Corpus: Subarachnoid space

The subarachnoid space is a cleft space between the arachnoid mater and the pia mater. As the cerebrospinal fluid circulates in it, it is also referred to as the external cerebrospinal fluid space.

The subarachnoid space is the largest contiguous external cerebrospinal fluid space. The numerous superficial arteries and veins of the brain and spinal cord are located in the subarachnoid space between the arachnoid and pia mater. The bridging veins, which run from the subarachnoid space through the arachnoid and dura mater into the sinus durae matris, are an exception.

The subarachnoid space is connected via the paired apertura lateralis and the unpaired apertura mediana of the 4th cerebral ventricle to the inner cerebrospinal fluid space, which is formed by the cerebral ventricles. It is separated by the pia mater from the Virchow-Robin space (spatium perivasculare), which continues along the vessels that run into the interior of the brain.

The arachnoid spans the depressions (sulci and fissures) of the cerebrum. This results in fluid-filled extensions of the subarachnoid space, the cerebrospinal fluid cisterns (cisternae subarachnoideae). The most important cisterns include

• Cisterna cerebellomedullaris (Cisterna magna) between the caudal surface of the cerebellum and the dorsal surface of the medulla oblongata
• Cisterna basalis
  • Cisterna interpeduncularis
  • Cisterna chiasmatis
  • Cisterna ambiens
  • Cisterna fossae lateralis cerebri
  • Cisterna pontocerebellaris superior
  • Cisterna pontocerebellaris inferior

Another important cistern lies at the caudal end of the spinal cord, the cisterna lumbalis.

According to recent studies (2023), the subarachnoid space is divided into two compartments by a "fourth meninges", the "Subarachnoid Lymphatic-like Membrane", SLYM for short.^[1]

Subarachnoid haemorrhage[Bearbeiten]

Under pathological conditions, blood accumulations can be detected in the cisterns in so-called subarachnoid haemorrhages (SAH). SAH is the most common non-hypertensive cerebral haemorrhage. It usually occurs as a result of aneurysm rupture and accounts for around 6-20% of all spontaneous intracerebral haemorrhages. A SAH occurs preferentially in the area of the Willisian artery and in the proximal sections of the anterior cerebral artery (especially the anterior communicating artery) and the middle cerebral artery. A rupture of subarachnoid arteries or an aneurysm can be triggered, for example, by sudden increases in blood pressure or violent squeezing. However, craniocerebral trauma can also cause SAH (traumatic SAH).

Lumbar puncture[Bearbeiten]

A cerebrospinal fluid puncture is performed to obtain cerebrospinal fluid for diagnostic purposes. The main site used for CSF collection is in the area of the cauda equina. The sample is taken from the cisterna lumbalis of the subarachnoid space. The needle is usually inserted between the laminae of the vertebral bodies L3 and L4 or L4 and L5. The area around the cauda equina is filled with fluid, as the spinal canal with the dura mater and arachnoid is longer than the spinal cord, which ends at LWK 1/2. There is therefore no risk of injury to the spinal cord when puncturing at the level of LWK 3/4. With careful puncture, the fibre cords of the cauda equina (anterior and posterior roots of the lumbosacral segments) avoid the cannula.

In small children, the spinal cord extends further caudally in the spinal canal. Here However, due to the risk of injury to the medulla oblongata, a suboccipital puncture may only be considered in exceptional cases. Cerebrospinal fluid can be extracted from the cisterna cerebellomedullaris by inserting the puncture needle directly caudally from the occiput. To collect the cerebrospinal fluid, the atlantooccipital membrane must be passed after the neck muscles, the dura mater and the arachnoid membrane in a parietal-rostral direction.

• Image source podcast (Packaging of the CNS: The Meninges): © 1195798 / Needpix
• Image source podcast (The ventricles of the brain): © Midjourney