Arachnoid

Ashley Davidoff MD

Definition

The arachnoid (cobweb) layer, is a thin, avascular membrane which adheres closely to the fibrous dura mater, creating between them. This space is of small capacity, and it is called the subdural space, . In addition to the connection with the dura, the arachnoid has projections referred to as trabeculae, which allow a connection with the pia mater. The subarachnoid space is a signifcant space as it contains the cerebrospinal fluid, and cerebral blood vessels. There are certain places, especially at the base of the brain, where the arachnoid layer is widely separated from the pia mater and results in the pooling of large amounts of CSF. Examples include the cerebellomedullary cistern, the pontine cistern, the cistern of the lateral sulcus, the superior cistern and the interpeduncular cistern.

General Organization of the Meninges

The coronal drawing reveals the three layers of the brain. The inner layer (maroon) is the pia mater and it is in intimate contact with the brain and faithfully follows the sulci and gyri. The second layer is the arachnoid (pink) which is a slightly thicker membrane and follows the pia in a general fashion but does not extend into the sulci. It is intimately attached to the inner layer of the dura (bright green) The space between the pia and the arachnoid is the subarachnoid space, and it is in this space that the CSF is present and surrounds the brain. The next layer is the dura which is a double layer. The inner layer (bright green) is intimately attached to the arachnoid and the outer layer (also bright green) is attached to the bone and functions as the inner periosteum. There is a potential space between the arachnoid (pink) and the inner layer of the dura (green). This space is called the subdural space. (combination pink and green) The CSF (light blue) is seen in the subarachnoid space and in the lateral ventricles (gray blue)

The Piamater, Subarachnoid Space, and the Arachnoid

The histological section of the cortex at 10X magnification reveals two of the three meninges of the brain. The inner layer (maroon) is the pia mater and it is in intimate contact with the brain and faithfully follows the sulci and gyri. At this magnification it is barely visible since it is very thin. The second layer is the arachnoid (pink) which is a slightly thicker membrane and follows the pia in a general fashion but does not extend into the sulci. It is intimately attached to the inner layer of the dura The space between the pia and the arachnoid is the subarachnoid space, and it is in this space that the CSF is present and surrounds the brain. A small blood vessel likely a vein is seen entering the brain via the subarachnoid space.

Image Courtesy of Thomas W.Smith, MD; Department of Pathology, University of Massachusetts Medical School. 97377b04.9

Dura and Arachnoid and Subarachnoid Space

MRI T1 Weighted Coronal View

The coronal T1 weighted MRI reveals demonstrates some of the anatomical features of the meninges and their relations The sulci are seen as T1 dark CSF containing fissures in the brain substance. The pia is too thin to see but would be adherent to the brain surface. The space that contains the CSF is called the subarachnoid space (light blue). The very fine T2 bright membrane seen on the surface of the brain is a combination of arachnoid (pink) adherent to the two layers of the dura (green). The outer layer of the dura acts as the inner periosteum and is adherent to the skull. The dura splits at the vertex to encompass the superior sagittal sinus and then extends down in the interhemispheric fissure to form the falx. The thin black T1 dark layer after the arachnoid and dura is the inner cortical bone, and is followed by the T1 bright fat containing marrow of the skull. Next is the thin black T1 dark outer cortical bone, followed by a thick layer of T1 bright subcutaneous fat, and then a thin layer of isointense (to soft tissue) skin layer.

Dura and Arachnoid and Subarachnoid Space

MRI T1 Weighted Coronal View

The coronal image of the brain using T1 weighted MRI sequence reveals the three layers of the brain. The inner layer (maroon) is the pia mater and it is in intimate contact with the brain and faithfully follows the sulci and gyri. The second layer is the arachnoid (pink) which is a slightly thicker membrane and follows the pia in a general fashion but does not extend into the sulci. It is intimately attached to the inner layer of the dura (bright green) The space between the pia and the arachnoid is the subarachnoid space, and it is in this space that the CSF that surrounds the brain is housed. The next layer is the dura which is a double layer. The inner layer (bright green) is intimately attached to the arachnoid and the outer layer (also bright green) is attached to the bone and functions as the inner periosteum. There is a potential space between the two layers of the dura. The CSF (light blue) is seen in the subarachnoid space and in the lateral ventricles (gray blue)

Applied Biology

Fractures – usually tear the dura and by implication since the arachnoid is intimately related to the inner dural layer the archnoid is involved as well.

Subdural hematoma is the most common dural trumatic injury occurring in the elederly since the brain is shrunken and the vessels are therefore relativley stretched and with trauma they bleed under the dura either immadiately or even subsequently.

The hematom occurs between the inner layer of the dura and the arachnoid.

A chronic subdural becomes hyperosmolar and the collection slowly draws in fluid and over a few weeks, gets larger and the patient may become unconscious as a result in the subacute phase.

One of the first surgical operations was the relief of subdural hematoma

Subacute Subdural hematoma with Enhancing and Thickened Arachnoid

CT scan

The CTscan with contrast shows a collection that conforms to the shape of the skull with an enhancing meningeal layer. The right parietal collection is a subdural hematoma (SDH) with the outer border conforming to the shape of the skull and lined by the two layers of dura (green) and the inner border thickened and enhancing lined by the arachnoid (pink). The arachnoid is unusually thick in this case suggesting some complexity to the subdural. In the appropriate clinical setting this could be an infected subdural. A subacute subgaleal subcutaneous hematoma is seen in the soft tissues overlying the SDH (white arrow). It also has a slightly enhancing outer layer In addition, as an unrelated abnormality, there is a chronic infarct of the right parietal lobe characterized by a hypodense triangular defect, with loss of gray white matter interface, all consistent with encephalomalacia.

The epidural hematoma also occurs as a result of blunt trauma to skull usually in younger patients and is more commonly an arterial bleed. A classical syndrome of momentary loss of conciousness manifesting clinically as a concussion, the patient becomes lucid again but with ongoing bleeding they become unconscious again as the hematoma expands.

The hematoma occurs beneath the skull and pushes the inner layer of the periostem (outer dural layer) off the skull resulting in both layers of the dura and the arachnoid being on the inner surface of the hematoma.

Epidural Hematoma Skull Fracture

The CTscan without contrast is from a 19 year old male who sustained head trauma and who is complaining of a severe headache. The scan shows a high density lenticular shaped hematoma (a,b,c),. The right parietal collection is an extradural or epidural hematoma, with the outer border conforming to the shape of the skull and lined by the bone) and the inner border lined by two layers of dura (green c,f) and the arachnoid (pink c,f). There are bubbles of air scattered along the inner bone surface (black bubbles best seen on the magnified views (c, f) a tiny fragment of bone (white arrow in f) but seen on images d, and e as a white speck. The fracture of the temporal bone is ringed in orange on the coronal reconstructions of the axial views (h,i)

Arachnoid Cyst

The CT scan of the brain shows a 4.5cms midline triangular mass in the midline in the posterior fossa between the cerebellar hemispheres. The mass is filled with fluid that is isodense to CSF

A dural reflection is noted in the middle of the soft cystic mass under low pressure.

This abnormality which has water density is characteristic of an arachnoid cyst.