AUTHORS: ANGEL M. HIDALGO-OVEJERO, SERAFIN GARCÍA-MATA, TOMAS IZCO-CABEZÓN, GREGORIO GARRALDA-GALARZA, MANUEL MARTÍNEZ-GRANDE.^*

Orthopaedic Surgery Department

Spine Section

(Publicado en Spine 23: 281-3, 1998)

Avda. PÍO XII, 16 - Esc. dcha., 3º C

31008 Pamplona. Spain

Telephone : 48-277609

FAX : 48-331162

Study Design. We present a patient suffering from an intradural herniated disc associated with the presence of epidural gas.

Objective. To advise spine surgeons of the possible association of intradural disc herniation and epidural gas in order not overlook some intradural disc fragments during the surgery.

Summary of Background Data. Three cases of this rare association have been previously published, something which is particularly surprising given both the relatively rare occurrence of intradural herniations and the presence of epidural gas.

Methods. We present a case where such an association occurred, on the basis of preoperative examinations and intraoperative findings. The literature is reviewed for cases of herniated discs associated with epidural gas, and for intradural herniations.

Results. During the open discectomy, following a negative epidural examination, an intradural examination was performed, revealing a disc herniation, which was removed. The patient's postoperative development was satisfactory.

Conclusion. The possibility of an intradural herniated disc must always be considered when performing an open discectomy on a patient whose CT scan reveals the presence of epidural gas. In the event that no clear disc herniation is found to justify the clinical symptoms or the previous radiological findings, an intradural exploration may be indicated.

(Key-words: Herniated disc, epidural gas, vacuum phenomenon, intradural herniation)

The association of epidural gas with a herniated disc has been regarded as an exceptional finding (1)(2)(4)(5)(7)(9)(12)(15)(16). However, in a study by the mentioned authors (8), it has been shown to occur much more frequently than had previously been thought, usually with limited clinical repercussions. The same study also alerted to the fact that herniations with gas may be associated with an intradural herniation, as occurred in 3 of the first 37 published cases of disc herniation with epidural gas (1)(4)(9). The authors regarded these findings as extremely interesting, given the relatively rare occurrence of intradural herniations.

Of the 32 cases of disc herniation associated with epidural gas reviewed by the authors, one was found to be intradural.

A 61-year-old male suffered from sciatica for several months. The condition worsened in recent months, and a slight motor deficiency of the right L5 had developed. The initial CT scan revealed the presence of gas within the medullary canal (Figures 1 A-B).

The patient improved spontaneously, but returned 3 months later, and was found to be suffering from a severe neurological deficiency indicative of a cauda equina syndrome. An emergency mielogram was performed, revealing the presence of a large fill defect distal to the intervertebral L4-L5 space (Figures 2 A-B).

The patient underwent surgery, and a laminotomy revealed no disc material in the canal. Upon mobilisation of the dura and the L5 nerve root a small gap was observed at the ventral level of the dura, from which CSF was exuding. A wide laminectomy and dorsal durotomy was performed and annulus and disc material was found within the dura. The dura was sutured with a fascia patch. The patient's subsequent evolution has been entirely satisfactory.

Intradural herniations are relatively rare, yet a total of one hundred patients have been reported (3)(6)(10)(13)(17)(18)(19)(20), accounting for about 0.27% of all herniations (6). These herniations may appear in any location, although at thoracic level their relative occurrence seems to be discreetly higher (12-15% of all thoracic herniations) (20). Intradural herniations at the thoracic level are usually calcified (20).

The mechanism for herniated disc penetration into the subdural space is unclear, but different theories have been suggested (10)(17). Adhesions between the dura and the common posterior vertebral ligament, secondary to local inflammation, can produce spontaneous perforation, even in the absence of a herniated disc. A second theory is that of a congenital adherence of the dura and the common posterior vertebral ligament. Finally changes brought about by previous surgery can also contribute to dural penetration by a herniated disc

Mielography generally shows the passage of contrast medium to be totally or subtotally blocked, usually proximal or distal to the intervertebral space (13). It may be difficult to differentiate an intradural herniation from a neoplasm using this technique (3). A CT scan does not usually reveal an intradural location adequately. MRI would seem to be useful (17)(18) for this purpose, particularly in coronal cross-sections in T2 sequences. In any event, and even with the aid of the different diagnostic means, it can be difficult to diagnose an intradural disc herniation precisely before surgery (6).

When a potential intradural process is suspected, the differential diagnosis should be include disc herniation, tumours, haemorrhage, meningeal cysts, infection and inflammatory processes, among other conditions (14).

The problem of surgical treatment for intradural herniations is that they may be easily overlooked by the surgeon, who may not entertain such a possibility, thus making it necessary to perform an additional, and potentially difficult, surgical procedure. In order to avoid incorrect surgical treatment, one author has recommended the use of intraoperative ultrasonography for the detection of free intra- or extradural fragments (11).

As occurred in the only three cases previously published of association of intradural disc herniation and epidural gas (1)(4)(9), in our clinical case the herniación was located at the lumbar spine.

Our finding further supports the possible association of epidural gas with an intradural herniation. This should be strongly suspected in the absence of epidural disc material at surgery in patients with a presumed herniated disc. In such cases, if a detailed visualisation of the spinal canal proves negative, we believe an intradural exploration is justified.

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Figures 1 A-B: Different CT scan cross-sections showing the presence of gas a the level of the medullary canal. Hypertrophy of the facet joints. No clear image of the herniated disc is apparent.

Figures 2 A-B : Sacculoradiography indicating the presence of a fill defect distal to the intervertebral space L4-L5. Severe discopathy at L4-L5.

A case of intradural disc herniation associated with epidural gas is presented. This finding is rarely described in the literature, but should be considered by the surgeon if during an open discectomy procedure no obvious disc herniation is found to justify the results of the patient's preoperative imaging findings and/or clinical symptoms.