the end of the spinal cord. It contains the cauda equina and is the space commonly accessed in spinal taps (L3-4). Filum terminale internum Clinical Correlation- Meningiomas Meningiomas are tumors of the meninges that can often form space occupying lesions that compress the brain in the dural compartments. They are most likely to arise from clusters of arachnoid cells in the villi (arachnoid cap cells). Meningiomas are most likely to form at points where cranial nerves or blood vessels transverse the dura, along the base of the skull and at the cribriform plate. They are most frequently diagnosed in women patients from 55 to 70 years of age. Patients with neurofibromatosis are particularly likely to present with multiple tumors. Meningiomas Meningiomas may be classified as benign (typical), atypical or malignant. Benign and atypical are the most common and are located outside the brain parenchyma. They may invade the adjacent skull, but rarely penetrate brain tissue. Neurological signs are generally due to compression or edema. Because of this, and the fact that the anterior fossa is the area most common, they may become quite large before they are detected. Surgical removal is usually the treatment of choice. Other Major Pathologies- Meningitis- bacterial Meningeal infections may be bacterial or viral in origin. Causes include trauma, septicemia, and metastasis from another site of infection (streptococcus pneumoniae and Neisseria meningitiis are most common). In bacterial meningitis, the infection is most often initially located in and spreads through in the subarachnoid space. It typically involves the arachnoid and pia (leptomeningitis). Because the infection spreads in the region of CSF flow, CSF is usually cloudy with many white blood cells, increased protein and bacteria. Meningitis- Bacterial In acute forms of bacterial meningitis, symptoms include alternating chills and fever, headache and patients may have a depressed level of consciousness. Rapidly progressing disease can result in death within 2 days. Thickening of the meninges may result in partial obstruction of CSF return flow and may result in signs of increased Intracranial pressure. Meningitis- Bacterial In subacute or more slowly progressing forms of meningitis symptoms may develop over a period of weeks (headache, fever, irritability,wakefulness at night) Prognosis of subacute forms with proper diagnosis and treatment is good (90% cure) Haines Chapt 7 Meningitis-viral Viral meningitis can be caused by a variety of viral agents and is usually seen in younger (