Dural Tears

Overview

In spinal surgery, the occurrence of dural tears, both incidental and nonincidental, is not uncommon. Dural tears are more common with certain spinal procedures, such as revision laminectomy surgery, and they can be encountered incidentally, as with burst fractures and lamina fractures. Durotomies are also necessary for the resection of intrathecal lesions and for tethered cord releases. Although an effort has been made to study the frequency with which dural tears occur, there does not seem to be a consensus. In one review of the records, based on operative reports of 641 patients who underwent a decompression of the lumbar spine, 88 (14%) sustained an operative dural tear. Other studies have observed dural tear rates of 5.3% for open diskectomies, 17.4% for reoperation of herniated disks, 7.6% during primary lumbar surgery, and 15.9% for revision cases. Higher incidences may be associated with larger procedures and revision procedures, in ankylosing spondylitis, and with advanced age; the risk for complications in spinal surgery, including dural tear, increases in older patients.

Although finding a durotomy intraoperatively can be a source of frustration and anxiety for the surgeon, it is extremely important to find and treat the tear efficiently. Fortunately, many effective techniques of treatment are available. In this chapter we will describe various methods for diagnosing dural tears, and we will discuss intraoperative and postoperative techniques available for treatment.

Anatomy Review

The dura mater is the outermost of the three layers of the meninges that surround the brain and spinal cord and confine cerebrospinal fluid (CSF). The Latin dura mater literally means “tough mother” or “hard mother,” and this layer is so named because of its leathery exterior, and because it serves to protect the other two meningeal layers, the pia mater and the arachnoid mater. The dura is more prone to longitudinal tears, because most of the internal fibers run in a longitudinal direction. The dura extends distally to the S2 segment, envelops the arachnoid mater, and forms a sac filled with CSF (the thecal sac). The spinal cord lies within the subarachnoid space with its terminal portion, the conus medullaris, at the L1–L2 level. CSF fills the space over the spinal cord and is reabsorbed into venous sinus blood via arachnoid granulations. About 500 mL of CSF are made per day, and CSF is turned over about three times daily; it flows from the lateral ventricles to the third and fourth ventricles, enters the basal cistern, and continues to the cortical and spinal subarachnoid spaces.

A dural tear can range in size from a tiny hole invisible to the naked eye to a large defect that requires dural regeneration or patching. Any maneuver performed near the dura puts it at risk for damage. For example, tears can transpire directly during both soft and sharp dissection of soft tissues or during removal of bony material. Most commonly tears occur during dissection with a Kerrison rongeur, but they may also occur as a result of adhesion of dural matter to removed bone, postoperative contact of the dura with a remaining spicule of bone from surgery, or erosion of the dura in chronic stenosis. Further, puncture of the thecal sac can occur during medical procedures other than surgery, such as epidural injection or myelography.

Often, a dural tear does not initially result in serious danger to the patient, but it can produce pain and discomfort until it is treated. Short-term symptoms of unrepaired dural tears may include a moderate to severe positional headache; nausea and vomiting; photophobia; and/or CSF drainage from the wound. There is also greater risk for deep infection. If left untreated or undiagnosed, long-term complications as a result of persistent dural tears and CSF leaks can occur, with the potential for significant morbidity. Specific complications include, but are not limited to, persistent CSF fistula, pseudomeningocele, nerve injury, arachnoiditis with subsequent chronic pain, and in rare cases meningitis. Also, intracranial bleeding and basilar herniation have been reported after dural tears as a result of the change in CSF pressure.

Clinical Diagnosis

Frequently, a dural tear is recognized intraoperatively as a visible leak of CSF at the durotomy site. It may be a gush of fluid, or it may be as subtle as blood getting washed off of the dural surface from a pinhole leak. Excessive, clear output through a chest tube or subfascial drain can also be a sign of durotomy. A collapsed thecal sac or excessive epidural bleeding indirectly implies a dural tear intraoperatively. If detected, the dural tear should be repaired by the attending surgeon at the time of surgery.

If not detected intraoperatively, imaging techniques can assist in diagnosing a dural tear. A magnetic resonance image (MRI), for example, can confirm a diagnosis of pseudomeningocele with the presumption of a dural tear. Conventionally, a T2-weighted MRI is used to detect extraarachnoid fluid collections. MRI is preferred as a noninvasive imaging technique, however, the interpenetration of fluid and solid compositions of the spine with MRI can make the detection of dural leaks difficult. It is also not feasible to directly visualize dural tears smaller than 1 cm using MRI. Further, epidural hematoma after a laminectomy procedure is quite common and may be impossible to differentiate from CSF.

In contrast, radionucleotide cisternography provides visualization of small leaks; radioactivity outside the subarachnoid space indicates a CSF leak. However, if there is no active leakage at the time of the test, or if the tear is smaller than the image resolution, identification of the leak via cisternography is impossible. Additionally, the injection is radioactive and therefore presents undue risk to the patient.

Computed tomographic (CT) myelography is another frequently used imaging technique that can identify multiple CSF leaks, is sensitive, and illustrates the relationship of bony structures to extradural CSF collections. In many practices, CT myelography is the modality of choice if a dural tear requires a definitive diagnosis. However, the invasive nature of an injection may result in pial irritation, and the x-rays present a risk of ionizing radiation. Complications may also arise from a new CSF leak at the injection site or from infections that can originate at the dural puncture.

Proteins are another option for confirming the existence of a dural tear: β-2 transferrin is a protein unique to CSF and inner-ear perilymph; and if in question, immunofixation electrophoresis is an option. The assay takes about 3 hours and requires one to two decontaminated drops of the fluid. Sensitivity of the assay is reported to be near 100%, and specificity is about 95%. The high concentration of another protein in CSF, β-trace protein, provides an alternative assay to β-2 transferrin. The β-trace protein assay takes about 20 minutes. A study of 176 samples found a sensitivity of 99% and a specificity of 100% for the β-trace protein assay. Unfortunately, many hospital labs do not offer these tests, and sample results may not be available for days.

Conservative and Nonsurgical Treatment of Dural Tears

Conservative treatment of a patient should begin with a thorough evaluation. If the patient is suspected to have a CSF leak but is asymptomatic and does not have CSF leakage from the wound, no treatment is immediately necessary, although the patient should continue to be monitored. Postural headache is a common symptom of a dural tear and is believed to occur both because of meningeal irritation and increased CSF leakage as the lumbar fluid pressure increases in the upright position. Thus bed rest is often prescribed after lumbar durotomy. Caffeine is a vasoconstrictor and is therefore often prescribed because of its ability to provide relief to patients with postural headaches. Acetazolamide, a carbonic anhydrase inhibitor, may be used to reduce the normal volume of CSF produced, theoretically allowing the tear to more effortlessly seal itself. Resewing the superficial skin sutures under local anesthetics and with antibiotic prophylaxis may seal the leak; however, this method has the potential to increase back pressure caused by the CSF. Alternatively, products such as Dermabond (Ethicon) can be used as a skin sealant.

Overall, the goals of the aforementioned solutions are to reduce CSF leakage, relieve pain, and enable the tear to resolve on its own. However, more substantial, nonsurgical alternatives should be considered if symptoms fail to resolve after conservative treatment. Daily or continuous CSF drainage through a closed subarachnoid catheter in a separate dural site may reduce subarachnoid pressure and facilitate healing of the tear. In a study of 107 patients, 94% reported that CSF fistula or pseudomeningocele were cured or prevented by a lumbar subarachnoid drain. Complications were found to occur that included an infection rate of 5% (meningitis, wound infection, and diskitis); CSF overdrainage with nonpermanent neurologic deterioration, headache, nausea, and vomiting in 3% of patients; and temporary nerve root irritation in 14% of patients.

Because of the risks of subarachnoid drain placement, some physicians recommend that subarachnoid drains should only be used if the leak is persistent and cannot be repaired operatively. When used, the lumbar drain is typically set at a height to encourage 10 to 15 mL/hour of drainage. Alternatively, the drain may be set at a height just below the level of the tear to ensure that low pressure is seen at the tear. For example, the drain can be opened at the cervicothoracic junction for cervical tears with the patient sitting in an inclined position. Overdrainage should be carefully guarded against, and frequent neurologic evaluations should be performed on any patient with a lumbar drain. Overdraining may mimic cerebral herniation, with altered mental status, cranial nerve deficits, respiratory anomalies, and papillary abnormalities. In such cases, the drain should be immediately clamped, and the patient should be placed in the Trendelenburg position. In the event of a lumbar tear, lumboperitoneal or ventriculoperitoneal shunting may be considered in refractory cases, however, the need for this is extremely rare.

An epidural blood patch offers another nonoperative solution. In this approach, blood is withdrawn from the antecubital vein and injected, typically under fluoroscopic or CT guidance, into the epidural space near the fistulous tract. Blood-clotting factors form a patch over any holes in the dura to prevent further leakage of CSF while enabling normal healing. Complications of epidural blood patch have been reported that include vertigo, dizziness, ataxia, and tinnitus during injection, as well as a temporary increase in temperature, mild backache and/or stiffness, and transient or residual paresthesia, especially in the legs and toes. In a study of 118 patients, epidural blood patches successfully relieved headache in 89% of patients with severe headache after lumbar puncture. Of the remaining patients, a second epidural blood patch was performed on 11 patients, and it was successful in 91% for an overall success rate of 97.5%.

A percutaneous fibrin glue injection is another nonoperative treatment for CSF leaks. A solution of cryoprecipitate is simultaneously injected with a calcium chloride and thrombin solution into the space overlying the CSF leak (Tisseel, Baxter Healthcare, Deerfield, IL; Fig. 73-1 ). Placement of the fibrin glue aggregate may be established using CT imaging. Using this procedure on six patients, a group of researchers successfully resolved CSF leaks in 50%; the three patients in whom leaks remained unresolved underwent surgery.

Intraoperative Surgical Repair

For tears found intraoperatively, or if nonoperative treatments are unsuccessful in postoperatively discovered tears, surgical treatment is advised. Techniques for surgical repair of dural tears vary based on the size, complexity, and accessibility of the tear. A simple, linear tear is typically repaired with a secure suture closure, using a running or an interrupted technique. A sealant may be added to reinforce the closure and eliminate leaks from the suture holes. A large tear can be patched with a dural substitute alone or in combination with sutures and/or a sealant ( Figs. 73-2 and 73-3 ). A complex, unapproachable tear, such as an anterior or lateral tear, can be repaired using an onlay graft with or without a sealant.

Surgical Methods and Materials

Suture

To suture the dura, small diameter, nonabsorbable sutures are typically used with a tapered needle. Suture materials include:

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Nurolon (Ethicon), a braided nylon
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Gore-Tex, a monofilament of expanded polytetrafluoroethylene (PTFE)
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Prolene (Ethicon), a monofilament of polypropylene
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Silk (Ethicon)

Some surgeons prefer Gore-Tex, because the needle hole is smaller than the suture itself, which decreases the risk of suture hole leaks. Others prefer silk ( Fig. 73-4 ), because fibrin and blood stick to the suture, which ultimately helps to seal the leaks. In a retrospective review, 338 dural tears were successfully repaired with silk sutures and a running, locking stitch; only 1.8% of patients developed a postoperative CSF leak that required surgical repair.

Prior to attempting repair using sutures, the nerves should be carefully protected and manipulated back into the intradural space. This can be facilitated by allowing some of the CSF to leak out, decreasing intradural pressure. Some surgeons also advocate for irrigation of any hematoma out of the intradural space prior to closure to reduce nerve irritation and possibly reduce arachnoiditis. However, one risk in the dural closure suture process is injury to the nerve root, because it can be difficult for the surgeon to distinguish between the nerve tissue and the dura mater. Therefore, use of an operating microscope is encouraged to minimize this risk. After sutures are placed, a procoagulant such as Floseal (Baxter; Fig. 73-5 ) or a patch such as DuraGen (Integra LifeSciences, Cincinnati, OH; Fig. 73-6 ) can be placed over the suture line to help seal the leak.