Arachnoid Granulations

ANATOMY AIIMS, GROSS ANATOMY, EMBRYOLOGY, NEUROANATOMY, MICROANATOMY, APPLIED/ CLINICAL ANATOMY

Arachnoid granulations, also known as Pacchionian granulations, are fascinating structures within the human brain that play a crucial role in the circulation of cerebrospinal fluid (CSF). Let’s delve into the details of these intriguing formations.

Anatomy and Function

1. Location and Structure:

   • Arachnoid granulations are small protrusions of the arachnoid mater, which is the thin second layer covering the brain.
   • They extend from the subarachnoid space (the space between the arachnoid and pia mater) through the dura mater (the thick outer layer) to enter the walls of dural venous sinuses.
   • These granulations are essentially pockets of arachnoid membrane that project into the outer membrane of the dura mater.

2. Function:

   • Arachnoid granulations serve as valves that allow CSF to pass from the subarachnoid space into the venous system.
   • The CSF, which bathes the brain and spinal cord, needs a way to return to the bloodstream. Arachnoid granulations facilitate this process by providing a direct connection between the subarachnoid space and the venous sinuses.
   • As CSF flows through the arachnoid granulations, it is absorbed into the venous blood, maintaining the delicate balance of fluid within the central nervous system.

3. Epidemiology:

   • Arachnoid granulations are more commonly seen in older patients.
   • They increase in size and number with age and are observed in approximately two-thirds of patients.
   • These structures are usually incidental findings during radiological imaging studies.

4. Radiographic Features:

   • On CT scans or MRI, arachnoid granulations appear as osteolytic, sharply circumscribed lucencies within the skull.
   • They can also be mistaken for dural venous thrombosis due to their location within the dural venous sinuses.
   • However, their round, well-defined shape and characteristic location help differentiate them from thrombosed veins.
   • On MRI, they exhibit signal characteristics similar to CSF: low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.

5. History and Etymology:

   • These granulations are named after Antonio Pacchioni (1665-1726), an Italian physician who extensively studied the anatomy of the dura mater.
   • Pacchioni provided the first written description of these eponymous granulations in 1705 in his monograph titled Dissertatio Epistolaris de Glandulis Conglobatis Durae Meningis Humanae.

In summary, arachnoid granulations are remarkable structures that bridge the gap between the subarachnoid space and the venous system, ensuring the proper circulation of cerebrospinal fluid. Their discovery and understanding owe much to the pioneering work of Antonio Pacchioni, whose name they bear to this day.