Cysts of several different types may develop within the spinal canal and may come to medical attention by causing myelopathy or radiculopathy or as incidental findings. Intraspinal ganglion cysts and synovial cysts, thought to be rare in 1985, are encountered not infrequently in current neurosurgical practice.

There is some element of confusion in the classification of spinal cysts, and several schemes have been used. Part of the confusion comes from the various terms that have been applied to the same entities by different authors, and part comes from the imprecise description of the cysts in some of the case reports.

Another source of confusion is our incomplete knowledge of the origins of some of these lesions. As an example, intraspinal epithelial cysts that are lined by a single layer of cuboidal epithelium could be of neurenteric or neuroepithelial origin. In some of the reported cases, standard light and electron microscopic techniques have not been sufficiently precise to make this differentiation. At present it is possible to supplement such assessments by immunohistochemical studies which have the potential of permitting more accurate classification.

A final source of confusion is the fact that there is some degree of overlap between the groups. For example, there is a continuum of cases from spinal epidermoid cysts at one end to spinal teratomatous cysts at the other, and a similar continuum from simple spinal neurenteric cysts at one end to spinal teratomatous cysts at the other. Another area of overlap is among spinal perineurial cysts, meningeal diverticula along spinal nerve roots, and lateral spinal meningoceles.

Discussion here does not consider syringomyelia or hydromyelia; cysts associated with astrocytomas, ependymomas, or hemangioblastomas of the spinal cord; or meningoceles and meningomyeloceles that extend out of the confines of the spinal canal.

Spinal Extradural Cysts

Spinal extradural diverticula and cysts are of various types, but those that develop spontaneously, have connective tissue walls, and contain cerebrospinal fluid are probably variations on a theme. They can be grouped as congenital extradural spinal cysts, spinal perineurial cysts (Tarlov cysts), spontaneous meningeal diverticula along spinal nerve roots, and occult intrasacral meningoceles. The congenital extradural spinal cysts (also called developmental extradural arachnoidal cysts or extradural arachnoid cysts) typically come to medical attention during the second decade of life, whereas the other three types are diagnosed most often during the middle of life.

Congenital Extradural Spinal Cysts

Between 1898 and 1970, at least 115 cases of congenital extradural spinal cyst were described in the medical literature, and since then many additional cases have been reported. These cysts are most often located in the thoracic area and are diagnosed most frequently in patients under 30 years of age. The male/female ratio of occurrence is about 4: 3. Scheuermann's kyphosis dorsalis or kyphosis without epiphysitis has coexisted in almost half the cases overall and in two-thirds of the patients between 11 and 20 years of age (all of whom had cysts in the thoracic region).

Cysts in the lumbar and cervical regions tend to occur in persons past the age of 20, suggesting that the cysts arising within the relatively small thoracic spinal canal become symptomatic earlier in life than those arising within the larger cervical or lumbosacral areas.

The congenital extradural spinal cyst, also referred to as an extradural arachnoid cyst, is attached by a narrow pedicle to the dura mater, usually either in the midline posteriorly or posterolaterally at the attachment of the dural sleeve covering a dorsal nerve root. The cyst may or may not communicate with the subarachnoid space. Several such cysts may be present in the same patient, though they usually are solitary. Microscopically, the cyst may be lined by a flattened layer of cells resembling arachnoidal cells, but the rest of its wall is composed of avascular collagenous connective tissue.

Most of the reported cases have been sporadic rather than familial. Several hypotheses have been advanced to explain their origin, all of which imply a congenital defect of some sort. No matter what their exact origin, such cysts appear to enlarge by hydrostatic pressure and CSF pulsations (perhaps with a ball-valve mechanism) until for some reason their communication with the spinal subarachnoid space becomes obliterated.

The clinical course of the patient with a congenital extradural cyst in the thoracic or cervical region is one of spastic paraparesis or quadriparesis of fluctuating but progressive severity. Sensory and autonomic functions are usually involved to a lesser degree than motor function. A congenital extradural cyst in the lumbar area may present with pain, especially pain of a radicular nature.

As the cyst enlarges, it causes enlargement of the spinal canal, which can be identified radiographically. Myelography will reveal an extradural defect. and occasionally the contrast medium will enter the cyst through its pedicle. CT scanning after Myelography enhances the value of the study. MRI is also valuable in establishing the diagnosis.

The overall prognosis of surgically treated cases is good. Usually the cyst can be excised or resected. and the dural defect closed. If the associated cord compression is not too severe or too long­standing, recovery is likely. The key is early recognition and treatment.

Spinal Perineurial Cysts (Tarlov Cysts)

Although such cysts were noted occasionally during the first four decades of the twentieth century, their nature was not elucidated until Tarlov's investigations, first published in 1938. At that time Tarlov was conducting an anatomic study of the filum terminale in about 30 human subjects for another purpose. and in five of the subjects he discovered cysts on the sacral or coccygeal posterior nerve roots in the region where they passed through the dura mater. By studying these and subsequent cases, Tarlov defined the nature of spinal perineurial cysts.

The spinal perineurial cyst arises between the arachnoid that covers a nerve root (perineurium) and the outer surface of its pia (endoneurium), and its walls are formed of these elements. The cyst may extend around the circumference of the nerve or into the substance of the nerve root and dorsal ganglion. It is filled with a clear fluid that probably initially is CSF in communication with the CSF within the spinal subarachnoid space; later this communication may close. Although a spinal perineurial cyst involves a single nerve at first, it may enlarge to the point where it compresses adjacent nerve roots as well.

These cysts typically involve the sacral nerve roots, and they usually were discovered incidentally during myelography, CT scanning, or MR imaging for the evaluation of another condition. However, in at least 60 cases reported between 1948 and 1970, a spinal perineurial cyst has been thought to be the cause of symptoms (especially radicular pain) and signs of nerve root compression.

In the rare instance in which a single Tarlov cyst is causing significant symptoms, surgical excision of the cyst (and its parent root if a water-tight dural closure cannot be achieved otherwise) may provide relief. Since most of these cysts are asymptomatic and are found incidentally, treatment is otherwise not necessary.

Spontaneous Meningeal Diverticula along Spinal Nerve Roots

Meningeal diverticula along spinal nerve roots are not uncommon findings on myelography, CT, or MRI examinations, or during detailed postmortem studies. Like perineurial cysts. these occur in adults of either sex and project laterally into the dorsal root ganglia. They appear to be arachnoid-lined outpouchings that communicate freely with the spinal subarachnoid space. Although they occur along the length of the spine, there is a cervicosacral size gradient. the more cephalad cysts being smaller than those located more caudad (probably a hydrodynamic effect in the upright human being).

On rare occasions a meningeal diverticulum may produce symptoms of nerve root compression. especially in the sacral region. Treatment under this circumstance would be the same as for a symptomatic Tarlov cyst. However. most meningeal diverticula along spinal nerve roots are incidental and do not require treatment.

Occult Intrasacral Meningoceles

The term intrasacral meningocele is actually inaccurate, since a meningocele is ordinarily defined as a herniation of the meninges through a defect in the skull or vertebral column. However, in common usage it denotes a sac composed of fibrous tissue resembling dura mater that is usually lined by arachnoid, lies within an enlarged sacral spinal canal. and is attached to the caudal termination of the dural sac by a narrow or broad pedicle that usually permits the free flow of CSF from the tip of the subarachnoid space into the meningocele.

At least 44 cases of occult intrasacral meningocele were reported between 1932 and 1972. Most of these meningoceles did not become symptomatic until adult life. which suggests that this lesion enlarges with time. probably because of the hydrodynamic effects of the CSF within it.

The intrasacral meningocele displaces the adjacent sacral nerve roots and compresses them against the bony walls of the sacral canal, which is enlarged about the meningocele. This nerve root compression is apparently the cause of the patient's symptoms. The most common complaint is pain in the back with radiating pain and paresthesia in the sciatic distribution of one or both legs. At times hypoesthesia is noted in the same sciatic distribution or in the perineal area. There may also be weakness in one or both legs. Urinary or fecal incontinence or retention are common symptoms and impotence has also been reported. Ordinarily the enlargement of the sacral canal can be seen or suspected on plain roentgenograms and confirmed by myelography, CT or MRI.

Surgical treatment consists of ligating and dividing the pedicle of the meningocele and deflating its sac. Such treatment should relieve the sacral nerve root compression. If the meningocele is associated with tethering of the spinal cord. that condition should also be treated by division of the filum terminale.

Meningeal Diverticula at the Site of Avulsed Spinal Nerve Roots

After traumatic avulsion of one or more cervical nerve roots from the spinal cord, meningeal diverticula are commonly demonstrated by myelography, CT, or MRI at the levels of injury. Such a pseudomeningocele results from a tear of the dura mater and arachnoid with subsequent loculation of the CSF or from a tear in the dura with herniation of the arachnoid through the defect. It may be confined within the spinal canal. but ordinarily it extends out through the intervertebral foramen along the course of the spinal nerve. Usually the traumatic meningeal diverticulum marks the level of the avulsed nerve root, but rarely the pseudomeningocele occurs around or adjacent to an intact nerve root. or an avulsed nerve root exists in the absence of a myelographically demonstrable dural injury.

This same type of traumatic diverticulum can occur in the lumbosacral region in association with avulsion or traction-disruption of one or more of the lower lumbar or upper sacral nerve roots. Traumatic lumbosacral diverticula of this sort are usually seen in association with a fractured pelvis. They are much less common than those in the cervical region.

When one or more meningeal diverticula are demonstrated radiologically after a traction injury to the brachial, lumbar or sacral plexus, this usually indicates not only which nerve root(s) are involved but the fact that the nerve injury is complete and permanent and is not amenable to surgical treatment. The pseudomeningocele(s) themselves ordinarily do not require treatment.

Postoperative or Post-Traumatic Spinal Meningeal Diverticula

A postoperative spinal meningeal diverticulum is ordinarily the result of an intentional or accidental opening of the dura mater land usually of the arachnoid as well) during an operation on the spine, without a watertight closure of the dura at the end of the operation. The CSF leaks out into the planes of operative dissection and becomes encysted by connective tissue as a pseudomeningocele. A post-traumatic spinal meningeal diverticulum may occur in association with any penetrating wound or fracture-dislocation that disrupts the dura mater and arachnoid.

The diverticulum may increase in size because of the direct hydrostatic pressure of the CSF it contains. Most of the reported postoperative diverticula have been in the lower lumbar area, which may simply reflect the distribution of the original condition that was treated surgically (e.g., a ruptured lumbar disc) or may also indicate the importance of CSF pressure in the development of these fluid-filled sacs. As the diverticulum enlarges, it may compress, distort, or entrap the adjacent neural structures. The resulting symptoms and signs will vary according to the specific structure that is compressed. In some cases the CSF enters the diverticulum to such an extent that when the patient assumes an upright position, postural headaches and even syncope may result.

If the presence of a postoperative or post-traumatic spinal pseudomeningocele is suspected, the diagnosis can be verified radiologically. Myelography with postmyelographic CT is the procedure of choice, but MRI may be helpful as well.

The key to the management of postoperative meningeal diverticula is their prevention by avoidance of dural tears and by water­tight closure of dural defects at the initial operation. Closure of such defects with gelatin sponge, oxidized cellulose, muscle stamps, and the like is far less effective than the direct reapproximation of the dural edges with sutures. If prevention fails, the successful treatment of the symptomatic diverticulum is secondary closure of the dural defect, combined with deflation of the diverticulum.

Spinal Ganglion Cysts and Spinal Synovial Cysts

There are two types of cyst that arise from periarticular tissue throughout the body. One has a synovial lining and is called a synovial cyst. The other has no specific lining and is called a ganglion cyst. Such cysts contain viscid or gelatinous material that can range from clear and colorless to turbid or grumous and colored (white to brown).

In 1968, Kao et al. reported two cases of ganglion cyst arising in the extradural periarticular tissues of the lumbar spine. In 1973, Sypert et al. described a patient with a true synovial cyst of a facet joint (which was unusual in that it seemed to be post-traumatic in origin). Since 1968, numerous additional examples of both types of cyst have been reported.

Usually the cyst is unilateral and is located in the posterolateral epidural space, attached to or adjacent to a facet joint. At times it may be adherent to the ligamentum flavum, or rarely, within the interspinous ligament. Most often it occurs in the lumbar area (especially at the L4-L5 level) and comes to medical attention by compressing one or more spinal nerve roots. Spinal ganglion cysts or synovial cysts have been described in bilateral locations and in other areas of the spine, but both situations are unusual.

On myelography a spinal ganglion or synovial cyst produces a posterolateral extradural mass effect. CT scanning or MR imaging shows a rounded posterolateral extradural cystic mass that may contain peripheral calcification. On occasion, such a cyst can be seen to contain gas, perhaps originating from a facet joint. A cyst that communicates with the facet joint (synovial diverticulum) can be imaged by vertebral arthrography, although this technique is not ordinarily required in the management of the usual case.

The standard treatment of a symptomatic spinal ganglion or synovial cyst involves the surgical excision of the cyst. a straight­forward operation that usually provides relief. However. two percutaneous CT-guided approaches to treatment have been reported. The first involves the opacification of a synovial diverticulum by vertebral arthrography. followed by the instillation of a corticosteroid preparation. The second involves the percutaneous biopsy and aspiration of the cyst. It is not yet known whether such approaches will provide better long-term results than surgical excision.

Miscellaneous Spinal Extradural Cysts

Other spinal extradural cysts are rarely encountered in current neurosurgical practice and are mentioned here only for completeness. They include multiple posterior diverticula into the laminae and spinous processes in the lumbo­sacral region in patients with ankylosing spondylitis, aneurysmal bone cysts (occurring more often in the neural arch than in the vertebral body). extradural spinal hydatid cysts t spinal echinococcosis), etc.

Spinal Intradural Cysts

Neurenteric Cysts

During the normal development of the human embryo. a trilaminar germ disc is formed during the third week of development. The outer layer, the ectoderm faces the amniotic cavity. The inner layer, the endoderm faces the yolk sac in the opposite direction. Between these lies the mesoderm. The mesodermal cells that will give rise to the notochord pass forward from Hensen's node under the ectodermal surface. By the eighteenth day, the floor of this tubular notochord and the underlying endoderm disintegrate. The notochordal canal, now known as the neurenteric canal. connects the yolk sac and the amniotic cavity through the blastopore. Thus there is a temporary direct connection between the anlagen of the spinal cord, spine and gastrointestinal tract.

Knowing this, one would predict that malfunctions at this stage of development might result in combined anomalies of the spinal cord, spine and gut. In fact, such combined anomalies have been recognized since the nineteenth century and were classified by Feller and Sternberg in 1929. In 1960, Bentley and Smith unified to some degree the previous hypotheses of the origin of neurenteric malformations, especially those of Feller and Sternberg 1929 and Saunders ( 1943), under the label the split notochord syndrome.

On the basis of different embryologic information. one would expect to occasionally encounter neurenteric cysts within the spinal canal, and such is the case. Between 1926 and 1972, at least 47 intraspinal neurenteric cysts (also known as enterogenous cysts, endodermal cysts, archenteric cysts, gastrocystomas, intestinomas, cystic teratomas, foregut cysts. etc.) were reported. Wilkins and Rossitch subsequently reviewed 72 additional cases of a similar nature. Of the 119 patients. 76 were male and 43 were female. In some instances. abnormalities were noted at birth. The diagnosis was established during the first decade in 41 (34 percent) patients and in the second decade in 27 (23 percent) patients. Only eight of the patients were 50 years old or older.

Of the 119 cysts. 63 were totally or partially in the cervical spinal canal. 53 were totally or partially in the thoracic spinal canal. and 27 were totally or partially in the lumbosacral canal (cysts at the cervicothoracic junction were counted in both cervical and thoracic categories. and cysts at the thoracolumbar junction were counted in both thoracic and lumbosacral categories). The most common cervical or thoracic cyst location (in 57 cases) was anterior or anterolateral to the spinal cord, perhaps with intramedullary involvement as well.

Pathologically, these cysts vary in composition and complexity and form a spectrum that merges with intraspinal teratomas on the one hand and intraspinal dermoids and epidermoids on the other. The simplest examples of intraspinal neurenteric cysts are those lined by a single layer of simple. pseudostratified. or stratified cuboidal or columnar epithelial cells. with or without cilia, lying on a basement membrane and supported by a layer of more or less vascular connective tissue. Varying numbers of the epithelial cells can be demonstrated to contain mucin by appropriate stains. The more complex types have a similar epithelial lining but, in addition, they have one or more of the other elements found along the course of the gastrointestinal tract or tracheobronchial tree.

Intraspinal neurenteric cysts cause the neurological symptoms and signs and the radiologic features one would expect from any slowly enlarging intraspinal mass. Pain in the affected region of the spine is a common and prominent symptom. which may be accompanied by appropriate radicular pain or by meningismus. Unless an early diagnosis is made, the total course of the illness is usually prolonged over many years, often marked by remissions and exacerbations.

Myelography with postmyelographic CT and/or MRI are the best techniques at present for visualizing intraspinal neurenteric cysts and their coexisting vertebral abnormalities. In addition, plain roentgenograms and tomograms may add to the identification and depiction of the vertebral abnormalities. such as anterior or posterior spina bifida, widened vertebral bodies, fused vertebrae, hemivertebrae, and diastematomyelia.

The treatment of intraspinal neurenteric cysts is surgical. They are frequently adherent to the spinal cord or cauda equina to the extent that complete excision is not feasible. but even so. evacuation and partial resection of these lesions is usually followed by the same good results as complete excision. In either case, neurological function usually improves and recurrence after partial resection may take years to become symptomatic.

Epidermoid and Dermoid Cysts

Epidermoid and dermoid cysts are also dealt with elsewhere. They account for 0.2 to 2 percent of the primary spinal tumors seen by neurosurgeons. Among children, however, epidermoid and dermoid cysts comprise 3 to I3 percent of such spinal tumors. and if only the spinal tumors pre­senting within the first year of life are considered. the incidence rises further to 17 percent.

The epidermoid cyst represents one end of a scale of increasing complexity, with the tridermal teratomas at the other end. The wall of an epidermoid cyst is composed of a connective tissue capsule lined by stratified squamous epithelium. The wall of a dermoid cyst is similar, but also exhibits dermal appendages (e.g.. hair follicles, hair, sebaceous glands, and occasional sweat glands). In the walls of both types of cysts, calcium deposits and cholesterol clefts may be found. The connective tissue capsule of those cysts that are attached to, or are located within. the spinal cord are frequently surrounded by a zone of reactive gliosis. The cavity of an epidermoid cyst is filled with white keratinized debris shed from the epithelial lining. The material in a dermoid cyst also contains hair and glandular secretions, and because of the latter the cyst contents may be yellowish with a buttery consistency. More complex cysts lined with stratified squamous epithelium can also be seen.

The contents of both epidermoid and dermoid cysts are quite irritating. If such material enters the spinal subarachnoid space, for example. during a surgical resection of the cyst, it may produce a sterile meningitis followed by a dense chronic arachnoiditis.

Many of the intraspinal epidermoid or dermoid cysts that arise "spontaneously" are seen in association with other congenital anomalies of the spinal cord, the vertebrae and the soft tissues overlying them posteriorly. Posterior spina bifida occulta is common: and posterior dermal sinuses. hypertrichosis. and other dermal abnormalities are also occasionally encountered. Because of these associations, it is thought that solitary intraspinal epidermoid and dermoid cysts probably occur through an error in embryologic development, such as the inadvertent inclusion of cutaneous ectoderm during closure of the neural tube heteroplasia of prospective neural tissue or deficient closure of the posterior portions of an early neurenteric communication.

However, there is also evidence that some intraspinal epidermoid cysts (especially multiple lumbar cysts in the same patient) may have their origin from fragments of skin that are implanted into the spinal canal. such as by lumbar punctures performed without a stylette in the needle. This assertion is supported by several types of evidence. First, the cysts in these cases have rarely been associated with spina bifida occulta. dermal sinus tracts or other congenital anomalies. Second. the cysts have frequently been multiple, have virtually always been extramedullary, and have occurred at the levels of previous lumbar punctures. Third, similar intraspinal epidermoid and dermoid cysts have been encountered in patients who have had epidermis implanted into the spinal canal during the surgical closure of a meningomyelocele. Fourth, it has been shown that fragments of epidermis will lodge within open needles. And finally, epidermoid and dermoid cysts have been produced in experimental animals by the introduction of tiny fragments of skin into the spinal subarachnoid space.

Takeuchi et al. analyzed the reports of 84 intraspinal dermoids and 114 intraspinal epidermoids from the literature.28 Wilkins and Rossitch added 24 intraspinal dermoids and eight intraspinal epidermoids from three other reports. Among the patients whose sex was stated, there were 52 males and 52 females with dermoid cysts, and there were 71 males and 49 females with epidermoid cysts. Of 104 patients with dermoid cysts, 33 were less than 10 years old and 17 were between 10 and 19 years of age: thus, about half presented during the first two decades of life. Similarly, of 120 patients with epidermoid cysts, 30 were less than 10 years old and 24 were between 10 and 19 years of age, again, almost half presented during the first two decades of life.

Using the same sources, it is apparent that most intraspinal dermoid and epidermoid cysts involve the thoracic or lumbar area. Only 15 of 108 dermoid cysts and 6 of 122 epidermoid cysts were located totally or partially within the cervical spine. In contrast, at least 67 of the dermoid cysts and 79 of the epidermoid cysts occurred at or below the thoracolumbar junction. Of 104 intraspinal dermoids, at least 31 were wholly or partially intramedullary in location. Of 114 intraspinal epidermoids, at least 31 were wholly or partially intramedullary.

In regard to associated defects, 25 of 100 patients with dermoid cysts and 17 of 85 patients with epidermoid cysts had posterior spina bifida, and 34 of 100 patients with dermoid cysts and 12 of 85 patients with epidermoid cysts had a posterior dermal sinus tract. When a patient presents with a dermal sinus tract in the lumbosacral or thoracic area, it is likely that the tract extends to an intradural congenital tumour. Among the cases collected by Wright, 11 of 12 sinus tracts in the thoracic region terminated in such a tumour (9 dermoids. 1 epidermoid, and 1 teratoid tumour). as did 31 of 72 lumbosacral sinus tracts.

Epidermoid and dermoid cysts tend to grow slowly within the spinal canal and are usually associated with a long latent period followed by gradually increasing neurological symptoms and signs that are appropriate to the specific location of the cyst. Scoliosis may develop as the cyst enlarges in a child. Frequently, these tumors occur at the level of the conus medullaris and cauda equina. and in addition to the usual symptoms and signs may cause lower spinal rigidity accompanied by spinal hyperlordosis. In these cases, pain is produced if the patient attempts to flatten his back. flex his trunk. or extend his legs. When associated with a dermal sinus, the cysts are usually diagnosed earlier in life, either through investigation of the obvious sinus tract or because of repeated bouts of meningitis.

Plain roentgenograms may reveal associated vertebral anomalies and will also frequently demonstrate focal enlargement of the spinal canal with pedicular erosion and scalloping of the posterior surfaces of the vertebral bodies. In cases in which the cysts are thought to be of iatrogenic origin from lumbar punctures, plain x-ray films are more likely to be normal. Ultrasonography, myelography, CT scanning. and MRI have all proved useful in the diagnostic workup of this lesion.

The treatment of intraspinal epidermoid and dermoid tumors is surgical, although antibiotics and glucocorticosteroids may also be necessary for the management of any concurrent bacterial or chemical meningitis. When a dermal sinus is present, the optimal treatment is complete excision of the entire tract, utilizing prophylactic antibiotics. This is usually best accomplished early in life, before infection has occurred and before any associated intraspinal cyst has reached extensive proportions. The sinus should not be probed or injected because of the danger of introducing bacteria or dermal debris into the spinal canal. If infection is present and there is no significant neurological deficit, operation should be deferred until the infection has been treated adequately. Since dermal sinus tracts in the lumbosacral area may lead upward to a cyst within the conus medullaris, the surgeon should be prepared to follow to that level and to operate intradurally if this is indicated.

Most intraspinal epidermoid and dermoid cysts are located intradurally. A large percentage are intramedullary, but even those which are extramedullary are frequently firmly attached to the spinal cord or cauda equina. especially if the cyst has been previously infected. The cyst should be evacuated, with care taken not to spill its contents into the subarachnoid space. and the accessible portions of the cyst wall should be excised. It is usually wiser to leave any adherent portion rather than risk the neurological damage that may result from attempts to dissect it free. This conservative approach is usually followed by marked and prolonged improvement. even though the cyst may recur years later.

Developmental Arachnoidal Diverticula and Cysts

Developmental intradural spinal arachnoidal diverticula and cysts have been recognized as pathologic entities since at least 1898. They are composed of normal or slightly thickened arachnoid and are filled with a clear colorless fluid that is most likely normal cerebrospinal fluid. There is debate about the origin of these diverticula and cysts, but it is likely that most arise from congenital "faults" in the distribution of the arachnoidal trabeculae, especially in the septum posticum, which divides the posterior spinal subarachnoid space longitudinally in the cervical and thoracic areas. Despite this apparent developmental origin. it is unusual for intraspinal arachnoidal diverticula and cysts to be associated with spinal dysraphism or other congenital anomalies. They rarely occur in families or as part of a congenital syndrome.

The arachnoidal diverticula are common anatomic variations that ordinarily cause no symptoms. They are encountered inciden­tally. The noncommunicating arachnoidal cysts are much less common but also are usually located posterior to the spinal cord. This type of cyst probably originates from the closure of a previously patent diverticulum. (In the present classification scheme, intradural arachnoidal cysts that form as a response to inflammation caused by spinal trauma or surgery, by the presence of blood or a myelographic contrast medium within the subarachnoid space, by bacterial infection, etc. are grouped in a separate category.) As it expands. the developmental spinal intradural arachnoidal cyst can cause progressive spinal cord compression.

The symptoms and signs of a patient with a noncommunicating intradural arachnoidal cyst typically progress over several years. Occasionally. kyphoscoliosis will develop as the cyst enlarges. Sometimes radiographic evidence of enlargement of the spinal canal may be seen. Myelography and postmyelographic CT demonstrate these cysts as nonfilling mass lesions. MRI is now the procedure of choice for the diagnosis of an intradural spinal arachnoid cyst.

Wilkins and Odom collected 67 cases of symptomatic developmental spinal intradural arachnoidal diverticula and cysts from the medical literature through 1971. Of the 66 patients whose sex was specified, 34 were male and 32 were female. The peak age of occurrence was in the fifth decade: only four patients presented in the first decade and only eight in second. Of the 50 cases in which the location of the lesion was specified. 47 were in the posterior or posterolateral aspect of the spinal canal. Only three were in an anterior or anterolateral location, and none was intramedullary. The most common area of the spine affected was the thoracic region, and the second most common area was the cervical region. Other reports of similar diverticula and cysts have appeared since 1971.

Treatment ordinarily consists of a laminectomy with resection of as much of the cyst wall as possible. A portion of the cyst wall may be fused with or consist of the pia mater, making attempts at its complete excision unwise. The patient usually improves and it is unusual for the cyst to recur. However, delayed deterioration of neurological function after initial improvement may occur, perhaps on the basis of spinal cord ischemia or spinal instability. If the cyst extends along many spinal segments is anterior to the cord or recurs after surgical resection, it may be treated to better advantage by shunting its contents into another space such as the peritoneal cavity. Whether the cyst wall is resected or the cyst fluid is shunted, histologic examination of a part of the cyst wall should be performed to establish the diagnosis, because what appears by gross inspection to be an arachnoidal cyst may prove to be a neurenteric or neuroepithelial cyst.

Miscellaneous Spinal Intradural Cysts

Other more unusual types of intradural spinal cyst include inflammatory arachnoidal cyst, ependymal (neuroepithelial) cysts, cystic dilatation of the terminal ventricle, cystic spinal subdural haematoma, spinal cysticercosis, cystic neurilemoma and spinal subdural cysts of undetermined type.

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