32 Cerebrospinal Fluid Fistula in Skull Base Surgery

Cerebrospinal fluid (CSF) leak, or CSF fistula, is the direct passage of CSF from the subarachnoid space to lower pressure areas, often the upper respiratory tract or skin. It manifests most commonly as either CSF rhinorrhea or otorrhea, or as a leakage directly through a surgical incision.1–3

• A CSF leak may be classified according to its location or to the known or presumed cause.⁴ Clinical communication regarding CSF rhinorrhea should include the site as well as the presumed cause.⁴

Classification

By Location

• Clivus (rare)

• Temporal bones: from the posterior or middle fossa, via mastoid air cells, eustachian tube, or petrous apex through an extensive lateral pneumatization of the sphenoid sinus. In the pediatric population, three usual routes of fistula are the facial canal, the petromastoid canal, and Hyrtl’s fissure.⁵

• Anterior fossa: through the cribriform plate, ethmoid, sphenoid, or frontal bones

• Persistence of lateral craniopharyngeal canal or Sternberg canal^6,7: results from incomplete fusion of the greater wing of the sphenoid bone with the basisphenoid, causing a weak region of the skull base

• Along the path of the internal carotid artery⁵

• Percutaneously

• Rosenmüller’s fossa (pharyngeal recess): from Meckel’s cave through the medial part of the sphenoid bone in the nasopharynx⁸

• Unknown

By Cause

Surgical (iatrogenic): planned or unplanned (acute and delayed)—10%9

Nonsurgical: penetrating or nonpenetrating (acute and delayed)—80–90%⁹

• Nontraumatic

• High-pressure leak: tumor, hydrocephalus, idiopathic intracranial hypertension

Normal pressure leak: congenital (Gorham-Stout disease, dehiscence of the footplate of the stapes, Mondini dysplasia; see Box 32.1), tumors, bone necrosis following chemotherapy or radiotherapy, arachnoid granulations, infection (osteomyelitis), empty sella—10%⁹

Primary spontaneous rhinorrhea (PSR)—3–4%⁹

• Pseudo-CSF rhinorrhea: a surgical complication that mimics CSF rhinorrhea.¹⁰ It has been described in patients who underwent mobilization or resection of the ipsilateral petrous or cavernous sinus segments of the internal carotid artery (ICA), presenting with ipsilateral rhinorrhea, typically exacerbated by exertion, an increase in room temperature, or feeling an intense emotion. In such patients, lacrimation is typically absent ipsilaterally to rhinorrhea, and β₂-transferrin testing is negative. The pathophysiological explanation for this nasal hypersecretion is probably related to the parasympathomimetic state caused by the surgical interruption of the sympathetic innervation of the nasal cavity during surgery.¹⁰

Diagnosis

The diagnosis of a suspected CSF leak has two end points:

1. To demonstrate the presence of CSF

2. To identify the precise site of the leak

Appearance

• CSF is generally clear and colorless (“crystal-clear water”).

• When it is blood tinged, allowing it to drip onto linen will leave a ring of blood with a larger concentric ring of clear fluid (ring sign or halo sign).⁵

• Patients with CSF leaks may complain of a salty taste or even a sweet taste because CSF contains two-thirds the glucose content of blood.

• CSF leak may be induced by a Valsalva maneuver, compression of the neck veins, or a change in position because the sphenoid and frontal sinuses may act as reservoirs.11

Biochemical Analyses

• β₂-transferrin: a carbohydrate-free (desialated) isoform of transferrin that is almost exclusively found in the CSF. It is not present in blood, nasal mucus, tears, or mucosal discharge. It has been reported to have a sensitivity of nearly 100% and a specificity of about 95%.^12,13 False-positive and false-negative results can occur in patients with chronic liver disease, inborn errors of glycoprotein metabolism, or genetic variants of transferrin.¹¹ It is detected using protein electrophoresis.¹⁴

• Beta-trace protein: another CSF marker that is reported to have high predictive value.¹¹

• Glucostix test: a traditional method for detection of the presence of CSF. It is not recommended as a confirmatory test due to its lack of specificity and sensitivity.¹³ Normal CSF glucose is > 30 mg%, whereas secretions are < 5 mg%.⁵ Interpretation of the results is confounded by various factors such as contamination from glucose-containing fluid (tears, nasal mucus, blood in nasal mucus) or relatively low CSF glucose levels (meningitis).

Fluorescein Nasal Endoscopic Evaluation

When the biochemical examination of a fluid sample, nasal endoscopy, or imaging techniques give positive results that confirm and identify the leak, then surgical repair of the dural lesion is indicated.¹⁵ When these preliminary tests are unable to identify the leak, the fluorescein test follows in order to locate and to confirm or exclude a CSF leak.¹⁶ When the fistula is evident, the nasal endoscopic approach is successful in treating the fistula—also without the use of intrathecal fluorescein.

• Fluorescein can be administered suboccipitally into the cisterna magna (rarely used) or through a lumbar injection.

• It should be administered in a 5% concentration up to 1 mL (50 mg) of total dose. Fluorescein should be mixed with previously withdrawn CSF (ideal quantity of 9 mL) and slowly injected into the lumbar subarachnoid space.^16,17

• Applying a blue filter on the light source and a yellow filter on the endoscope lens is a useful technique to enhance fluorescein in very low flow fistulas. Fluorescein appears greenish.

• Rare complications including seizures, grand mal epilepsy, opisthotonos, and peripheral nerve palsy have been reported for doses > 100 mg.¹⁶

Radiological Investigations

• High-resolution computed tomography (CT): useful screening examination for the initial workup of CSF rhinorrhea or otorrhea. The study should include thin coronal cuts and can be performed with or without intravenous contrast. CT offers superb bony detail and can reveal bone defects and opacification of sinuses or air cells. The site of the leak is usually associated with abnormal enhancement of the brain.5,11

• Magnetic resonance imaging (MRI): reserved for characterizing underlying pathology (e.g., inflammatory tissue, meningoencephalocele, or tumor). MRI can demonstrate brain herniation into the ethmoid, sphenoid, or frontal sinuses.¹³

• Radionuclide cisternography: an old test that does not provide precise anatomic localization of CSF leaks but is very sensitive to small leaks. It requires that nasal pledgets be placed in the nasal cavity adjacent to each foramina before the test.

• CT or MR cisternography: With active leaks, cisternography demonstrates movement of contrast through the defect in 85% of the cases.¹¹ It is considered the procedure of choice when a patient is leaking clinically. If the patient is not leaking, there is no point in giving intrathecal contrast, as the test is almost always negative in the absence of active CSF leak.

Treatment

• Antibiotics: The role of antibiotic prophylaxis in patients with CSF leaks has been studied extensively, yet remains controversial.¹⁸ Some reports suggest that the use of antibiotic therapy in patients with a CSF leak may increase the risk of meningitis, rather than decrease it, via eradication of commensal organisms and colonization of pathogenic flora (that may be antibiotic resistant).¹⁹

• Acetazolamide: a carbonic anhydrase inhibitor and a sulfonamide derivative. Following an initial dose of acetazolamide, more than 99% of brain carbonic anhydrase activity is inhibited, thus decreasing CSF production by as much as 48%. Early acetazolamide administration as part of the conservative measures in patients with skull base fractures can be useful in preventing CSF leakage and shortening the duration of leakage in patients who already have either otorrhea or rhinorrhea. However, no decrease in the incidence of meningitis or in the number of patients needing surgical intervention to terminate the leakage have been demonstrated. Therefore, the drug is considered to be of limited value.1

Dosage: 25 mg/kg/day by mouth to be continued for 48 hours after cessation of the leakage.¹

Traumatic Cerebrospinal Fluid Leak

A CSF leak is a complication in 2% of all patients who have sustained a head injury and in 12 to 30% of all cases of basilar skull base fractures. Fractures involving the frontal or ethmoidal sinuses and longitudinal temporal bone fractures are most commonly associated with CSF leakage.¹⁸ Although penetrating head trauma is an obvious cause of CSF leak, most traumatic CSF leaks occur as a result of blunt trauma.

Most traumatic CSF leaks resolve spontaneously, with the majority resolving within the first 24 to 48 hours. The mechanism of CSF leak cessation is thought to involve blood products, inflammatory adhesions, or herniation of brain at the site of the dural breach and associated skull fracture. However, persistent fistulas do occur, particularly in patients with fractures involving the anterior cranial fossa. The rate of spontaneous cessation of traumatic CSF leaks has been reported to be 53 to 95%.^18,20 CSF leaks associated with temporal bone fractures have historically been thought to have a higher propensity to cease spontaneously.²¹ Delayed CSF leaks occur at an average time interval of 13 days and a range of up to 30 days.¹⁸

• Some authors have suggested that lumbar drainage may be a safe and effective treatment in patients with traumatic CSF leakage, but it entails the risk of cerebral transtentorial herniation, which must be assessed.²²

• Traditionally, anterior fossa CSF leaks are approached intracranially via a bifrontal craniotomy for both intradural and extradural approaches (see Chapter 14). After the craniotomy is performed, loose bone fragments are removed. The dural tear is identified and closed primarily, whenever possible. Modest resection of herniated brain tissue may be required. When the dural tear is extensive, an autologous graft, such as fascia lata or pericranium, is preferable to nonautologous material, given the contaminated nature of the field. Frequently, fibrin glue is used as a sealant, although its true efficacy in preventing CSF fistulas is unproven.¹⁸

• The endoscopic endonasal approach has largely replaced the traditional open craniotomy approaches as the primary mode of repair due to its minimal invasiveness and to the optimal results obtained (≥ 90% success in sealing the dural defect after the first endoscopic intervention and ≥ 97% success after the second endoscopic attempt).¹⁶ These outcomes have reduced the indications for conservative treatment, thus limiting the use of CSF drainage for acute posttraumatic fistulas. To optimize the endoscopic approach, the following steps must be taken preoperatively: define precisely the site and size of the fistula; determine the presence of multiple fistulas; and adequately prepare the area surrounding the gap in order to guarantee good adhesion of the graft.16

• For defects of the middle cranial fossa, an intracranial approach via a temporal craniotomy and primary dural closure, or an extracranial approach via a mastoidectomy, can be used. The extracranial approach offers the advantage of enabling extensive packing of the middle ear with fat and cartilage, but it may be contraindicated if hearing is to be preserved.¹⁸

Iatrogenic Cerebrospinal Fluid Leak

An iatrogenic CSF leak can occur in pituitary and posterior fossa surgery.

• Anterior skull base surgery: There has been a rapid evolution in the surgical approach to many anterior skull base pathologies. The endoscopic route is now a preferred option in many surgical centers when managing both benign and malignant disease. The majority of small defects (< 1 cm) in the skull base are reliably repaired using multilayered free grafts, with success rates surpassing 90% and minimal differences among the methods or material used.²³

Multilayer free graft reconstruction can be tailored according to the CSF leak grading systems, such as the one in Table 32.1.²⁴ Similar algorithms for skull base reconstruction have also been proposed by other authors.²⁵

Collagen seems to be effective as a dural substitute because it provides scaffolding for fibroblast ingrowth and stimulates the coagulation cascade and platelet aggregation.²⁴ Tissue glues do not stop CSF leaks, but they can help prevent migration of the repair construct.²⁶

For larger skull base defects (> 3 cm), materials used for free graft repairs have included cadaveric pericardium, acellular dermis, fascia lata, and titanium mesh.²³

With multilayer reconstruction, the estimated postoperative CSF leak rate is considered to be less than 5% for traditional transsphenoidal surgery (< 1-cm defects) and around 15.6% for the extended surgery (> 3-cm defects).

A nasoseptal flap has been proposed to reconstruct large dural defects.²⁷ Subsequently, other intranasal and regional vascular flaps were described and made available for skull base reconstruction. Options for skull base reconstructions includes avascular grafts, nasoseptal pedicled flaps, turbinate flaps, and novel endoscopic regional flaps²⁸(Table 32.2). See also Chapter 16.

A systematic review of the literature showed that vascular flaps decrease the CSF leak rate in large skull base defects from 15.6% to 6.7%.²³ Although the current literature suggests that skull base repair with vascularized tissue is associated with a lower rate of CSF leak compared with free tissue graft,^27,29 this study was limited by its search strategy. Selected trials included subjects of any age, with any comorbidity, and of varied duration of follow-up.²³

Table 32.1 Example of a Cerebrospinal Fluid (CSF) Leak Grading System in Endonasal Transsphenoidal Surgery

A CSF leak may occur following lateral skull base craniotomies for the excision of cerebellopontine angle (CPA) neoplasms or microvascular decompression (MVD). CSF leakage can occur in lateral skull base surgery, regardless of the approach used.30 In these cases, CSF leakage can present as leakage through the surgical wound or by the presence of rhinorrhea or otorrhea, as the CSF escapes into the eustachian tube to the nasopharynx and nasal cavity or from the mastoid cavity to the middle ear and external auditory canal, respectively.31

Table 32.2 Flaps Used for Dural Defect Repairs

• The CSF leak rates for the translabyrinthine (TL), suboccipital (SO), and middle fossa (MF) approaches have been found to be 12%, 12%, and 13%, respectively, with no significant difference in leak rates between the approaches.32 However, combined approaches have a significantly higher leak rate when compared with singular approaches.³² A wound leak is the most common CSF leak presentation following the TL approach (54%), but rhinorrhea is the most common CSF leak presentation following the SO (68%) and MF (70%) approaches. This can be explained by the meticulous obliteration of the eustachian tube and middle ear space with fat and muscle grafts during the TL approach. Also, routine MF and SO approaches require only bone waxing of the air cells and occasional use of muscle graft in the internal auditory canal (IAC). If an air cell tract persists at the end of the IAC or into the mastoid, then CSF has an unobstructed path into the middle ear space and the eustachian tube, resulting in rhinorrhea.³²

• Conservative treatment includes any combination of acetazolamide, elevation of the head of the bed, oversewing the wound, bed rest, or placement of a lumbar drain. Surgical treatment includes reexploration of the wound, mastoid obliteration, eustachian tube obliteration, or shunt placement.³² It is imperative to be more aggressive with CSF rhinorrhea when initial conservative management fails due to the decreased success when treating rhinorrhea conservatively after initial failure.³²

• Shunting should be considered as the primary treatment when the diagnosis of hydrocephalus has been established.³³

• In cases of persistent or recurrent CSF leak, a subtotal petrosectomy can be considered. This is accomplished by converting the mastoid and petrous elements of the temporal bone into a single cavity while the ossicles and bony external canal are sacrificed. The likelihood of subsequent ascending infection is diminished by occluding the eustachian tube, blind sacking the external meatus, and obliterating the temporal bone cavity. This procedure results in a conductive deafness. Other techniques may be considered if the hearing on the opposite side is also compromised.34 For persistent leaks, consideration should be given to the fact that the patient may in fact have increased intracranial pressure, and the leak is a manifestation of hydrocephalus. In this case, once the leak is stopped, typically with a lumbar drain at minimum, a ventriculoperitoneal shunt should be considered.

Primary Spontaneous Rhinorrhea

This is the term used for rhinorrhea without an identifiable cause.^4,35 A careful history, combined with endoscopic, radiological, and surgical examination will commonly result in the identification of a probable cause of the leak.³⁶

• Primary spontaneous rhinorrhea often occurs in middle aged or older women, with a body mass index (BMI) > 30 in most patient series.³⁷

• The so-called focal atrophy theory proposes that atrophy of the normal content of the cribriform plate or sella turcica results in reduced bone tissue bulk through which arachnoidal pouches exert a local erosive effect. However, this theory lacks anatomic studies to support it.³⁵

• The arachnoid granulation theory, which is supported by anatomic studies, proposed that skull base arachnoid granulations are not covered by endothelium but by an extension of the dura. This creates a closed system of CSF within the arachnoid granulation that transmits pulsations through its capsule to cause destruction of the surrounding bone.³⁸

• Primary spontaneous rhinorrhea may be due to an elevated intracranial pressure (ICP) and is a rare symptom of idiopathic intracranial hypertension (IIH).³⁷ As with PSR, IIH is a diagnosis of exclusion. High ICP is the most important sign in the diagnosis of IIH. Normal ICP values registered in PSR might be due to the fact that rhinorrhea may reduce the ICP due to CSF leakage. In one study, pressures obtained after repair of the skull base and resolution of the leakage were indeed significantly higher.³⁷

• Conservative management has little effect on PSR and may increase the risk of meningitis; thus, surgery should be performed as soon as the condition is diagnosed. Even though conservative treatment is sometimes effective, ICP could again become elevated, and the already weakened skull base may be destroyed again, thereby causing a recurrence. PSR is an indication for surgery and should be followed by postoperative control of high ICP.³⁷

Complications

The significance of a persistent CSF fistula is not the leak itself but its sequelae: posture-related headache, pneumocephalus, and, most significantly, bacterial meningitis. CSF rhinorrhea and otorrhea are independent predictors of post-traumatic meningitis.39 Additionally, they may lead to a significantly increased cost of hospitalization.³⁹ CSF leaks have been associated with about a 10% risk of developing meningitis per year.¹³

• Overall, between 7% and 30% of all patients with posttraumatic CSF leakage develop meningitis, and this rate increases as the duration of CSF leakage increases.^40,41Streptococcus pneumoniae is by far the most common pathogen.⁴²

Related posts:

Hi-Tech Tools in Skull Base Surgery Pediatric Skull Base Functional Anatomy of the Cranial Nerves Skull Base Infections Skull Base Reconstruction Techniques Transcranial Approaches to the Skull Base

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