The brain is bathed in fluid during life. The name of this substance is cerebrospinal fluid (CSF). It provides the brain with nutrients, allows for solute exchange, and provides basic mechanical and functional support to the organ.
There are areas where the CSF will accumulate due to spaces between the two innermost brain layers (meninges). These areas are known as the subarachnoid cisterns. In this article we will discuss the anatomy, contents, function and clinical relevance of these structures.
- Clinical notes
- The pia mater is closely adherent to the brain, and closely follows the contours of the sulci and gyri on the cerebral cortex.
- The arachnoid mater (spider-like, hence its name) is loosely adherent to the brain.
- The dura mater is in close contact with the periosteum that forms the innermost layer of the cranium. The CSF is found in the subarachnoid space.
The subarachnoid space is not of uniform depth across the entire brain. It varies from place to place within the central nervous system. In some of these areas the pia mater and arachnoid mater are not in close contact. In these locations, the subarachnoid tissue is not as dense or plentiful as in other areas. Here we can find expanded areas filled with cerebrospinal fluid (CSF) known as the cisterns from the Latin word for ‘box.’
Although the subarachnoid cisterns are frequently described as distinct compartments, they are not in fact anatomically separate. In reality only a porous wall that has numerous openings or various sizes separates the cisterns. All the arteries and veins of the brain as well as the cranial nerves must pass through the subarachnoid space to leave the skull, and carry their meningeal layers until the very point at which they leave the skull.
The cisterna magna (also known as the cerebellomedullary cistern) is the largest of the subarachnoid cisterns. It is located between the medulla anteriorly and the cerebellum posteriorly. The cistern receives CSF from the diamond shaped fourth ventricle via the median and lateral apertures. The cistern contains numerous structures, including the following:
- vertebral arteries (the first branches of the subclavian arteries) that form the posterior circulation to the circle of Willis
- glossopharyngeal nerve (cranial nerve 9 ), which provides sensation to the pharynx and is the afferent limb of the gag reflex
- vagus nerve (cranial nerve 10), which provides parasympathetic innervation down to the splenic flexure
- accessory nerve (cranial nerve 11), which innervates the sternocleidomastoid and trapezius
- choroid plexus, which produces CSF
The pontine cistern can be found anterior to the pons. It contains the following:
The chiasmatic, or suprasellar cistern is located above the sella turcica and below the hypothalamus. It contains the optic chiasm and pituitary stalk.
The interpeduncular cistern is a relatively wide and cone-shaped cistern occupying the interpeduncular fossa. It is located at the confluence of the supra- and infratentorial subarachnoid space between the two temporal lobes. The cistern is surrounded by the cerebral peduncles and contains the following:
- the bifurcation of the basilar artery
- proximal parts of the posterior cerebral arteries, together with some of their perforating branches
- proximal parts of the superior cerebellar arteries
- oculomotor nerves, which pass between the posterior cerebral and superior cerebellar arteries
- mammillary bodies, which are found in the anterior area of the deep portion of the cistern
The quadrigeminal cistern is also known as the superior cistern, ambient cistern or cistern of the great cerebral vein. The precise location is between the splenium of the corpus callosum and the upper surface of the cerebellum, i.e. dorsal to the midbrain. The cistern extends laterally around the midbrain, from the great cerebral vein to the third ventricle. This cistern contains:
- the pineal gland
- part of the trochlear nerves (cranial nerve 4)
- the great cerebral vein
- the third portion of the posterior cerebral arteries, together with some of their perforating branches
- the third portion of the superior cerebellar arteries, together with some of their perforating branches
The infratentorial portion contains the posterior cerebral artery and the 4th cranial nerve (the trochlear nerve ). This nerve innervates the superior oblique muscle, which moves the eye down and out. It is vulnerable to damage in raised intracranial pressure due to its long winding course from the back of the brainstem to the superior orbital fissure, as well as its relatively thin caliber.
Crural and carotid cisterns
The crural cistern can be found surrounding the ventrolateral aspect of the midbrain, between the parahippocampal gyrus and the cerebral peduncle. Within it, the medial posterior choroidal artery can be found.
The cisternal segment of the anterior choroidal artery runs through the basal cisterns (carotid, crural and ambient cisterns). The carotid cistern is located anterior to the crural cistern between the internal carotid artery and the ipsilateral optic nerve. It contains the first part of the posterior communicating artery, together with the internal carotid artery, which supplies the anterior part of the circle of Willis.
The Sylvian or insular cistern can be located between the temporal lobes and the frontal lobes. It is bordered by the arachnoid surface of the Sylvian fissure and contains several arteries, including the middle cerebral artery.
Cerebellopontine angle cistern
Another relatively small cistern is the cerebellopontine angle cistern. The precise location is between the pons and the cerebellum. It contains the following:
- the facial nerve (cranial nerve 7), which supplies the muscles of facial expression and taste to the anterior two thirds of the tongue (via the chorda tympani branch). It also gives off the nerve to stapedius, and many more
- vestibulocochlear nerve (cranial nerve 8), which has a vestibular branch for balance, and a cochlear branch for hearing
- trigeminal nerve (cranial nerve 5), which relays sensation from the face and innervates the muscles of mastication i.e. masseter, medial and lateral pterygoids, and the temporalis
- posterior inferior cerebellar arteries
Cistern of the lamina terminalis
Next we have the cistern of the lamina terminalis, which can be found in the shape of a high-top tent just anterior to the lamina terminalis. It contains:
Finally we have the lumbar cistern, which is not located within the skull, but rather within the spinal canal at its very termination. It can be found extending from the conus medullaris , (the termination of the spinal cord) around vertebral level L1-L2, to the S2 sacral foramina level.
It contains the filum terminale (a thin cord of non neural tissue that anchors the spinal cord to the sacrum), as well as the cauda equina (from the Latin for ‘horse tail’, which means the sacral spinal nerves). This is the precise location that CSF is drawn from for lumbar puncture.
A lumbar puncture is performed to obtain a sample of CSF. The hollow bore needle is passed through the skin , subcutaneous fat and pierces the posterior longitudinal membrane of the spine. Next, the ligamentum flavum is pierced which gives a popping sound. This ligamentum is yellow due to a high elastin content. The dura mater must also be pierced, and the needle must continue to pass through the arachnoid mater. Now we are in the subarachnoid space, and CSF can be drawn. This may be required in suspected cases of:
- meningitis (in bacterial meningitis the CSF may appear cloudy, tests will show low glucose and high levels of white blood cells)
- subarachnoid haemorrhage (xanthochromia or yellowish discolouration of CSF due to the breakdown of red blood cells)
- multiple sclerosis (for instance, oligoclonal banding or several IgG bands in CSF electrophoresis).
If a lumbar puncture is ineffective, the cisterna magna can be tapped for a CSF sample.
This is the accumulation of CSF in the brain. This may be due to overproduction from the choroid plexus (from a tumour), or more commonly a blockage, from a separate tumour, infection or other cause. The condition may also not present with raised intracranial pressure. Children who have this condition have grossly enlarged skulls, as their skull bones have not yet fused.
Adults will experience headaches, dizziness and cranial nerve issues. Late stage symptoms may include sudden death, due to compression of the vital respiratory and cardiac centres in the medulla. This process is called ‘coning’.