The thecal sac or dural sac is the membranous sheath (theca) or tube of dura mater that surrounds the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

and the cauda equina. The thecal sac contains the

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless Extracellular fluid#Transcellular fluid, transcellular body fluid found within the meninges, meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricular system, ven ...

which provides nutrients and buoyancy to the spinal cord. From the

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

the tube adheres to bone at the foramen magnum and extends down to the second sacral vertebra where it tapers to cover over the filum terminale. Along most of the spinal canal it is separated from the inner surface by the epidural space. The sac has projections that follow the

spinal nerve A spinal nerve is a mixed nerve, which carries Motor neuron, motor, Sensory neuron, sensory, and Autonomic nervous system, autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each s ...

s along their paths out of the vertebral canal which become the dural root sheaths.

Clinical significance

The lumbar cistern is part of the subarachnoid space. It is the space within the thecal sac which extends from below the end of the spinal cord (the conus medularis), typically at the level of the first to second lumbar vertebrae down to tapering of the dura at the level of the second sacral vertebra. The dura is pierced with a needle during a lumbar puncture (spinal tap). For epidural anesthesia an anesthetic agent is injected into the space just outside the thecal sac and diffuses through the dura to the nerve roots where they exit the thecal sac. For

spinal anaesthesia Spinal anaesthesia (or spinal anesthesia), also called spinal block, subarachnoid block, intradural block and intrathecal block, is a form of neuraxial regional anaesthesia involving the injection of a local anaesthetic with or without an opi ...

in general, an injection can be given intrathecally into the subarachnoid space, or into the spinal canal. This

route of administration In pharmacology and toxicology, a route of administration is the way by which a medication, drug, fluid, poison, or other substance is taken into the body. Routes of administration are generally classified by the location at which the substance ...

may also be used for the delivery of

drugs A drug is any chemical substance other than a nutrient or an essential dietary ingredient, which, when administered to a living organism, produces a biological effect. Consumption of drugs can be via inhalation, injection, smoking, ingestio ...

which will evade the

blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...

. Disruption of the dural sac may occur as a complication of a medical procedure, or as a consequence of trauma causing a cerebrospinal fluid leak, or spontaneously resulting in a spontaneous cerebrospinal fluid leak. If the spinal cord is not free to move within the thecal sac due to abnormal tissue attachments, especially during growth, tethered spinal cord syndrome may occur. In a split cord malformation, some portion of the spinal cord is divided into parallel halves. The thecal sac may be divided and surround each half with a spike of cartilage or bone dividing the halves (Type I), or both halves may be present within the same sac where the dura is bound to a band of fibrous tissue (Type II).

References

{{Spinal cord Spinal cord