Incidence

The incidence of CSF fistula is relatively rare because most dural tears heal spontaneously and only a small percentage of patients develop symptoms. In a study of 3038 spinal surgeries, the incidence of dural tears during the course of bone removal or during dural sac or root retraction was noted to be 5.9%.¹⁰ In Mayfield’s review of 1408 laminectomies, the incidence of CSF fistula requiring reoperation was 0.3% and the incidence of pseudomeningocele was 0.8%.¹

The incidence of pseudomeningoceles is difficult to determine because most cases are asymptomatic. Swanson and Fincher reported a 0.068% incidence of pseudomeningocele in a review of 1700 exploratory laminectomies.⁹ Schumacher et al. reported the incidence of pseudomeningoceles to be less than 0.1% in 3000 patients who had undergone a lumbar discectomy.¹¹ Teplick et al.¹² reported a 2% incidence of pseudomeningocele in a series of 400 symptomatic postlaminectomy patients examined with CT. None of these patients required reoperation.

The relatively low incidence of CSF wound complications in these series may be because a majority of the patients underwent uncomplicated laminectomy for discectomy.^1,9–12 The incidence is much higher and has not been well reported in patients who have undergone laminectomy for spinal dysraphism or in patients with a history of prior spinal irradiation or surgery. Zide et al. reported that 43% of patients with intramedullary spinal cord neoplasms previously treated with radiation developed a CSF fistula or pseudomeningocele after surgery.¹³ They also found a high incidence of pseudomeningocele (43%) and CSF fistula (13%) in patients after surgical correction of the tethered spinal cord.¹⁴

Shapiro and Scully reviewed 39 patients with CSF fistula after spinal surgery.⁵ Sixteen leaks occurred after intradural procedures, despite a primary closure or dural patch graft. Of the remaining 23 cases, 19 occurred in lumbar surgeries. In 6 out of 19 lumbar cases (33%), a dural tear was identified and repaired at the time of surgery. In 13 of the lumbar cases (66%), no tear or leak was identified at the time of surgery. A myelogram was performed the day before surgery in 5 out of these 13 cases (38%). Three cases occurred after cervical spine surgery.

Pathophysiology

A dural tear, either occult or recognized, is the initial event that leads to postoperative CSF cutaneous fistula and pseudomeningocele.¹⁵ Tears may result from excessive traction or inadvertent disruption during surgical decompression. A myelography needle puncture performed shortly before lumbar surgery can also be the cause of postoperative CSF leaks.⁵ Durotomies that are recognized and repaired may still result in postoperative CSF leakage due to inadequate closure, particularly of tears that are difficult to access (i.e., ventral and lateral dura). Resection of dural-based tumors may create dural defects that are impossible to close in a watertight manner.

Cutaneous CSF fistulas most commonly occur in the immediate postoperative period (1–7 days). CSF typically passes through an incompletely healed area of the surgical wound or through a drainage tract. Fistulas may be aggravated by an upright posture, coughing, sneezing, or straining during bowel movement.

Pseudomeningoceles are caused either by herniation of the arachnoid through a dural tear, which subsequently forms an arachnoid-lined sac filled with CSF,⁸ or by direct extravasation of CSF into the soft tissues, with eventual development of a fibrous capsule.^8,12 CSF pulsations force the fluid into the muscular and superficial subcutaneous tissues (Fig. 200-1). The size, shape, and location of the sac depend on the nature of the soft tissue into which the fluid is forced. In rare cases the capsule may ossify.¹⁶ Entrapment of nerve roots in the pseudomeningocele may be a barrier to dural healing^6,7 (Fig. 200-2).

FIGURE 200-1 Lumbar postmyelogram CT demonstrating a postoperative pseudomeningocele.

FIGURE 200-2 Intraoperative photograph during lumbar surgery demonstrating herniation of nerve rootlets (arrow) through a dural defect.

The majority of dural tears heal uneventfully after primary repair. Although the most likely cause of a CSF fistula or pseudomeningocele is an inadequate repair at the time of the durotomy, other factors may contribute to persistent CSF leakage. These include factors that delay or prevent healing of the dura mater and overlying soft tissue.¹⁵ Dural and wound healing may be compromised by scar tissue, irradiation, localized infection, or foreign body reaction. Systemic factors that impair healing include nutritional deficits, endocrine disorders (e.g., diabetes), chronic disease, and steroid administration.

CSF leakage through a repaired dural tear may be exacerbated by elevated CSF pressure. Cutaneous CSF fistula after myelomeningocele repair is often caused by hydrocephalus (CSF pressure 350–450 mm H₂O) and is typically treated with ventricular shunting to correct the abnormal CSF dynamics. Excessive straining can transiently elevate CSF pressure (to >400 mm H₂O) and should be avoided in the perioperative period.¹⁷ Lumbar intradural pressure is markedly elevated with an erect posture (350–450 mm H₂O), compared with supine recordings (70–170 mm H₂O).^18,19 For this reason, patients at risk for CSF leakage are kept flat in bed in the immediate postoperative period. Less commonly, CSF leakage following a durotomy can be exacerbated by the lowering of paraspinal tissue pressure created by the placement of drains under suction adjacent to the dura.

Diagnosis

The diagnosis of a cutaneous CSF fistula is most often established by inspection of the patient’s wound. A watery discharge is assumed to be CSF, particularly if leakage is augmented by upright posture or Valsalva maneuver or is associated with postural headaches. When headaches occur, they are typically more severe with an erect posture and are relieved in a recumbent position. Headaches are secondary to the reduction of the CSF volume when CSF loss through the fistula exceeds its production. The lowered intracranial pressure induces traction on pain-sensitive structures, such as meninges and blood vessels. In the recumbent position, traction is reduced and the pain is relieved.²⁰ Fever or evidence of meningismus suggests bacterial meningitis. When leakage is profuse and clear at the incision site, the diagnosis is unmistakable. Small and intermittent leaks may be overlooked or misinterpreted, especially if they are mixed with blood. If the leaking fluid produces a clear halo that surrounds a central pink stain on an absorbent surface (e.g., sheets or cotton gauze), the fluid is most likely CSF.

Laboratory analysis of the fluid may be helpful in making a diagnosis. Although determining the glucose content of the fluid has been described as a potentially helpful diagnostic test, it is not consistently reliable in being able to identify CSF.²¹ A more specific test is immunofixation of β₂-transferrin.²² A high proportion of transferrin in CSF exists as a carbohydrate-free isoform (β₂-transferrin) that is not present in sweat or serous fluid. Detection of β₂-transferrin in such fluids is indicative of CSF leakage. Only a small sample (<1 mL) is required, and no special handling or refrigeration is required.

Pseudomeningoceles may present clinically with localized back pain and postural headaches. Localized nerve root entrapment or adhesions of roots to the dural edges of the pseudomeningocele can produce radicular symptoms.⁶ Symptoms may occur several weeks to months after surgery. The clinical syndrome in the lumbar region may mimic the symptoms of lumbar disc herniation. Cervical and thoracic pseudomeningoceles may be palpable as boggy masses. Lumbar pseudomeningoceles are usually not palpable on physical examination, but occasionally the collections track into the subcutaneous tissues, producing a noticeable swelling of the wound site.

Imaging Studies

MRI and CT will adequately localize the CSF fistula tract or pseudomeningocele.⁷ MRI is the study of choice because of its superior imaging of soft tissue compared with CT. To best define the fistula tract for operative planning, iopamidol is injected into the subarachnoid space, followed by CT scanning. Suspected pleural CSF fistula^23–25 or slow and intermittent leaks, such as those occurring after a lumbar puncture, are often best evaluated with radionuclide myelography.^23,26

Conservative Management