Postdiscectomy lumbar subdural hygroma with a concurrent cerebrospinal fluid leak

Description

A woman in her 30s underwent a bilateral laminar fenestration and discectomy for an L5–S1 left paramedian disc herniation migrating inferiorly and to the right, which caused subacute sciatica along the right S1 dermatome (figure 1). Surgery was complicated by a small dural tear, but no correction was attempted since the arachnoid layer looked intact, and no cerebrospinal fluid (CSF) efflux was noted. Symptoms abated, and she was discharged the following day.

Figure 1.

Preoperative lumbar spine MRI, T2 weighted. (A) Sagittal section showing a bulky L5–S1 disc herniation with an inferiorly migrating fragment. (B, C) Axial sections showing the left paramedian herniated disc (B) migrating inferiorly and to the right (C), with associated meningoradicular compression. Note the absence of subdural fluid collections exerting mass effect on the conus medullaris or cauda equina nerve roots.

On postoperative day 7, the patient was admitted to our emergency service for intense bifrontal headache, nausea and vomiting, which were relieved by recumbency. Symptoms had started on postoperative day 2. No signs of wound dehiscence or cutaneous leakage of CSF were found. Lumbar spine CT suggested a CSF leak, and the patient was admitted to a neurosurgery ward, where she remained on bed rest. Lumbar spine MRI on postoperative day 12 revealed two CSF leaks with associated bilateral pseudomeningoceles, and an unforeseen intradural CSF collection extending from the conus medullaris to the L5 vertebral body (figure 2). The latter caused the anterior displacement of the conus medullaris and compaction of the cauda equina nerve roots, which were displaced anteriorly but also seen residing centrally within the thecal sac, acquiring what has been dubbed the ‘flying bat’ sign for lumbar subdural hygroma.¹

Figure 2.

Postdiscectomy day 12 lumbar spine MRI. (A) T2-weighted sagittal section revealing a subdural hygroma extending from the conus medullaris to the L5 vertebral body (asterisk). The conus medullaris and cauda equina nerve roots can be seen displaced anteriorly. Associated subfascial and subcutaneous CSF collections are also shown. (B–F) T2-weighted three-dimensional constructive interference in steady state (3D CISS) axial reconstructions. Both a left (B, arrow) and right (C, arrow) posterolateral CSF leaks were identified. The cauda equina nerve roots can be seen compacted and anteriorly displaced (D, E), as well as residing centrally in distal levels, acquiring the ‘flying bat’ sign (F). CSF, cerebrospinal fluid.

Later the same day, the patient underwent surgical correction of the CSF leak. An abundant bilateral outflow of CSF was noted on opening of the subcutaneous and subfascial layers. A right 2 mm dural tear was found on the L5–S1 level, with an associated arachnoid blister. No obvious arachnoid effraction was evident, but on pressure, CSF could be seen flowing from the subdural space. The dural tear was closed with Vicryl 5-0. No active CSF dural leak was found to the left. The symptoms subsided, and a postoperative day 7 MRI showed no evidence of the subdural hygroma, with the intrathecal structures reacquiring a normal position (figure 3). Repeat MRIs at postoperative days 45 and 191 showed no recurrence of the hygroma, the latter also documenting complete resorption of the paravertebral CSF collections.

Figure 3.

Postoperative day 7 lumbar spine MRI, T2-weighted three-dimensional constructive interference in steady state (3D CISS). Sagittal (A) and axial sections (B–D) showing remission of the subdural hygroma and normally positioned conus medullaris and cauda equina nerve roots.

The incidence of spinal subdural hygromas is unknown.² They have been seldom reported, and even more rarely as a complication of spinal surgery.^1–3 Its pathophysiology is controversial. A commonly cited hypothesis suggests pinhole breaches of the arachnoid layer result in a ball-valve type phenomenon that leads to the entrapment of CSF in the subdural space.^1–3 Symptoms vary from none to as severe as cauda equina syndrome.² There are no current guidelines on management. Resolution has been described with approaches as different as watchful waiting,^{1 2} lumbar puncture,⁴ durotomy with or without subdural drain placement,^{1 3} and, as in our case, closing of the dural tear and thorough reclosing of the overlying tissues.²

Learning points.

• Spinal subdural hygromas have been rarely reported following spine surgery. Its pathophysiology is controversial, and there are no guidelines on optimal management.

• Spinal subdural hygromas should be suspected in patients developing postoperative worsening of compressive cord symptoms. MRI of the spine is the mainstay of diagnosis.

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