Epidural space
Encyclopedia
- For a discussion about the anesthetic procedure, see EpiduralEpiduralThe term epidural is often short for epidural analgesia, a form of regional analgesia involving injection of drugs through a catheter placed into the epidural space...
.
In the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
, the epidural space (also known as "extradural space" or "peridural space") is the outermost part of the spinal canal
Spinal canal
The spinal canal is the space in vertebrae through which the spinal cord passes. It is a process of the dorsal human body cavity. This canal is enclosed within the vertebral foramen of the vertebrae...
. It is the space within the canal (formed by the surrounding vertebrae) lying outside the dura mater
Dura mater
The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from Mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for...
(which encloses the arachnoid mater
Arachnoid mater
The arachnoid mater, literally from Latin "spider -like mother", is one of the three meninges, the membranes that cover the brain and spinal cord...
, subarachnoid space
Subarachnoid space
In the central nervous system, the subarachnoid cavity is the interval between the arachnoid membrane and pia mater....
, the cerebrospinal fluid, and the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
). In humans the epidural space contains lymphatics, spinal nerve roots, loose fatty tissue, small arteries, and a network of large, thin-walled blood vessels called the epidural venous plexus.
In humans
The upper limit of the epidural space is the foramen magnum, which is the point where the spine meets the base of the skullHuman skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
. The lower limit is at the tip of the sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, at the sacrococcygeal membrane
Sacrococcygeal membrane
The sacrococcygeal membrane is a tough fibrous membrane about 10mm long which extends from the inferior tip of the sacrum to the body of the coccyx in humans...
.
In the head, the dura is continuous with the periosteum
Periosteum
Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....
, the tough fibrous lining of the inside of the skull
Human skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
. This means that, in the head, the epidural space is known as a potential space
Potential space
In anatomy, potential space refers to a space or cavity that can exist because two adjacent features that are not tightly adjoined, but does not appear during normal functioning.-Examples:* subdural space* vagina* pericardial cavity...
, which means that normally it does not exist. In rare circumstances, a torn artery (e.g. the middle meningeal artery
Middle meningeal artery
The middle meningeal artery is typically the third branch of the first part of the maxillary artery, one of the two terminal branches of the external carotid artery...
) may cause bleeding which is sufficient to separate both the dura and periosteum
Periosteum
Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....
from the bone; this is an epidural hematoma
Epidural hematoma
Epidural or extradural hematoma is a type of traumatic brain injury in which a buildup of blood occurs between the dura mater and the skull. The dura mater also covers the spine, so epidural bleeds may also occur in the spinal column...
.
The space between the dura and the arachnoid (in both head and spine), the subdural space
Subdural space
The subdural space is an artificial space created by the separation of the arachnoid mater from the dura mater as the result of trauma, pathologic process, or the absence of cerebrospinal fluid as seen in a cadaver. In the cadaver, due to the absence of cerebrospinal fluid, the arachnoid mater...
, is also a potential space. Bleeding may also occur here.
In other mammals
In other mammals, the relationship between the spinal canal and its contents is similar to that in humans, although many species possess a tail into which the epidural space is prolonged.A unique property of the epidural venous plexus is that the veins are prevented from collapsing due to external pressure because the bony spinal canal prevents that pressure being transmitted. This means that for many diving mammals, e.g. whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, when diving a large fraction of venous return to the heart takes place via the epidural space, as veins such as the vena cava may be substantially compressed by the pressure at depth.