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Categorizing cerebrospinal fluid leaks: A framework for understanding
Introduction
Cerebrospinal fluid (CSF) rhinorrhea is the drainage of spinal fluid from the intracranial space through the nose via the paranasal sinuses. The condition was initially described in the second century by Galen of Pergamon (130–200 ad ), who identified the ventricles to be reservoirs for the animal spirits produced in the brain. He postulated that this liquid carrying the spirits was mixed with air entering the brain through the cribriform plate and would then undergo a process of purification. The purified parts were thought to enter the brain through pores, and the debris waste was supposed to leave the brain again through the pituitary body into the nose as “pituita.”
Based on Galen’s concept, free communication of fluid between the brain and the nose was considered a physiologic process for hundreds of years until the 17th century. The leaking of CSF through the nose after trauma was first described in the 17th century by a Dutch surgeon, Bidloo the elderly. In 1826, Charles Miller was the first to report on hydrocephalus in a child with an intermittent nasal discharge of CSF. In 1884, Chiari described an air-filled cavity in the frontal lobe and the ethmoid sinus during an autopsy in one of his patients and assumed that this was caused by the explosive force of sneezing. The term CSF rhinorrhea appeared for the first time in 1899 when St. Clair Thompson, of the Royal Ear Hospital, London, described a series of patients presenting with nasal CSF leaks of various causes. The importance of exclusion of the CSF space from the external environment was emphasized in the early 20th century. In 1929, Walter Dandy, an American neurosurgeon, published the first report on the surgical intracranial repair of a frontal sinus CSF leak using a muscle and fascia graft via a bifrontal craniotomy. The main advantages of this transfrontal–intracranial or –intradural approach are the preservation of olfaction; a good exposure; and the possibility to simultaneously treat dura, brain, and vascular injuries caused by complex fractures. Similar to the extradural approach, the main disadvantage of this procedure is the need to mobilize the frontal lobes. Although this approach is associated with significant morbidities, it remained the workhorse for CSF leak repair for several decades.
In 1948, the Swedish physician Gustav Dohlmann first published the closure of a lamina cribrosa CSF leak via a transfacial–extracranial approach. Fractures in the inferior wall of the posterior table with potential involvement of the anterior part of the ethmoid roof can be approached after incision in the medial corner of the eye. The disadvantages of this external procedure are sensitivity impairment or loss of sensation in the supply area of the supraorbital nerve, a cosmetically disturbing scar, and potential occlusion of the frontal drainage pathway caused by loss of the bony structure in the area of the frontal recess.
In general, open surgical repair via transfacial, subcranial, or transfrontal craniotomies is reserved for management of patients with extensive frontobasal fractures with large bony defects, open trauma with intracranial lesions, or severe disruption of the sinuses. Craniotomies are also indicated as salvage surgery when endoscopic repair techniques have failed or are contraindicated. Taken together, even though providing favorable access for larger defects, craniotomy approaches are associated with significant morbidity. Despite the 60% to 80% success rate, patients usually suffer from anosmia and may be at higher risk for hematoma and postoperative epilepsy caused by frontal lobe retraction during the craniotomy.
The Austrian neurosurgeon Oskar Hirsch (1952) described the first repair of a CSF leak in the sphenoid sinus using an endonasal transseptal approach. The era of contemporary endoscopic sinus surgery (ESS) began later in the 1970s with Messerklinger, who postulated that most of the sinus pathology originates at the level of the ostiomeatal unit and then affects the other sinuses. According to this concept, the clearance of the obstructive areas with mucosa preservation leads to restoration of drainage and ventilation through the natural ostia, resulting in resolution of the diseased sinuses. This revolutionary concept for treating the paranasal sinus disease was not published in the English literature until 1978. In 1985, David Kennedy and colleagues popularized ESS in the United States.
In the next decades, the development of a new suction–irrigation endoscope and refined instruments allowed the endonasal endoscopic treatment of small to medium-sized lesions of the anterior base of the skull with lower complication rates.
Based on this technical progress, the first endoscopic approach to repair an anterior skull base CSF leak was published in 1981. For the first time, disadvantages of the external approach such as scars, disruption of olfaction, and impaired sinus drainage could be circumvented by the minimally invasive endoscopic approach.
Over the past several decades, the gold standard for CSF leak repair, the open intracranial approach, has been replaced with minimally invasive endoscopic techniques. This is mainly a result of advances in endoscopic tools and techniques allowing otolaryngologists to manage CSF leaks endonasally. This, along with the high success rates of greater than 90% , and low morbidity and mortality have ensured that transnasal endoscopic closure has become the standard of care for the vast majority of skull base defects in the sinus and nasal cavities. This has been reinforced by the growing body of literature supporting endoscopic CSF leak closure techniques as the gold standard.
Classification of cerebrospinal fluid leaks
The first intent to classify CSF leaks was published in 1937 (Cairns) and proposed a classification of acute, delayed, traumatic, operative, and spontaneous causes. During the next several decades, a further categorization was undertaken, differentiating between primary spontaneous (idiopathic) CSF rhinorrhea and secondary spontaneous CSF rhinorrhea resulting from different ethiologies. The most commonly used classification, the Ommaya classification (1968), divides CSF rhinorrhea into traumatic and nontraumatic (spontaneous) etiologies.
In 1999, Har-El further modified Ommaya’s classification according to the site of the leak and also differentiated between nontraumatic with normal CSF pressure and nontraumatic with increased CSF pressure ( Fig. 1.1 ).
Nontraumatic, high-pressure CSF leaks are usually located in the thinnest bony area of the olfactory fossa, lateral wall of the sphenoid or the sellar region as a result of high CSF pressure from underlying conditions such as osteoporosis (Albers-Schönberg) or Crouzon’s disease. In this classification, nontraumatic normal pressure leaks can be caused by tumor invasion, infection (fungal, osteomyelitis, tuberculosis), congenital anomalies (meningoceles and meningoencephaloceles), or focal atrophy of the olfactory bulbs that exposes the vulnerable bone of the olfactory fossa to CSF pulsations. Traumatic causes are further divided into nonsurgical (accidental) trauma and surgical (iatrogenic) injuries. In the group of CSF leaks caused by surgery ( Fig. 1.2 ), a differentiation is made between unplanned injury during sinus surgery and planned dural defects with failed reconstruction attempts resulting in an ongoing leak. Planned and unplanned posttraumatic leaks can be categorized in immediate onset and delayed onset, the latter when rhinorrhea commences weeks to months after the injury.
Even though earlier studies state that accidental trauma is the most common reason for CSF rhinorrhea, newer data indicate that iatrogenic trauma after endoscopic sinus and skull base surgery (ESS) (46%) are a more common cause followed by spontaneous CSF leaks (∼40%), accidental trauma (8%), and congenital defects (4%).
We believe that it is useful to categorize CSF leaks into four categories depending on the cause ( Fig. 1.3 ).
Traumatic cerebrospinal fluid leaks
Closed anterior skull base fractures are the most common accidental cause of CSF leaks.
The onset of CSF rhinorrhea is usually within the first 48 hours of the trauma. Historically, these leaks are managed conservatively with bed rest and occasionally lumbar drains because the vast majority (50%–90%) close spontaneously.
Surgical management is indicated when CSF rhinorrhea persists for more than 10 days to reduce the risk of meningitis. Usually, a bone fragment that tents the edges of the torn dura is the reason for the ongoing leakage. In contrast, iatrogenic leaks are caused by displaced bone fragments and consequent prolapse of the torn dura. The fovea ethmoidalis–olfactory fossa, the fovea ethmoidalis, and sphenoid are the most common sites. If the posterior table of the frontal sinus ( Fig. 1.4 ) is affected by continuous leakage, the defect has to be addressed with a frontal drillout procedure, allowing a direct access to the site of the leak.
Spontaneous cerebrospinal fluid leaks
If no specific cause of CSF rhinorrhea can be determined, the CSF leak is classified as spontaneous. Recent data indicate that many patients with “spontaneous” leaks have increased intracranial pressure (ICP). It is hypothesized that intermittent leaks occur to release elevated ICP. Because increased ICP is a risk factor for successful repair, it is important to identify these patients. The highest incidence of spontaneous CSF rhinorrhea occurs in obese, middle-aged women. In these patients, the computed tomography (CT) scan often reveals skull base defects and associated meningoencephaloceles. Even though failure rates for endoscopic CSF leak repair are less than 10%, patients with spontaneous leaks have higher recurrence rates. Therefore, it is important to address an increased ICP to improve success rates in this subset of patients. Data suggest that medical therapy with acetazolamide in the postoperative period if CSF pressure exceeds 15 cm H 2 O and a ventriculoperitoneal shunt for pressures higher than 35 cm H 2 O can improve success rates (89%) of endoscopic repairs should they fail. An important consideration in these patients is to ensure that the closure is performed on the internal surface of the dura so that the higher CSF pressure pushes the repair onto the dural defect rather than an external repair where the high-pressure CSF pushes the repair away from the dura. The bath plug repair technique and underlay repair technique are examples of such an internal repair.
These high-pressure CSF leaks usually occur in the cribriform plate or the lateral wall of the sphenoid sinus, often in a very well pneumatized lateral sphenoid sinus recess. This thin bone can be eroded with prolapse of middle cranial fossa dura into the sphenoid with subsequent CSF leak.
Meningoencephaloceles and meningoceles with associated cerebrospinal fluid leaks
Whereas meningoencephaloceles are brain and dura protruding through a skull base defect into the nasal cavity or paranasal sinus, meningoceles are a dural prolapse alone. These are often associated with CSF leaks. They can result from trauma but can also be spontaneous (congenital or acquired). In meningoencephaloceles, the brain tissue is usually nonfunctional and can be reduced with bipolar cautery before closure of the defect. In cases of posttraumatic meningoencephaloceles, it is important to properly visualize the edges of the bony defect to guarantee successful repair as the defect can often be funnel shaped.
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