Management of Cerebrospinal Fluid Leaks

Conventions and Definitions

CSF leaks are classified based on the location of observed fluid, rather than the location of fistula, such as rhinorrhea (from the nose) and otorrhea (from the ear). Transcranial CSF leaks fall into two major categories: traumatic and spontaneous (nontraumatic) leaks. The traumatic group is subdivided into acute or delayed; acute leaks present within 1 week of injury and delayed leaks occur months or years later. The nontraumatic group is also subdivided, including leaks associated with intracranial mass lesions, congenital defects of the skull base, osteomyelitis, osteonecrosis (including other etiology of bony erosion), and focal cerebral atrophy. Bony dehiscence can lead to leaks associated with an ill-defined group of acquired hernias, encephaloceles, meningoceles, and meningeal diverticula. Furthermore, nontraumatic fistulas are divided into high pressure and low pressure. ^, There is a practicality to applying an analogous nosology to the traumatic group as well. Iatrogenic or postoperative leaks are usually included in the category of traumatic fistulas.

Spinal CSF leaks can be classified similarly. Most spinal CSF leaks are postoperative (i.e., traumatic). A number of rare congenital anomalies can give rise to meningopleural or meningoperitoneal fistulas. The distinction between high-pressure and low-pressure fistulas is particularly important in the management of spinal leaks. In children with spinal dysraphism or other anomalies, the leak may be the first expression of hydrocephalus or shunt failure. ^,

Causes and Epidemiology

Traumatic Leaks

The most common cause of CSF leak is head trauma, particularly skull base fractures. A CSF leak was detected in 2.8% of 1250 head injuries and in 11.5% of the basilar fractures studied by Brawley. In another study of 1077 skull fractures, including a particularly large proportion of high-speed road traffic accidents, 20.8% of 168 basilar skull fractures had an acute CSF leak. The association of incidence with high speed, although not rising to the level of a correlation, is certainly suggestive. The proportion is somewhat higher with transventricular penetration.

In childhood, the incidence of traumatic CSF leaks is far lower, at 1% or less of closed head injuries. This disparity may be caused by differences in fragility between the adult and the pediatric skull, as well as by the lack of development of the air sinuses in children. As a guideline, the frontal sinuses become visible between the 4th and 12th year, and they are always detected by the 15th year. These sinuses are often asymmetric until age 20 years. The ethmoids are present at birth, enlarge by age 3 years, and are fully formed by age 16 or 17 years. The cavity of the sphenoid sinus is usually recognizable by age 4 years and is fully developed by puberty. In the pediatric age range, the interpretation of sinus x-ray studies is often difficult because of small size, variations in development, and normal calcification and clouding.

Nontraumatic Leaks

The characterization of “nontraumatic” should be restricted to leaks explained neither by trauma nor by any other identifiable cause. Because there have been few, if any, collected series of nontraumatic leaks rigorously studied, there are insufficient data to extrapolate quantitative estimates of incidence or etiology. Nontraumatic leaks, however, are correlated with tumors and increased intracranial pressure (ICP). Anecdotal series suggest that pituitary tumors are the most common cause of spontaneous CSF leaks; increased ICP may also be present. Because of the structures eroded by sellar masses, such leaks generally present as rhinorrhea. ^, ^, Other presentations, including a serous otitis media, have also been reported. There is an intriguing report of pituitary hyperemia in the context of nontraumatic CSF leak masquerading as pituitary adenoma in three patients. After surgical repair of the leak, the magnetic resonance imaging (MRI) abnormalities, including an enlarged pituitary resembling pituitary tumor, normalize.

Postoperative Leaks

Large and potentially dangerous subgaleal CSF pseudomeningoceles were common before the modern era of neurosurgery. These represented the most visible manifestations of altered postoperative CSF flow, increases in intracranial pressure, or unrecognized or untreated hydrocephalus. Incisional fistulas also occurred. Consequently, postoperative incisional CSF leaks were understood as common neurosurgical complications. Attempts to prevent this complication were focused on careful dural closure, buttressing the suture line, multiple-layer incisional repair, and other techniques to reconstruct and reinforce the tissue planes. Particular attention was paid to the air sinuses after frontal and posterior fossa surgery.

Radical approaches to cerebellopontine angle lesions, tumors straddling the nasopharynx and the anterior and middle fossa, and other regions of the skull base appear to have increased the incidence of CSF leaks. However, this incidence of leaks has been addressed with improved technique, including waxing and plugging mastoid air cells as they are opened and placing a graft of adipose tissue in the opened porus acusticus. The use of endoscopy to inspect the craniectomy site for unsealed air sinuses has also been advocated. Several recent series demonstrate that the incidence can be reduced below 11%, and that much lower rates are achievable, but that the incidence of leak seems relatively consistent irrespective of surgical approach (e.g., posterior fossa, transmastoid, or middle fossa). ^, ^, It is noteworthy that rhinorrhea, a classic false localizing sign in this setting, may be the presenting sign in up to 50% of leaks.

In transsphenoidal approaches, postoperative leak rates for endonasal resection of tumors are estimated to range from 0% to 35%. Patient factors including body habitus, tumor type, tumor location, etc., can have an impact on this risk. In the largest series reported, the CSF leak rate was 16.75% for patients with intradural tumors. Those at highest risk were overweight patients and/or were harboring posterior fossa tumors.

Pneumocephalus

Intracranial air, a pathognomonic sign of CSF fistula after trauma or spontaneous rhinorrhea (although not pathognomonic after surgery), is demonstrable in approximately 20% of patients with CSF leaks. Pneumocephalus is posttraumatic in 75% of these patients and is spontaneous, or otherwise unexplained, in 10%.

Meningitis

Meningitis occurs in approximately 20% of acute post-traumatic leaks and in 57% of delayed leaks. The incidence of meningitis in nontraumatic CSF leaks has not been well documented. Anecdotally, copious, continuous leakage of the high-pressure type is less likely to be associated with meningitis than is intermittent leakage. The overall risk of meningitis associated with traumatic CSF leaks is approximately 25%. In neurosurgical postoperative leaks, the incidence of meningitis is roughly 20%, but this figure is complicated by the problem of distinguishing aseptic from bacterial meningitis, and by the complexity introduced by factors such as steroid administration, chronic illness, and immunosuppression that might impair wound healing, predisposing to both leaks and meningitis. The high incidence of delayed infection in military penetrating head injuries is demonstrably correlated to CSF fistulas, actively leaking or not. ^,

Defining and Localizing a Fistula

The management of CSF leaks involves three steps : (1) confirming that the leaking fluid is CSF, (2) delineating the site of the fistula, and (3) defining its mechanism.

Confirming the Presence of Cerebrospinal Fluid

Glucose

The presence of glucose in clear, leaking fluid has been historically used to differentiate CSF from nasal secretions and other sources of drainage. The concentration of glucose in CSF equals or exceeds 50% of the serum concentration, with notable exceptions, such as concomitant meningitis or following subarachnoid hemorrhage. The glucose concentration in nasal secretions, in contrast, is 10 mg/dL or less.

Quantitative measurements of glucose concentration are diagnostic. Qualitative spot tests, such as those provided by chemical testing strips (e.g., Clinistix, Dextrostix, Uristix, or Tes-Tape), are not definitive for two reasons. First, the glucose oxidase test on which they are based is highly sensitive, turning positive at values less than 20 mg/100 mL of glucose ; second, normal nasopharyngeal secretions often elicit false-positive reactions even in the absence of glucose. ^, Thus, although a negative glucose oxidase reaction effectively eliminates the possibility of CSF rhinorrhea, a positive result does not diagnose it unequivocally.

Reservoir Sign

It is widely held that true CSF leaks produce quantities of fluid sufficient for collection and quantitative analysis. The reservoir sign (the ability of a patient to produce CSF by positioning the head in a certain way) is generally quite specific for a fistula with pooling in the sphenoid sinus. Although Dandy believed that this sign would differentiate leakage through the frontal sinus from ethmoidal and sphenoidal leaks, it is not reliably localizing.

Target Sign

The target sign refers to the pseudochromatographic pattern produced by the differential diffusion of CSF mixed with blood or other serosanguinous fluid on filter paper or bedclothes. CSF migrates further, creating a bull’s-eye stain with blood in the center. This is a convenient but unreliable sign, because whenever watery nasal secretions and blood are mixed, the same phenomenon occurs.

Headache

CSF leaks can be accompanied by high-pressure or low-pressure headaches. Intermittent high-pressure leaks are characterized by high CSF pressure headaches that are relieved by the sudden discharge of fluid and that build up again over time. Normal-pressure leaks, in contrast, are characterized by postural low CSF pressure headaches, relieved by reclining or otherwise allowing pressure in the subarachnoid space to rise to normal levels.

Other Confirmatory Evidence

The finding of unusually low opening pressure in the lumbar subarachnoid space is corroborative evidence for CSF leak. Unilateral or bilateral anosmia is associated with defects or leaks in the region of the cribriform plate and the fovea ethmoidalis. Olfaction may be preserved, however, in cases of spontaneous CSF rhinorrhea with congenital defects of the cribriform fossa. ^, Optic nerve lesions point to the tuberculum sella, the sphenoid sinus, and the posterior ethmoids as the likely site of injury. Impaired vestibular function, facial nerve palsy, and cochlear damage accompany fractures in the temporal bone.

Imaging Techniques

Imaging techniques are used to detect intracranial air, fractures and defects in the skull base, mass lesions, and hydrocephalus, and to demonstrate flow through fistulas or skull defects. Plain films, multiplanar tomography, computed tomography (CT), and MRI have been used to delineate the anatomy and pathology of the skull base, sinuses, and calvaria. Radiographic data must be interpreted in the context of clinical findings. As always, positive data obtained from radiography are helpful, but negative data are often equivocal.

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