Conus Medullaris/The spinal cord measures roughly 45 cm in the adult male and 42 cm in the adult female.[rx] It is a downward extension of the medulla oblongata after it passes through the foramen magnum. The lowermost tapering extremity of the spinal cord is called the conus medullaris, which is around the first or second lumbar vertebra and can sometimes be lower.[rx] The upper border of the conus medullaris is usually poorly defined.
The variation in the position of the conus in adults was studied by Saifuddin et al. via magnetic resonance imaging (MRI), and in children by Wilson et al.[rx][rx] It can be involved in several pathologies such as injury, ischemia, tethered cord syndrome, neoplastic, and non-neoplastic tumors; all of which produce a variety of clinical manifestations. This activity attempts to provide a comprehensive overview of the anatomy of the conus medullaris; which includes the structure, embryology, blood supply, and nerves along with its clinical importance.
Structure and Function
As mentioned above, the conus medullaris is the distal tapering end of the spinal cord. It is continuous with the epic bonus (L4 to S1 segments) superiorly and consists of S2 to S5 as well as the coccygeal segments.[rx] The pia mater of the tapering end of the CONUS continues downward as the filum terminale, which is a delicate strand of fibrous tissue about 20 cm in length. This structure serves to stabilize the spinal cord by connecting the conus to the coccyx via the coccygeal ligament. The lumbosacral nerve roots continue inferiorly to this as the cauda equina. On average, the conus terminates at the middle third of the L1 vertebra but can be located as high as the middle third of the T11 vertebra or as low as the middle third of L3 vertebra. On cross-section, the left and right halves are found to be separated by the ventral median fissure and posterior median sulcus. According to Grogan et al., on computed tomogram (CT) imaging, the conus appears oval in shape, with an anterior sulcus and posterior promontory.[rx] The length of the anteroposterior diameter is 5 to 8 mm, and that of the transverse diameter is 8 to 11 mm. The ventriculus terminalis or the fifth ventricle is an incidentally found asymptomatic cerebrospinal fluid (CSF) containing ependymal lined cavity within the conus which is formed during embryogenesis and regresses in early childhood.[rx] Sometimes, this benign imaging finding may be mistaken for a cystic neoplasm or syringohydromyelia.
The conus medullaris give rise to the lumbar sympathetic, sacral somatic and sacral parasympathetic nerves which continue downward within the cauda equina. These nerves have important functions which can be impaired by injury or ischemia.
Embryology
The spinal cord, along with the rest of the CNS, develops from the neural plate which forms during the third and fourth weeks of intrauterine life.[rx] Secondary neurulation develops the caudal spinal cord, distal to the S2 and filum terminale. Around post ovulation day 27, the conus medullaris ascents opposite of the embryonic coccygeal spinal segments with the nerve roots exiting directly opposite to their vertebral levels.
Between post ovulation days 27 and 54, the caudal end of the neural tube experiences retrogressive differentiation causing it to become thinner, less well developed, and eventually it contains only a rudimentary mantle zone and no marginal zone. The caudal neural tube thinning makes it appear that the end of the conus, relative to the adjacent vertebral column is gradually ascending.[rx]
Beyond post ovulation day 54, retrogressive differentiation ends, and the further ascent of the conus is caused exclusively by a disparity in growth of the vertebral column compared to the spinal cord. Therefore the conus medullaris inhabits a position that is opposite to the progressively higher vertebral levels and nerve roots exiting a specific spinal cord level thus have to travel farther caudal in the thecal sac; towards their eventual exit foramina.[rx][rx][rx]
Eventually, the conus medullaris occupies its “adult level,” by birth or within 2 months after birth. This location is most commonly cranial to or opposite the L1–L2 disk space.[rx]
Blood Supply and Lymphatics
The conus medullaris receives vascular supply via the following vessels-
- The anterior spinal artery, which traverses the anterior median fissure and ends at the filum terminale.
- Two posterior spinal arteries; these traverse the posterolateral aspect of the spinal cord along with the posterior nerve roots.
- One or two small arteries arise from the anterior spinal artery and circumferentially connect it with the posterior spinal arteries at the lower end of the cord, this is called the arterial basket of the conus medullaris. This structure is frequently involved in arteriovenous fistulas and malformations of the conus.
- Radicular arteries: 8 anterior and 12 posterior arteries form longitudinal arterial trunks along with the anterior and posterior spinal arteries and supply the corresponding nerve roots. The anterior arteries at the T1 and T11 segmental levels are very large, termed the arteries of Adamkiewicz, which supply the lumbosacral region of the cord. These radicular arteries are clinically important as they are end arteries, and occlusion leads to spinal cord ischemia.
Nerves
The spinal nerves S3-S5 originate in the conus and provide motor and sensory innervation to the lower extremities, bowel, bladder, and perineum. They are also crucial for sexual function. Spinal nerves L2-L5, S1-S5, and Co1 continue inferiorly as the cauda equina. Compression of these nerves can produce cauda equina or conus medullaris syndromes.[rx][rx]
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