The usual spinal meningocele or meningomyelocele is an obvious posterior protrusion of meningeal-neural elements out of the spinal canal, most commonly in the lumbosacral area. However, lateral or anterior spinal meningoceles also occur, albeit much less frequently. Typically, lateral and anterior meningoceles are occult lesions that are not visible externally. Yet, patients with these unusual lesions come to medical attention in characteristic ways, and the diagnosis can be confirmed readily by myelography and postmyelographic computed tomography (CT) or by MRI, if the physician simply thinks of it.

An anterior defect can form in the vertebral column as a result of faulty embryogenesis. Such faulty development also can result in coexisting abnormalities in the skin, subcutaneous tissues, spine, and internal organs. For example, when one considers the close embryologic associations of the caudal end of the spinal cord, the adjacent notochord, the tailgut, and the cloaca, one can understand the greater than chance simultaneous occurrence of two or more developmental defects, such as sacral agenesis, anterior sacral meningocele, imperforate anus, duplication of the ureter and renal pelvis and duplication of the vagina and uterus. In like fashion, one or more of these conditions may be associated with a presacral or sacral epidermoid, dermoid, or teratomatous tumour. One can also understand how in some instances such associated abnormalities can be found within the same family, apparently transmitted genetically.

Another important factor that can influence the development of anterior and lateral spinal meningoceles is the balance between both the hydrostatic pressure and the pulsations of the cerebrospinal fluid (CSF), and the resistance of the arachnoid and dura mater to deformation by such pressure, especially at the intervertebral foramina. If there is a developmental bony defect in the sacrum, the dura and arachnoid may bulge out through it. especially since the hydrostatic pressure of the CSF at this level is increased when the individual is sitting or standing. von Recklinghausen's disease can lead to both bony dysplasia and dural ectasia, permitting the development of a meningocele at an intervertebral foramen, particularly in the thoracic area. Similarly, Marfan's syndrome can also result in dural ectasia and a lateral meningocele.

As a generalization. an anterior vertebral meningocele may be viewed as the result of an embryologic malformation, and a lateral vertebral meningocele as the result of structural weakness related to dysplasia. especially in the setting of neurofibromatosis or Marfan's syndrome. Occasionally, an anterior or lateral spinal meningocele results from trauma or surgery. These are actually, however. pseudomeningoceles formed by a connective tissue response to cerebrospinal fluid that has escaped through a tear in the arachnoid and dura mater, they are dealt  elsewhere.

Anterior (Lateral) Sacral Meningocele; Intrapelvic Meningocele

The first such lesion was recognized in 1837, and more than 227 were reported in the medical literature through 1988. Anterior sacral meningoceles have been discovered in women almost three times as often as in men. Of the patients reported through 1988 for whom the sex was specified, 165 were female and 60 were male (an 8: 3 ratio). In women, this type of meningocele has been diagnosed primarily during the childbearing years, and especially during the third decade of life; in males, the diagnosis has been made primarily during the first decade.

An anterior sacral meningocele is a unilocular or multilocular extension of the dura mater and arachnoid out of the sacral spinal canal into the retroperitoneal and infraperitoneal space, either anteriorly through the body of the sacrum or anterolaterally through an enlarged intervertebral foramen or coalesced foramina. It contains CSF, which in some of the reported cases has been discolored slightly yellow and has had an elevated protein content. Its wall is composed of fibrous connective tissue, which may contain some of the adjacent sacral nerve fibers and which occasionally contains poorly differentiated neuroglial tissue. It may be covered by a network of epidural veins.

The anterior sacral meningocele is accompanied by some type of bony defect in the anterior surface of the sacrum, varying in severity from an enlarged intervertebral foramen to complete agenesis of the lower sacrum and coccyx. Usually the first and second sacral segments are spared, and the bony pelvic ring remains intact. The most distinctive sacral abnormality is a lateral defect in the lower sacrum that is accompanied by a sickle-shaped deformity of the remaining sacrum and coccyx. This is sometimes referred to as a scimitar sacrum because of the resemblance to a scimitar.

In cases reported through 1988 in which the side of the anterior sacral defect was specified, 70 occurred solely or primarily on the right side, 49 on the left side, and 24 in the midline or in bilateral 10cations.l~ The pedicle of the anterior meningocele is often smaller in diameter than the bony defect, but it involves the same portion of the sacrum.

Other vertebral abnormalities, such as hemivertebra and posterior spina bifida also occur occasionally in association with an anterior sacral meningocele. A posterior bony sacrococcygeal defect is common, usually in the same general location as the anterior defect, but through 1971 a coexisting intrasacral meningocele had been described in only two cases, and a posterior sacral meningocele in just one case.

It must be pointed out, however, that partial or total sacral agenesis may occur in the absence of an anterior sacral meningocele, and is itself frequently associated with neurological deficits in the distribution of the sacral nerves with posterior myelomeningocele and other vertebral anomalies. and with congenital abnormalities in the genitourinary apparatus, rectum and anus. The combination of sacral agenesis and a presacral mass does not always mean an anterior sacral meningocele. Similarly, an anterior vertebral defect (anterior spina bifida) may occur at any level of the vertebral column without an anterior meningocele; it may instead be accompanied by a neurenteric or enterogenous cyst.

Sacral laminectomy with aspiration of the meningocele through its pedicle, followed by closure of the pedicle, is an excellent method of treatment if the meningocele pedicle is small enough and the dura mater is firm enough to permit a water-tight closure. Otherwise the meningocele will refill.

Exposure of the meningocele sac by an abdominal or retroanal approach may allow a better closure of the pedicle since the walls of the pedicle and sac can be used to bridge larger defects. But an abdominal or retroanal operation is somewhat more difficult for at least three reasons: the sac of the meningocele is often closely adherent to the rectum, and the surgical separation of the two is somewhat hazardous because of the risk of bacterial contamination if the rectal wall is inadvertently opened; the epidural veins over the sac may cause annoying or dangerous bleeding during or after the operation; and the blood or serum that collects in the presacral space left after the meningocele has been excised is an excellent culture medium for the bacteria that may enter from the rectum or through postoperative drains.

It seems that the most logical surgical treatment involves the water-tight closure of the meningocele pedicle. by whatever approach (including endoscopy), with prior aspiration but not excision of the meningocele sac. This should be performed with strict aseptic technique, and surgical drains should be avoided.

Lateral or Anterior Lumbar Meningocele

Through 1987, only about 21 lateral or anterior lumbar meningoceles had been reported. In general, these fall into two categories. The first type is a single large retroperitoneal meningocele that presents as an abdominal or lumbar paravertebral mass, frequently in an infant or young child and that usually is associated with other congenital anomalies, especially involving the spinal column at the level of the meningocele. The second. more common, type is a lateral lumbar meningocele that involves a relatively small meningeal herniation or herniations, at one or several levels on one or both sides, causing back or loin pain in an adolescent or young adult, who may have neurofibromatosis. Perhaps in these second cases an underlying weakness in the vertebral bones and connective tissues allows a gradual herniation of the meninges to occur under the influence of CSF pressure.

Among the 21 patients reviewed by Wilkins and Rossitch, the main meningoceles were right-sided in 10 patients, left-sided in 5. and bilateral in 4. Additional vertebral abnormalities were present in at least 12 of the 21 patients. Of the 21 patients, 8 presented during the first decade of life. and 3 during the second decade; among these 11 patients, only one had neurofibromatosis. In contrast, among the 10 patients who presented after the age of 20, 7 had neurofibromatosis. Overall, the patient's sex was given for 19 patients: 12 were female and 7 were male.

As is true with the intrapelvic meningocele. plain roentgenograms. ultrasonography. and MRI are useful in establishing the diagnosis. but the key diagnostic test is myelography combined with postmyelographic CT. If the symptoms are sufficient to justify surgical treatment, either an anterior or posterior approach can be made, with the goal being a water-tight closure of the neck of the meningocele.

Lateral or Anterior Thoracic Meningocele; Intrathoracic Meningocele

More than 150 cases of intrathoracic meningocele have been reported since 1933. One hundred such cases were reviewed in 1978, and 45 more were collected from the literature through 1988. An intrathoracic meningocele is a unilocular or multilocular herniation of the dura mater and arachnoid out of the spinal canal into the retropleural space in the region of the posterior mediastinum. The lateral meningocele typically exits from the spinal canal through a single enlarged intervertebral foramen or two coalesced foramina. whereas the less common anterior meningocele extends through or between the vertebral bodies.

When a kyphoscoliosis is present, the lateral meningocele usually arises at the apex of the deformity. on its convex side. Of 137 cases of intrathoracic meningocele compiled by Wilkins and Odom and Wilkins and Rossitch for whom lateralization data were available, 66 were on the right. 52 were on the left. and 13 were bilateral. The remaining 6 extended anteriorly in the midline. At least 99 of the total group of 145 patients had a single intrathoracic meningocele. The meningoceles were fairly evenly distributed from T1 to T12.

With time. the intrathoracic meningocele usually increases in size. This has been well documented in at least 20 cases by repeated roentgenograms of the chest over periods ranging from 2 months to 28 years. If the lateral thoracic meningocele does enlarge, it usually erodes the ipsilateral bony structures adjacent to the intervertebral foramen, such as the posterior and lateral aspects of the vertebral bodies, the pedicles, the transverse processes and the ribs immediately superior and inferior to the foramen. It also stretches the adjacent nerves and vessels over its dural surface. In addition. the vertebral canal and the proximal intercostal space may be widened at the level of the lesion.

The meningocele finally assumes the form of a mushroom with a narrow stalk (pedicle) and a rounded intrathoracic sac. The wall of the sac is covered by parietal pleura. It is composed of dura mater and arachnoid with varying numbers of epidural veins. intercostal nerves, intercostal vessels and sympathetic chains coursing between the pleura and dura. Occasionally a neurofibroma is also present in the wall of the meningocele. The sac is filled with CSF that usually communicates freely with the spinal subarachnoid space. However, in two of the reported cases, the fluid within the sac had a yellow or brown tint.

In 1979. Erkulvrawatr et al. reviewed 88 cases of intrathoracic meningocele and found that 75 (85 percent) of them had evidence of neurofibromatosis. In one review of 145 cases, at least 89 (61.4 percent) of the patients with intrathoracic meningocele also had cutaneous neurofibromatosis. This figure may be compared to the incidence of neurofibromatosis among 227 patients with intrapelvic meningocele (1.8 percent). 2I patients with lateral or anterior lumbar meningocele (38 percent). and 5 patients with lateral or anterior cervical meningocele (60 percent).

Vertebral abnormalities not ordinarily associated with neurofibromatosis were present in a few of the reported patients with intrathoracic meningocele. Anomalies were noted occasionally in a number of other organ systems as well, but not to the same extent as in patients with an intrapelvic meningocele. In the majority of cases. the lateral thoracic meningocele appears to be a gradual outpouching of the dura mater and arachnoid through a weakened area in the vertebral column. This weakening seems to be due to a regional dysplasia related to neurofibromatosis, but its exact nature remains a subject for debate.

In a few of the reported cases. in which the intrathoracic meningocele was discovered during infancy or was noted to project anteriorly in the midline, it seems more likely that the meningocele was a congenital malformation similar to a posterior meningocele. In these patients there was no evidence of neurofibromatosis and at least four of them had other congenital anomalies.

Of the reported cases of lateral or anterior thoracic meningocele in which the patient's sex was specified. 85 (60.7 percent) were females and 55 (39.3 percent) were males. The peak age at diagnosis for both sexes was in the fourth and fifth decades.

Many patients with intrathoracic meningoceles are asymptomatic. Of the 100 patients reviewed by Wilkins and Odom 40 had no symptoms. The presence of a mass in the chest was detected by chest roentgenograms made in conjunction with a preemployment physical examination as a routine study during hospitalization or for some other unrelated reason. Another 30 patients had chest or back pain, which was often radicular in nature (probably due to the distortion of the intercostal nerves adjacent to the meningocele). Symptoms of pulmonary compression (dyspnoea, cyanosis) were present in 17 cases and cough in 7. Although many of the patients had some degree of kyphosis or scoliosis, this was the presenting symptom of eight patients. Myelopathy or leg weakness was present in 11 cases, but these did not seem to be caused by the meningocele itself.

Intraspinal neoplasms of various kinds develop occasionally in patients with neurofibromatosis. Among these is a dumbbell­shaped neurofibroma or schwannoma having intrathoracic and intraspinal masses connected by a cylindrical stalk that lies within (and expands) an intervertebral foramen. In many of the reported cases of intrathoracic meningocele. the meningocele was thought initially to be this type of tumour until myelography or thoracotomy was performed.

Diagnosing an intrathoracic meningocele usually involves establishing the true nature of a symptomatic or asymptomatic posterior mediastinal mass that has been demonstrated by a roentgenogram of the chest. A wide variety of neoplasms and cysts occur in this location, some of which may be encountered in association with von Recklinghausen's neurofibromatosis.

Percutaneous aspiration of the intrathoracic mass. with analysis of the contained fluid and with the introduction of a positive or negative contrast medium for roentgenographic examination will confirm the diagnosis of a meningocele. However, it is ordinarily easier and safer to establish the diagnosis by myelography aided by postmyelographic CT scanning. MRI is also a useful diagnostic procedure.

Intrathoracic meningoceles are often asymptomatic, so the fact that one exists does not necessarily mean that it should be treated. The probability of its gradual enlargement with time and the possibility that it may cause pain and dyspnoea must be weighed against the risks of anaesthesia and surgery. which may be significant in a cachectic patient with severe kyphoscoliosis and poor pulmonary ventilation.

If surgical correction of the meningocele is decided upon, the surgeon's primary goal should be the closure of its pedicle. This is usually easier to perform from an anterolateral approach via thoracotomy or from a posterolateral approach than by means of a laminectomy. It is usually also feasible to excise the sac of the meningocele, but, of course care must be taken not to injure important adjacent structures. For example, if the sympathetic chain is damaged in a high thoracic meningocele. a Horner syndrome may result.

Lateral or Anterior Cervical Meningocele

In their 1978 review, Wilkins and Odom could find no reports of such lesions. Between 1978 and 1986. at least four cases were documented. In addition, a case originally categorized in 1976 as a meningeal diverticulum along the left C7 nerve root can be classified as a lateral cervical meningocele. In all five cases, these were lateral meningoceles. As with the intrathoracic meningoceles, they involved females more often than males (3: 2) and the right side more often than the left (3:1). Three of the five patients had evidence of von Recklinghausen's neurofibromatosis, and in these cases there were multiple lateral meningoceles.

Conclusion

Spinal meningoceles and meningomyeloceles most often occur as posterior lesions that are obvious at birth. However, a meningocele may form at a lateral or anterior defect in the vertebral column. The lateral defect ordinarily involves a single intervertebral foramen or coalesced adjacent foramina. The anterior opening usually represents a developmental defect in one or more vertebral bodies and intervertebral discs. Patients with lateral meningoceles often have neurofibromatosis, and those with anterior vertebral defects often have other associated embryologic abnormalities.

Overall, female patients are more often affected than male patients [8: 3 in the sacral region; 3:2 in the lumbar, thoracic, and cervical regions; roughly 11:5 overall (265: 124)]. More of the meningoceles are on the right (149) than on the left (107) or in the midline (25). Although the majority are solitary lesions, many patients have multiple unilateral or bilateral meningoceles. Among the cases surveyed, the incidence of neurofibromatosis ranged from 1.8 percent of the patients with an intrapelvic meningocele, to 38 percent of those with a lateral lumbar meningocele, to 60 percent of those with a lateral cervical meningocele. to 61 percent of those with an intrathoracic meningocele.

In general, lateral and anterior meningoceles gradually enlarge with time. The diagnosis can be verified easily by myelography. In the individual case, the decision about surgical treatment must be based on the severity of the symptoms and signs. the likelihood of clinical progression, and the risks posed by the operation. The key to successful surgical treatment is a water-tight closure of the meningocele pedicle. If that can be achieved, the symptoms and signs caused by the mass effect of the meningocele often can be relieved.