Surgical Management of Cysts: Intraventricular Cysts, Intraventricular Septations, and Extraventricular Arachnoid Cysts

Most of the intracranial cystic lesions are related to neoplasms, bacterial or parasitic infections, or loss of tissue due to malformation, infarction, or injury, including that resulting from surgical resection of brain tissue. These topics are discussed in other chapters of this book; however, an additional group of cystic intracranial lesions is encountered in neurosurgical practice, and in this chapter we discuss intraventricular cysts, intraventricular septations, and extraventricular arachnoid cysts. Of particular interest is that the management of these lesions continues to evolve with the development of endoscopic neurosurgical techniques.

Arachnoid Cysts

Arachnoid cysts are intra-arachnoid benign cystic lesions filled with cerebrospinal fluid (CSF). These lesions are probably developmental in origin and may become symptomatic either because of their progressive enlargement or because of hemorrhage into the cyst, though many are asymptomatic and incidentally found. The enlargement of arachnoid cysts has been discussed controversially and is at this point in time a matter for discussion. There are various hypotheses to explain the growth of arachnoid cysts:

1.
Active fluid secretion from the cyst wall ^,
2.
Fluid accumulation caused by an osmotic pressure gradient
3.
Pumping of CSF through a persistent communication between the cyst and the arachnoid space due to vascular pulsation
4.
The so-called slit-valve mechanism, which is described later ^, ^,

Arachnoid cysts occur throughout the neuraxis ( Fig. 84.1 ), and generally, no communication is demonstrable between the cyst and the subarachnoid space, although occasionally during surgery an arachnoid cyst is observed being filled through an apparent one-way valve. ^, The arachnoid cyst wall is histologically indistinguishable from normal arachnoidal membrane. Moderate thickening of the arachnoid and an increase in connective tissue are common ( Fig. 84.2 ). Arachnoid cysts may be asymptomatic throughout life, and rarely, they may spontaneously regress. However, if they do become symptomatic, the progression results from compression on the underlying brain, overlying bone, or both. There is debate as to whether space-occupying asymptomatic cysts should be operated on to prevent a hindrance to normal brain development and function, particularly in the pediatric population.

FIGURE 84.1, Typical localizations of intracranial arachnoid cysts. Supratentorial: sylvian fissure (1) , cerebral convexity (5) , sellar and suprasellar (7) , and interhemispheric fissure (8) . Optic nerve: intraorbital, intracranial. Tentorial: quadrigeminal plate (9) . Infratentorial: vermis (2) , cisterna magna (3) , cerebellopontine angle (4) , clival (6) , and posterior midline (10) .

FIGURE 84.2, Lining of subarachnoid cysts consisting of a single layer of arachnoid and adjacent loose subarachnoid network and psammoma body. H&E stain.

If it is unclear whether the cyst is leading to headaches, one should evaluate for papilledema. Additionally, for the most objective assessment, intracranial pressure (ICP) monitoring should be performed to prove ICP elevation or pathologic pressure waves ( Fig. 84.3 ). ^, The clinical symptoms resulting from these arachnoid cysts depend greatly on their location—whether over the sylvian fissure; over the cerebral convexity ( Fig. 84.4 ); in the interhemispheric region; in the sella and suprasellar region ( Figs. 84.5 and 84.6 ); around the optic nerve, the quadrigeminal plate, or the cerebellopontine angle; in the region of the clivus; over the cerebellar vermis or cerebellar hemisphere; or within the lateral or fourth ventricle ( Fig. 84.7 ). Arachnoid cysts have also been described extending across the region of the foramen magnum from the posterior cranial fossa into the upper cervical spine posterolateral to the spinal cord. The midline lesions often lead to an obstruction of the CSF flow and result in focal symptoms and raised ICP. There is a continuing discussion about whether intracranial arachnoid cysts are related to a specific seizure type and electroencephalographic focus.

FIGURE 84.3, Epidural measurements of intracranial pressure (ICP) in patients with arachnoid cysts. diastBP , Diastolic blood pressure; syst BP , systolic blood pressure.

FIGURE 84.4, Axial T2-weighted MRI demonstrating left sylvian fissure arachnoid cyst.

FIGURE 84.5, Anatomy of a suprasellar cyst. (A) Illustration of sagittal section of normal brain and sellar region, looking to the right. (B) The membrane of Lillequist has ballooned forward and backward, compressing the floor of the third ventricle to the level of the massa intermedia. DS , Dorsum sellae; LM , Lillequist’s membrane; M , mammillary body.

FIGURE 84.6, (A) Sagittal and axial T2-weighted MRI images demonstrating suprasellar arachnoid cyst. (B) Intraoperative endoscopic view of cyst call at the foramen of Monro. (C) Postoperative sagittal and axial T2-weighted MRI images demonstrating significant reduction of cyst size and some improvement of hydrocephalus. (D) Post-fenestration intraoperative view of pituitary stalk following the cystocisternostomy.

FIGURE 84.7, Axial (A) CT and (B) T2-weighted MRI demonstrating left hemispheric periventricular cyst. A right frontal endoscopic approach was used (C) to open the cyst and allow for communication with the left lateral ventricle. (D) Postoperative CT scan demonstrating the reduction of mass effect and cyst size.

In many cases, arachnoid cysts are incidental findings noted on computed tomography (CT) scanning or magnetic resonance imaging (MRI) of the head performed for a reason unrelated to the cyst. Such patients are informed about the radiographic finding, are provided with a copy of the study so they can present it to a physician at a later date, if necessary; and are followed up annually. In approximately 15% of middle fossa arachnoid cysts, an asymptomatic lesion may become symptomatic as a result of hemorrhage (often subdural) in association with the cyst and raised ICP ( Fig. 84.8 ). This event may occur after minor head trauma.

FIGURE 84.8, (A) Axial MRI demonstrating left hemispheric chronic subdural hematoma following head trauma, related to a left temporal arachnoid cyst (B). (C) Postoperative axial CT scan following evacuation of the subdural hematoma, but no intervention performed on the arachnoid cyst.

Cerebral convexity cysts occurring in adults present as seizures, headache, raised ICP, and, sometimes, marked reactive thickening of the overlying skull with erosion of the inner table. These cases can be managed by the wide excision of the membranes and the establishment of communication between the cyst interior and the CSF of the subarachnoid space. The same approach has been used in the treatment of symptomatic interhemispheric arachnoid cysts and cysts in the region of the quadrigeminal plate that produce aqueductal obstruction and resultant hydrocephalus. Cysts of the cerebellopontine angle may mimic other lesions in this location and may cause hearing loss and cerebellar signs. These cysts may present as intermittent downbeat nystagmus with an associated hydrocephalus or as vague symptoms, including hearing loss and disequilibrium, contralateral trigeminal neuralgia, or hemifacial spasm.

Surgical options for the management of symptomatic arachnoid cysts include endoscopic resection of the cyst wall with opening of the membranes, which establishes communication with the hemispheric or ventricular CSF pathways. Levy et al. described their results using the microsurgical keyhole approach for middle fossa arachnoid cysts, which can be performed with minimal morbidity. Compared with an endoscopic approach, better control of hemostasis can be obtained. The operative time and length of hospital stay were not excessively increased. Other options are stereotactic cyst aspiration, cystoperitoneal shunt drainage or drainage of the lesion through a burr hole, craniotomy with resection or marsupialization of the cyst walls, and craniotomy and ventriculostomy of the cyst. ^, In a cooperative European study of the management of arachnoid cysts in children, total excision or marsupialization emerged as the first-choice surgical procedure, and shunting procedures were often applied to cysts located in deeper locations. Among the 285 patients, from birth to 15 years of age, there was a resultant reduction of the size of the cyst in approximately two-thirds of the cases, and in 18%, the cyst had disappeared completely on follow-up CT scanning. Another study of the relative merits of different approaches to the management of arachnoid cysts in children is based on an analysis of 40 children treated between 1978 and 1989 at the University of California, San Francisco. Of 15 patients with cysts that were treated initially by fenestration alone, 67% showed no clinical or radiographic improvement and subsequently required cyst-peritoneal or ventriculoperitoneal shunting. All of these patients improved postoperatively, although shunt revision was required in approximately one-third of cases as a result of the recurrence of a cyst. These authors concluded that, irrespective of the location of the lesion, cystoperitoneal or cysto-ventriculoperitoneal shunting is the treatment of choice. However, there is often considerable risk of overdrainage.

Several groups reported about their results in neuroendoscopic treatment of arachnoid cysts. In a prospective study, Schroeder and coworkers treated seven consecutive patients with symptomatic arachnoid cysts in different locations endoscopically. The authors performed cystocisternostomies and ventriculocystostomies via burr holes with the aid of a universal neuroendoscopic system. Symptoms were relieved in five patients and improved in one patient, whereas the size of the cyst decreased in six patients. Although the follow-up period was short (15 to 30 months), the authors recommended neuroendoscopic treatment as the first therapy of choice. Hopf et al. evaluated 24 patients with intracranial arachnoid cysts that were treated endoscopically. Their surgical strategy was to create broad communication between the cyst and the subarachnoid space. Various techniques were used: endoscopic fenestration (10 cases), endoscopic controlled microsurgery (5 cases), and endoscopy-assisted microsurgery (9 cases). In all patients, sufficient fenestration of the cysts could be achieved, with a favorable outcome in 17 patients. Operative complications included infection (three patients), bleeding into the cyst (one patient), and subdural fluid collections (four patients). However, the complication rate in this series was quite high in comparison with other reported series that demonstrated improved safety of different cyst fenestration techniques. The authors concluded that different endoscopic techniques do provide sufficient treatment of selected arachnoid cysts. Recent advances in neurosurgical techniques and neuroendoscopy continue to favor cyst fenestration over shunt insertion as the method of choice for initial cyst decompression.

Neuroendoscopic Instrumentation and Operative Technique in Treatment of Arachnoid Cysts

Endoscopes

Various rigid and flexible endoscopes are available to perform cystostomy, cystoventriculostomy, or cystocisternoventriculostomy ( Figs. 84.9 and 84.10 ). The advantages of rigid-lens scopes are the brilliant and bright pictorial quality, and angled rigid scopes are used together with microscopes and neuronavigational devices in endoscopy-assisted microsurgery ( Fig. 84.11 ). Flexible neuroscopes have the advantages of steerability and maneuverability, which make inspection and interventions on multifocal and multiseptated cystic lesions easier.