41.2.1 Incidence and Epidemiology

In 1829, the French pathologist Cruveilhier was the first to offer a systematic description of a series of epidermoids, which he termed “pearly tumors.”1 Cruveilhier reported the incidental finding at autopsy of a large epidermoid tumor in a man who had died of a head injury. In describing this case, along with two cases previously reported by Dumeril and Le Prestre, Cruveilhier commented on the notable size and extent that epidermoid tumors may attain without producing symptoms.1 ^,​ 2 These lesions were known as “Cruveilhier’s pearly tumors” until Muller in 1838 first used the term cholesteatoma to describe them after noting the presence of cholesterol crystals within the matrix of the lesions.3 The prolific German pathologist Rudolf Virchow in 1855 concluded that pearly tumor was the more correct term to be used in the description of these lesions, for he found presence of cholesterol crystals to be inconsistent.4 Bostroem in 1897 coined the term epidermoids to describe these lesions, and this term is the most commonly used today.5

Epidermoids represent 0.2 to 1.5% of all intracranial tumors and 6 to 14% of all tumors of the cerebellopontine angle (CPA), where they are the third most common lesion, after only schwannomas and meningiomas.6 ^,​ 7 ^,​ 8 Approximately 30 to 60% of all epidermoids occur in the CPA, followed, in decreasing order of frequency, by the parasellar region, paraclival area, lateral recess of fourth ventricle, and petrous apex.6 ^,​ 9 ^,​ 10 ^,​ 11 ^,​ 12 ^,​ 13 In addition to the intracranial locations already listed, epidermoids can occur in the diploë of the calvarium (intraosseous), where they can present as a painless swelling with an associated calvarial defect.14 Intraosseous epidermoids represent up to 25% of cases in two larger series and are to be differentiated from the intracranial type of these lesions.15 ^,​ 16

The age of incidence of intracranial epidermoids is from birth to 80 years, with the majority identified by the third or fourth decades of life.6 ^,​ 17 A male preponderance has been reported, with a male:female ratio of 5:4.9 ^,​ 18 ^,​ 19 Multiple epidermoid tumors almost never occur, and no familial predisposition to the development of these lesions has been reported. In 2005, intracranial epidermoid cysts have been reported in three patients who had craniovertebral junction anomalies—the first known association of these lesions with other congenital malformations.20

41.2.2 Embryology

In 1854, Von Remak proposed that epidermoids occurred due to an error in embryological development leading to entrapment of keratinizing ectodermal elements, with subsequent growth of the aberrantly located tissue. The timing of this embryological event is postulated to have occurred at the time of closure of the neural groove, between the third and fifth week of development.21 ^,​ 22 ^,​ 23 ^,​ 24 Von Remak’s theory continues to be the most widely accepted theory for the formation of these lesions, though the timing of the embryological error and subtype (neuro vs. cutaneous ectoderm) of the entrapped tissue has been the source of debate through the decades.25 ^,​ 26 ^,​ 27 Other theories regarding the origin of epidermoids have been proposed, including Virchow’s theory of squamous metaplasia and Fleming and Botterel’s multipotential embryonic cell rest theory.4 ^,​ 28 More recent publications have proposed a modification of Fleming and Botterel’s theory whereby multipotential cells are carried from a medial to lateral position along with migrating otic capsular elements during embryogenesis.29 ^,​ 30 ^,​ 31 Kountakis demonstrated migratory properties of CPA epidermoid cells in vitro similar to those of acquired cholesteatomas and, because these properties are unique to epithelium of the first branchial groove, concluded that epidermoids originate from the first branchial groove.32 Though this theory does not rule out theories such as Virchow’s or Fleming and Botterel’s, it does indirectly support the concept that entrapment of would-be first branchial cleft epithelium during embryogenesis ultimately leads to epidermoid formation.

41.2.3 Pathology

Epidermoids’ characteristic appearance makes them easily recognized on gross inspection. The external surface is silky with a white-gold, mother-of-pearl appearance and sheen, often with multiple delicate vessels evident on the surface. The capsule can be lobulated or smooth and tears easily with application of a shearing force. The tumor is malleable and compresses easily owing to the caseous core of squamous debris. Section of the cyst reveals a capsule consisting of a thin layer of stratified, keratinizing squamous epithelium, often with multiple foci of calcification, surrounded by a thin layer of fibrous soft tissue. The cyst is filled with desquamated keratin debris and cholesterol crystals having a soft, waxy texture. The keratin debris within the core has a lamellar or onion-skin gross appearance and is essentially avascular (Fig. 41.1).6 ^,​ 9 ^,​ 33

Some epidermoids have a high triglyceride content in addition to the presence of cholesterol crystals. The growth rate of epidermoids is linear and to be differentiated from the exponential pattern seen with a neoplasm, either benign or malignant.24 It is only the basal layers of an epidermoid that undergo cell division, followed by progressive maturation and ultimately death of the overlying layers of the epithelium, which eventually slough into the central core of the lesion. With time the central, nonviable core represents the bulk of the epidermoid, with the viable cells displaced circumferentially toward the periphery of the lesion. Because of this growth pattern and the slow rate of expansion, epidermoids characteristically envelop surrounding nerves, vessels, and other critical structures rather than displacing them like most benign neoplasms. After filling the intracranial subarachnoid space from which it originated, the epidermoid then extends to adjacent spaces, eroding bone in the process.31 The capsule of the cyst typically insinuates itself into surrounding structures as it erodes the bone in the area, conforming to anatomical features in the area as it expands. In addition to engulfing cranial nerves and vessels in the area, epidermoids can cause atrophy, ischemic injury, and paresis due to the interaction with the cerebral and cerebellar parenchyma.36 This pattern of growth, combined with the relative friability of the epithelial layer, makes complete extirpation of these lesions difficult and, not infrequently, impossible without the sacrifice of the involved vessels and nerves, which is to be avoided.

41.2.4 Clinical Manifestations

Because of their slow rate and pattern of growth, epidermoids can be asymptomatic, found incidentally on imaging performed for other reasons. More commonly epidermoids will present with a long, protracted course of sometimes mild and vague clinical manifestations. Early series of patients who had epidermoids reported some who had duration of symptoms attributable to the lesion of as long as 53 years.34 Modern imaging has enabled earlier identification of these lesions, with some reports of duration of symptoms similar to those reported in series of acoustic neuromas.11 ^,​ 16 ^,​ 37 ^,​ 38 ^,​ s. Literatur Even so, some modern series report delays in diagnosis of 20 years or longer.11 ^,​ 13 ^,​ 30 ^,​ 37

Epidermoids present with a spectrum of symptoms similar to those seen with an acoustic neuroma or other expansile lesion of the CPA—namely, cranial nerve deficits, which occur in 80 to 90% of patients.30 ^,​ 31 Cranial nerve VIII is most commonly involved (40–93%) in most series of intracranial epidermoids and may manifest as hearing loss, vertigo, tinnitus, disequilibrium, or gait disturbance.16 ^,​ 30 ^,​ 31 ^,​ 37 ^,​ 39 ^,​ 40 ^,​ 41 ^,​ 42 Some have found the facial nerve to be more frequently affected on presentation.36 In contrast to the stretch injury to the facial nerve that can occur in cases of acoustic neuroma, epidermoids are thought to engulf the facial nerve and cause axonotmesis or neurotmesis due to the resulting ischemic injury, generally manifesting as facial weakness or hemifacial spasm.43 Trigeminal nerve involvement, producing symptoms of facial pain, numbness, corneal reflex abnormalities, and masticator muscle weakness, is more common on presentation (25–52%) in epidermoids than acoustic neuromas.13 ^,​ 16 ^,​ 30 ^,​ 31 ^,​ 37 ^,​ 39 ^,​ 41 The facial pain seen in cases of epidermoid tumors is atypical for trigeminal neuralgia in that it is longer in duration and may not be accompanied by sensory or motor dysfunction of cranial nerve V.44

Three mechanisms have been proposed to account for the trigeminal neuralgia seen with epidermoids: direct compression of the nerve root, indirect compression of the nerve due to vessel displacement from the enlarging cyst and toxic neuritis as a result of cyst content leakage.6 ^,​ 37 ^,​ 45 Larger epidermoids may involve cranial nerves III, IV, VI, IX, X, and XI, producing visual deficits, diplopia, hoarseness, and dysphagia. Headache occurs frequently with epidermoids and may be due to the expansile nature of the lesion (which may also produce papilledema secondary to increased intracranial pressure) or to a toxic effect from leakage of the cyst contents.30 ^,​ 31 ^,​ 36 ^,​ 37 Seizures due to epidermoids have been described but are an uncommon manifestation of the disease.46

41.2.5 Radiology

Please refer to Chapter 4 of this book for a detailed discussion of the imaging characteristics of epidermoid cysts of the skull base. Fig. 41.2 shows representative imaging of a right-sided CPA epidermoid.

41.2.6 Treatment

Epidermoids of the skull base are best treated using surgical excision. Chemotherapy has no role in the treatment of these lesions, for they are not true neoplasms. The use of external beam radiation therapy has been reported in one case of a recurrent epidermoid but should be reserved for symptomatic treatment of patients who are not surgical candidates, if any.47 In general, complete cyst removal should be pursued in all cases but, because of the infiltrative nature of these lesions, may not be possible without incurring significant morbidity and even mortality. Epidermoids engulf and become intimately involved with surrounding neurovascular structures, making complete excision difficult if not impossible without risking potentially devastating consequences from sacrifice of cranial nerves, vascular injury, or damage to the cerebellum, brainstem, or temporal lobe. Incomplete excision, leaving cyst matrix on cranial nerves and vasculature intimately involved with the lesion, is an acceptable alternative in some cases, for “recurrence may occur slowly and reoperation may not be required for many years.”7 ^,​ 37 ^,​ 44 The rates of total cyst excision vary considerably in published series, with rates as low as 0% and as high as 80 to 97%.11 ^,​ 30 ^,​ 31 ^,​ 36 Second operations are required in up to a third of patients, generally many years after initial resection.44

Multiple approaches have been employed to access and excise these lesions, including suboccipital, translabyrinthine, transotic, transcochlear, subtemporal, and petrosal routes.31 ^,​ 36 ^,​ 37 ^,​ 48 ^,​ 49 These approaches are beyond the scope of this chapter, but detailed descriptions of these approaches can be found in Chapter 18 (suboccipital), Chapters 18, 21, and 26 (translabyrinthine, transotic, transcochlear), Chapter 19 (subtemporal), and Chapters 18, 24, and 26 (petrosal).

41.2.7 Outcome and Prognosis

The rates of total cyst removal vary from 0 to 97% in modern published series, with rates of recurrence requiring a second operation ranging from 0 to 36%. Overall published recurrence rates range from 0 to 55% of patients, and the reported time to recurrence of epidermoids ranges from 36 to 264 months, highlighting the importance of long-term follow-up with periodic MRI surveillance in all patients who have epidermoids, regardless of whether total excision was achieved at the initial surgery.44 Mortality rates in modern series range from 0 to 16%, a significant improvement from the premicrosurgical era.44

Complications of surgery for epidermoids of the skull base are similar to those seen in the treatment of other CPA neoplasms, including cerebrospinal fluid (CSF) leak, cranial nerve injury, infection, aseptic meningitis, hydrocephalus, pulmonary embolus, headache, seizures, and aspiration.31 ^,​ 37 ^,​ 49 Cranial nerve VIII is the most frequently injured during excision of these lesions, though many series employ a translabyrinthine, possibly transcochlear approach to address these lesions, which may skew the interpretation of the rates of postoperative cranial nerve VIII dysfunction. It is difficult to assess the rates of such dysfunction postoperatively from published series owing to variability in the reporting of preexisting dysfunction, lack of mention of whether hearing preservation was attempted, and absence of clear descriptions of the nature of the postoperative dysfunction (vestibular vs. auditory, conductive vs. sensorineural hearing loss, means of auditory assessment). Nevertheless, reported rates of hearing preservation (as loosely defined) when attempted range from 15 to 72%.17 ^,​ 31 ^,​ 37 ^,​ 49 Similarly, interpretation of the rates of cranial nerve VII dysfunction postoperatively is difficult owing to variability in reporting but ranges from an 8 to 50% incidence of some level of facial nerve dysfunction.17 ^,​ 31 ^,​ 37 ^,​ 39 It is important to note that most series include patients who had some degree of preoperative facial nerve dysfunction or who had the facial nerve mobilized to enable complete cyst removal, both of which would clearly bias the reported rates of postoperative dysfunction.

In general, the rates of postoperative cranial nerve dysfunction after removal of intracranial epidermoids are highly variable and potentially subject to a number of factors, including age, coexistent medical problems, level of preoperative dysfunction, extent of nerve involvement by the epidermoid, meticulousness of cyst dissection and tissue handling, level of scrutiny in the evaluation of postoperative dysfunction, and administration of corticosteroids. Furthermore, the risk of injury to the facial and other involved cranial nerves is higher than for acoustic tumors of similar size because of epidermoids’ tendency to encircle the nerves.46 Intraoperative neurophysiological monitoring of involved cranial nerves, especially the facial nerve, is recommended.50

When assessing the outcomes of surgical interventions for epidermoids of the skull base, it is important to appreciate the natural history of these lesions with their associated morbidity and mortality. Reported complications associated with untreated intracranial epidermoids include hearing loss, facial weakness, facial numbness, lower cranial neuropathies, diplopia, blindness, headaches, cerebellar dysfunction, encephalitis, seizures, hydrocephalus, and death.31 ^,​ 36 ^,​ 46 Recurrent aseptic meningitis occurring spontaneously in the setting of an epidermoid has been described in a number of reports—in some as the initial presentation of the lesion.44 In addition, aseptic meningitis occurring after excision of epidermoid tumors has been reported in many series, thought to be secondary to a toxic effect of spilled keratin debris within the subarachnoid space.30 ^,​ 31 ^,​ 36 ^,​ 37 Evaluation and management of patients who have epidermoid cysts requires vigilance for the development of aseptic meningitis and initiation of treatment using corticosteroids, and possibly antibiotics, if suspected.

A rare but feared complication of intracranial epidermoids is the development of squamous cell carcinoma, whether a de novo lesion discovered at the first operation for epidermoid, incidentally at autopsy, or, more frequently, in the setting of an incompletely resected lesion.31 ^,​ 51 ^,​ 52 ^,​ 53 ^,​ 54 ^,​ 55 ^,​ 56 ^,​ 57 ^,​ 58 ^,​ 59 ^,​ 60 Most reports describe a delay of 3 months to 33 years before discovery of carcinoma in the setting of an incompletely excised lesion.22 ^,​ 53 ^,​ 61 ^,​ 62 ^,​ 63 ^,​ 64 ^,​ 65 ^,​ 66 ^,​ 67 There appeared to be a male predominance, and patients typically had rapidly progressive and more severe symptoms than seen in benign epidermoids. Some patients had an episode of aseptic meningitis prior to the development of carcinoma within the epidermoid, supporting the concept that chronic inflammation of the cyst matrix ultimately leads to malignant transformation, analogous to the pattern seen when carcinoma develops within a burn scar or area of chronic ulceration. Enhancement of a portion of an incompletely resected epidermoid tumor on CT or MRI, especially in the presence of atypical or severe symptoms, should signal the possibility of carcinoma.31 ^,​ 68 Improved clinical outcomes have been described with the use of adjuvant treatment with radiation.52 ^,​ 53 ^,​ 54 ^,​ 55 ^,​ 57 ^,​ 64 ^,​ 65 ^,​ 69 ^,​ 70 ^,​ 71 ^,​ 72 Asahi described an improvement in mean survival from 4 months to 15 months with the addition of external beam radiation therapy to surgical resection of carcinoma arising within an intracranial epidermoid.61 Tamura reviewed the use of stereotactic radiotherapy in the setting of carcinoma arising within an intracranial epidermoid and found median survival times of 1, 18, and 44 months with the use of surgery alone, surgery plus external beam radiation, and surgery plus stereotactic radiotherapy, respectively—differences that the authors found to be statistically significant.73 Murase stressed the importance of combination chemotherapy and radiation in the treatment of these lesions, though the benefit of adding chemotherapy in these cases remains unproven.61 ^,​ 69 Despite these measures, the prognosis overall for carcinoma in the setting of an epidermoid of the skull base remains poor.31 ^,​ 46 ^,​ 61

41.3.1 Incidence and Epidemiology

Dermoids, like epidermoids, are cysts lined by stratified, keratinizing squamous epithelium but differ from epidermoids in that they also contain mesodermal elements such as hair, sebaceous glands, sweat glands or, rarely, teeth, bone, or cartilage.44 ^,​ 74 ^,​ 75 Dermoids are to be differentiated from teratomas, which are true neoplasms that originate from a misplaced rest of embryonic germ cells which progress to form a tumor composed of well-differentiated tissue derivatives of all three germ layer in an organlike pattern.76 Like epidermoids, dermoids grow through the division of the outer lining of the cyst with progressive expansion of the core of the cyst as cellular debris accumulates. The growth pattern again is linear, not exponential as with a true neoplasm.30

Verratus in 1745 was the first to identify a dermoid on autopsy of a 40-year-old woman who died from a febrile illness.77 Masses of hair, in addition to abundant squamous debris, were found in pathological examination of the lesion. In 1860 Lannelogue and Achard reported the first posterior fossa dermoid occurring in a child.78 Six years later, Toynbee and Hinton described cystic lesions with “masses of hair” within the mastoid and middle ear space.79 ^,​ 80 The first reported attempt at surgical removal of an intracranial dermoid was by Horrax in 1922, but not until 1934 was a dermoid successfully removed from the posterior fossa, by Tytus and Pennybacker.81 ^,​ 82 Intracranial dermoids are rare lesions, accounting for 0.04 to 0.7% of all intracranial neoplasms, with incidence a quarter to half that of intracranial epidermoids.33 ^,​ 76 ^,​ 83 ^,​ 84 ^,​ 85 ^,​ 86 ^,​ 87 ^,​ 88 Dermoids can occur in many intracranial locations both intra- and extradurally and are thought to typically occur in more midline locations than epidermoids do.89 Above the tentorium, dermoids occur most frequently in the frontobasal, suprasellar, parasellar, cavernous sinus, and temporal regions.85 ^,​ 90 ^,​ 91 ^,​ 92 Infratentorial locations for dermoids include the occiput, CPA, and prepontine areas. Dermoids have also been reported within the temporal bone in the mastoid complex, tympanum, and petrous apex.93 ^,​ 94 ^,​ 95 Dermoids can occur in all age groups but are found more frequently in children than epidermoids are, especially when considering dermoids of the posterior fossa.96 In reviewing multiple case reports of posterior fossa dermoids, one study found a median age of 2 years (range 6 months to 27 years), with relatively equal sex distribution.89 Dermoids of the posterior fossa have been associated with Klippel-Feil syndrome in a number of reports.97 ^,​ 98 ^,​ 99 ^,​ 100 ^,​ 101 ^,​ 102 ^,​ 103 ^,​ 104 There are three case reports in the literature of dermoids in children who had Goldenhar syndrome, though this association is less clear.105 ^,​ 106 ^,​ 107 No familial predilection to the development of intracranial dermoids has been reported.

41.3.2 Embryology

Similar embryological events are thought to lead to the formation of dermoids and epidermoids (see previously). Some have postulated that dermoids form as a result of the aberrant adherence of primitive mesodermal cells to developing intracranial veins.108 The predilection of dermoids for a midline position has been theorized to occur when cutaneous ectoderm is drawn intracranially as the falx cerebri or tentorium are forming from the fusion of two leaflets of dura.89

41.3.3 Pathology

Dermoid cysts are similar in appearance to epidermoids but have a variable number of hairs or dermal appendages (sebaceous or sweat glands) present. Fat is often present within dermoid cysts in addition to desquamated debris. Teeth, bone, cartilage, salivary glands, nerves, and lymph nodes have been reported in dermoids but are rare.76 ^,​ 109 Dermoids have a variable growth rate and size on presentation, with some reports of large posterior fossa masses in children and others of small, limited tympanic lesions in adults.96 ^,​ 110 Dermoids, especially those occurring in the posterior fossa, can have an associated dermal sinus that can be complete or incomplete in its connection. In 1952, Logue and Till proposed a classification based on the position of the cyst relative to the dura and connection to an associated occipital dermal sinus: (1) extradural cyst with complete dermal sinus, (2) intradural cyst without an associated sinus, (3) intradural cyst with an incomplete dermal sinus, and (4) intradural dermoid cyst with a complete dermal sinus.111 In addition, dermoids may be either extra axial or intra axial in location, the most common intra-axial sites being the fourth ventricle or cerebellar vermis.89 Lunardi found that the mean age of children who had dermoid cyst of the posterior fossa with an associated dermal sinus was 2.4 years, whereas dermoids without a dermal sinus tended to be discovered in older children (age 9 on average), indicating that the presence of a dermal sinus will often lead to an earlier diagnosis of the intracranial component.96 In addition to intradural and extradural locations, dermoids have been classified as interdural when they are located between dural layers within the cavernous sinus, an uncommon location generally presenting with an oculomotor palsy.90 ^,​ 112 ^,​ 113

Rupture of dermoid and, less commonly, epidermoid cysts occurring spontaneously or during surgical removal, is a well-documented complication that can result in significant morbidity. The mechanism of spontaneous rupture of dermoid cysts is unclear, though Stendel hypothesized that glandular secretion under age-dependent hormonal changes leads to rapid enlargement with subsequent rupture and spillage of fatty contents in the subarachnoid space and ventricular system.76 Rupture of dermoid cysts after head trauma and during surgery have also been described.114 ^,​ 115 Aseptic meningitis is the most frequent complication that can result from rupture of an intracranial dermoid and may lead to cranial nerve fibrosis or obstructive hydrocephalus due to occlusion of the ventricular outlets.21 ^,​ 22 ^,​ 115 ^,​ 116 ^,​ 117 ^,​ 118 ^,​ 119 Rupture of intracranial dermoids can also cause an acute or delayed cerebral vasospasm with resultant ischemia, though the pathophysiology of this process is unclear.21 ^,​ 120 ^,​ 121 ^,​ 122 ^,​ 123 The presence of fat within the subarachnoid space and ventricular system following rupture of dermoids has been well described and has been known to persist for years (Fig. 41.3).21 ^,​ 118 ^,​ 124 ^,​ 125

41.3.4 Clinical Manifestations

Intracranial dermoids cause a spectrum of symptoms similar to those seen with epidermoids (see previously). Dermoids located in the cavernous sinus can cause paresis of cranial nerves III, IV, VI, V1, and V2. Ruptured intracranial dermoids most commonly present with headaches, seizures, and, when ischemia has occurred, sensory or motor defects.21 However, rupture of an intracranial epidermoid may occur without symptoms.75 ^,​ 124 Martinez-Lage and colleagues suggest that, at least in the case of dermoids of the posterior fossa, tumors reach a “critical size” of 3 cm at which most patients develop symptoms and because of this recommend removal prior to the lesion reaching this point.

41.3.5 Radiology

Please refer to Chapter 4 for a discussion of the imaging characteristics of dermoid cysts of the skull base. Fig. 41.3 shows representative imaging of a left middle and posterior fossa dermoid with evidence of rupture.

41.3.6 Treatment

Intracranial dermoid cysts are optimally treated with complete surgical removal, including the total removal of any associated dermal sinus, if present. Given the difficult anatomical relationships often encountered with these lesions, complete removal is sometimes impossible. When rupture of an intracranial dermoid has occurred, thorough irrigation of the subarachnoid space is recommended.21 ^,​ 76 The use of a corticosteroid containing solution to irrigate the operative field has been advocated as a way to prevent the development of aseptic meningitis when rupture of a dermoid or epidermoid cyst has occurred.31 ^,​ 39 ^,​ 40 ^,​ 41 ^,​ 115

41.3.7 Outcome and Prognosis

Published reports with follow-up of 6 to 22 years have shown freedom from recurrence following removal of dermoid cysts, including subtotal resection in one patient.82 ^,​ 96 ^,​ 126 Recurrence of incompletely resected dermoids has been reported three times in the literature, in contrast to subtotally resected epidermoids.16 ^,​ 33 ^,​ 45 ^,​ 85 ^,​ 89 ^,​ 127 Whether this reflects a lower propensity for recurrence with dermoids rather than simply a lower incidence of the disease is unclear. Periodic monitoring with contrast-enhanced MRI scanning after removal of dermoids is recommended.

41.4.1 Incidence and Epidemiology

Cholesterol granulomas, though the most common cystic lesion of the petrous apex, are rare, affecting approximately 0.6 per million people in the general population.128 ^,​ 129

41.4.2 Pathophysiology

Their etiology is debated, with two alternative theories of formation.128 ^,​ 129 ^,​ 130 One theory invokes the obstruction of the air cells of the petrous apex as the inciting event. This leads to resorption of the air and the creation of a vacuum into which blood is drawn and trapped. A second theory proposes that extensive pneumatization of the petrous apex leads to the exposure of bone marrow–filled spaces, which bleed into and obstruct the outflow tract. Although the etiology is debated, the crucial event is bleeding into the air cells and a reactive inflammatory response.

41.4.3 Pathology

The cyst has a fibrous capsule and contents that are viscous and motor oil–like and that contain cholesterol crystals. There is also evidence of chronic granulomatous inflammation with giant cell reaction.

41.4.4 Clinical Manifestations

Headache and dizziness are common symptoms, but many are incidentally identified on cranial imaging. Occasionally specific symptoms of facial paresthesia and/or pain, hearing loss, and facial weakness occur.

41.4.5 Radiology

CT scans reveal a non-enhancing cyst with smooth-edged erosion of bone and loss of internal bony structure. Cholesterol granulomas are typically hyperintense on both T1- and T2-weighted sequences. Their hypointensity on diffusion-weighted sequences further differentiates them from epidermoids (Fig. 41.4).

41.4.6 Treatment

The two management options that currently exist for cholesterol granuloma are observation and surgical drainage. Many incidentally identified cholesterol granulomas do not become symptomatic or grow. In a collaborative natural history study from Vanderbilt University and the Mayo Clinic–Rochester, 85.7% of patients had no reported symptom progression or radiographic growth over a median follow-up of 27.8 months (range 6.4–221.5 months). Of the 10 patients (14.3%) who experienced symptom progression or radiographic growth, 3 went on to surgical drainage and 7 continued with observation. In this study, patients who had large lesions (average 3 cm diameter) and multiple symptoms were more likely to be managed with surgery rather than observation.130

The goal of surgical management is to create an avenue of persistent drainage from the cyst into an air-containing region of the head. The most common procedures are an endonasal endoscopic drainage into the sphenoid sinus, or a transtemporal infralabyrinthine/infracochlear approach.130 ^,​ 131 ^,​ 132

The choice of approach requires a careful assessment of the availability of an adequate and safe surgical corridor for drainage. For endonasal approaches there must be space between the medial wall of the cyst and the lateral wall of the paraclival internal carotid artery; see case example 1 in Fig. 41.5. Lesions behind the internal carotid artery are best approached via the infralabyrinthine/infracochlear route; see case example 2 in Fig. 41.6.

41.4.7 Outcome and Prognosis

In well-selected patients the surgical outcomes are similar between approaches. Symptom improvement can be expected in 90% of patients. Cyst recurrence has an incidence of 7 to 13%. The need for stents to maintain the drainage pathway is an unresolved issue.

41.5.1 Incidence and Epidemiology

The first report of an intracranial arachnoid cyst was by Bright in 1831, describing them as “serous cysts forming in connection with the arachnoid.”133 Maunsell in 1899 first described an arachnoid cyst in the posterior fossa, but not until 1932 did Mullin report the first one such lesion in the CPA, followed by Aubry in 1937.134 ^,​ 135 ^,​ 136 Since then there have been many reports of arachnoid cysts in the posterior fossa.137 ^,​ 138 ^,​ 139 ^,​ 140 ^,​ 141 ^,​ 142 ^,​ 143 ^,​ 144 ^,​ 145 ^,​ 146 ^,​ 147 ^,​ 148

Arachnoid cysts represent 1% of all intracranial lesions and occur most commonly in the middle fossa at the sylvian fissure.149 ^,​ 150 The CPA is the second most common site for the development of arachnoid cysts. Temporal bone involvement with arachnoid cysts, though well described, is rare.145 ^,​ 148 ^,​ 149 ^,​ 151 ^,​ 152 ^,​ 153 ^,​ 154 ^,​ 155 ^,​ 156 ^,​ 157 There appears to be a male preponderance with arachnoid cysts, at least with regard to those involving the middle fossa.146 ^,​ 150 ^,​ 158 The mean age at surgery for arachnoid cysts in one review was 31 years, although 70% of patients who have arachnoid cysts become symptomatic in childhood and 60 to 90% of reported patients are children.145 ^,​ 146 ^,​ 159 ^,​ 160 ^,​ 161 This discrepancy is most likely a reflection of the vague symptoms that typify these lesions and the consequent delay in diagnosis and/or treatment that results from the often nonspecific symptoms. Sinha and colleagues have reported a left-sided predominance for arachnoid cysts.

Arachnoid cysts have been associated with congenital anomalies such as polycystic kidney disease as well as with developmental syndromes such as Kabuki, Goldenhar, Chudley-McCullough, trisomy-12, and Criduchat.162 ^,​ 163 ^,​ 164 ^,​ 165 ^,​ 166 ^,​ 167 In addition, bilateral temporal arachnoid cysts have been well documented in association with glutaric aciduria type I patients, in whom the increased catabolism associated with surgical interventions may produce devastating worsening of neurological status.168 ^,​ 169 ^,​ 170 ^,​ 171 ^,​ 172 Familial arachnoid cysts have been described by multiple groups, but the pattern of inheritance remains unclear.150 ^,​ 165 ^,​ 170 ^,​ 173 ^,​ 174 ^,​ 175

41.5.2 Embryology and Pathophysiology

Primary (or “true”) arachnoid cysts are congenital malformations thought to form at gestational week 15 when the roof of the fourth ventricle opens through the foramina of Luschka and Magendie into the cisterna magna.148 ^,​ 153 ^,​ 155 Secondary arachnoid cysts can result from trauma, neoplasia, infection, radiation, or hemorrhage and are thought to develop due to an adhesive arachnoiditis potentially caused by any of these insults.46 ^,​ 156 ^,​ 176 With regard to primary lesions, Starkman hypothesized that as the fourth ventricle opens into the cisterna, aberrant flow of CSF causes a splitting or duplication of the arachnoid membrane that, with further filling, results in the formation of an arachnoid cyst.177 Petrous apex arachnoid cysts are thought to arise from CSF pulsations through arachnoid granulations in areas of weakened dura overlying congenital bony dehiscences or irregularities in the petroclival fissure. With arachnoid cyst formation and continued, chronic pulsation further bone erosion of the petrous apex occurs, creating a smooth, scalloped defect.147 ^,​ 153 ^,​ 156

A similar but distinct entity is a CSF cephalocele of the petrous apex, which is a diverticulum of all of the layers of the meninges creating a similar bony defect (Fig. 41.7).178 Isaacson and colleagues hypothesized that a CSF cephalocele may result from increased intracranial pressure transmitting into Meckel’s cave via a patent porus trigeminus.148 In their series, three of four patients who had a CSF cephalocele were found to have an empty sella, a finding that others have associated with increased intracranial pressure.179 ^,​ 180 ^,​ 181

Three mechanisms have been proposed to explain the expansion of arachnoid cysts. One theory is that intracystic hemorrhage results in the establishment of an osmotic gradient, causing subsequent enlargement of the cyst due to fluid shifts. Another mechanism implicates active secretion of fluid by the internal lining of the cyst as the cause of expansion, a theory supported by the finding of Na⁺/K⁺-ATPase pumps in the cells lining the cyst.182 Further support for this theory was provided by the finding of ectopic choroid plexus within some arachnoid cysts.183 The most widely accepted theory regarding the expansion of arachnoid cysts describes a ball valve mechanism of CSF trapping within the cyst, driven by intermittent increases in intracranial pressure.151 ^,​ 184

41.5.3 Pathology

Histologically arachnoid cysts are CSF spaces surrounded by a thin membrane a few cell layers thick. The cyst lining resembles normal arachnoid tissue that has been split at its membrane and encloses the fluid cavity.176 The lining consists of pseudostratified epithelial cells with surface microvilli evident on electron microscopy. Rengachary and Watanabe described four histological findings to differentiate the wall of an arachnoid cyst from normal arachnoid: (1) splitting of the arachnoid membrane at the margin of the cyst, (2) a very thick layer of collagen in the cyst wall, (3) the absence of traversing trabecular processes within the cyst, and (4) the presence of hyperplastic arachnoid cells in the cyst wall, which presumably participate in collagen synthesis.185

41.5.4 Classification

Vaquero proposed a classification system for arachnoid cysts of the posterior fossa on anatomical location as follows: laterocerebellar, supracerebellar, retrocerebellar, clival, and mixed.186 Laterocerebellar cysts occupy the CPA and petrous apex and are generally smaller than those seen in other locations. Differentiation of arachnoid cysts in this area from epidermoids requires MRI scanning with fluid-attenuated inversion recovery (FLAIR) and diffusion-weighted image (DWI) sequences. Retrocerebellar cysts occupy the midline posterior to the cerebellum and can be quite large, with significant compression of the cerebellar parenchyma. Supracerebellar arachnoid cysts originate from the quadrigeminal cistern and extend posteriorly along the tentorium with a tendency to cause hydrocephalus. Clival cysts displace the brainstem posteriorly, can extend laterally to the CPA, and are rare.

41.5.5 Clinical Manifestations

Arachnoid cysts are highly variable in the spectrum and severity of the symptoms they cause, much like epidermoids and dermoids. Incidental, asymptomatic arachnoid cysts have been encountered commonly since the advent and widespread use of MRI and CT scanning in the setting of head trauma and as part of the work-up of other neurological complaints.187 ^,​ 188 The most frequent presentations of an arachnoid cyst are headache (sometimes associated with nausea and vomiting) and signs of cerebellar dysfunction, including dysmetria and gait disturbance.145 Arachnoid cysts can cause both communicating and noncommunicating hydrocephalus through compression of the ventricular system or blockage of CSF outflow, causing the typical spectrum of associated symptoms seen in hydrocephalus from other causes. Cysts in the CPA or petrous apex typically present with tinnitus, hearing loss, vertigo, and dizziness.148 ^,​ 151 ^,​ 189 ^,​ 190 ^,​ 191 Facial paralysis due to an arachnoid cyst has been described but is uncommon.157 ^,​ 192 ^,​ 193 ^,​ 194 Other unusual presentations include hemifacial spasm, trigeminal neuralgia, narcolepsy, seizures, visual loss, otalgia, meningitis, and otorrhea.148 ^,​ 151 ^,​ 160 ^,​ 195 ^,​ 196 ^,​ 197 ^,​ 201 A review by Jallo found that the time of onset of symptoms to diagnosis varied from 4 weeks to 12 months, reflecting the vague symptomatology on presentation of these lesions.148

41.5.6 Radiology

Please refer to Chapter 4 for a discussion of the imaging characteristics of arachnoid cysts of the skull base. Fig. 41.8 shows representative imaging of a right posterior fossa arachnoid cyst.

41.5.7 Treatment

In general, asymptomatic patients who have an arachnoid cyst do not require treatment and can be followed clinically and through periodic radiological examinations to monitor for growth or the development of symptoms.145 ^,​ 148 ^,​ 199 ^,​ 200 However, some authors argue that in the case of a large, asymptomatic lesion, the potential for rapid deterioration and even death from intracystic hemorrhage after minor head trauma is significant enough to justify operative intervention.201 ^,​ 202 ^,​ 203 Indications for surgery to treat arachnoid cysts include demonstrated growth, neural compression, hydrocephalus, and refractory symptoms referable to the lesion.145 ^,​ 149 Surgical treatment options include shunting (cystoperitoneal or ventriculoperitoneal for associated hydrocephalus), fenestration (either open or endoscopic), and microsurgical resection or marsupialization. Cystoperitoneal shunting has the advantage of low morbidity and mortality but leads to shunt dependency. Even so, many authors feel that cystoperitoneal shunting should be the initial surgical procedure for most arachnoid cysts, especially if hydrocephalus is present.139 ^,​ 159 ^,​ 186 ^,​ 204 ^,​ 205 ^,​ 206 ^,​ s. Literatur Ventriculoperitoneal shunting is recommended in cases of communicating hydrocephalus due to an arachnoid cyst but is not used as the sole treatment modality in the case of posterior fossa lesions, owing to the risk of upward tentorial herniation with progressive enlargement of the primary lesion.150 ^,​ 206 ^,​ 207 ^,​ 208

Open surgical interventions for arachnoid cysts include fenestration, marsupialization, and/or resection. Fenestration has been advocated by many surgeons, for it avoids shunt dependency, allows direct inspection and biopsy of the cyst, and seems to provide long-term results with few reported failures.145 ^,​ 159 ^,​ 161 ^,​ 186 ^,​ 205 ^,​ 209 However, some authors have reported that cyst fenestration alone does not reliably treat associated hydrocephalus, if present—presumably because there is blockage of the CSF flow in the subarachnoid space, perhaps because of the presence of blood and cellular debris. Accordingly, many patients who have had cyst fenestration still require ventriculoperitoneal (or lumboperitoneal) shunting to treat continued ventriculomegaly.209 ^,​ 210 In the treatment of CPA epidermoid cysts, some authors advocate resection of the medial, lateral, and posterior walls of the cyst with fenestration of the remaining portion. Advantages to this approach are that it provides optimal treatment by preventing subarachnoid blood and debris from causing future arachnoiditis and obstruction.145 ^,​ 148 ^,​ 208 With this approach, any cyst membrane that may be adherent to brainstem, cerebellum, major vessels, or cranial nerves is left in place. Arachnoid cysts of the petrous apex can be approached through retrosigmoid, middle fossa, or infracochlear approaches. The infracochlear approach provides decompression of petrous apex cysts only. In contrast, an extradural middle fossa approach has been advocated by many groups, because it allows for removal of cyst lining and obliteration of the defect to prevent future infection or CSF leakage in cases of both arachnoid cysts and CSF cephaloceles.148 ^,​ 178 ^,​ 211 ^,​ 212

41.5.8 Outcome and Prognosis

Ciricillo and colleagues reported on a series of 40 pediatric patients who had arachnoid cysts over a 10-year period.206 Five patients did not require intervention, but 15 underwent cyst fenestration, of whom 12 required shunting for improvement after a median follow-up of 8 years. The remaining 20 patients underwent shunting procedures, and 6 of them required subsequent shunt revision. Based on these results, the authors recommended shunting as the initial procedure for children who have arachnoid cysts. Levy reported on 39 patients who had middle fossa arachnoid cysts and who underwent cyst fenestration. Fifteen patients had either hydrocephalus or macrocephaly and required ventriculoperitoneal shunting in addition to fenestration.210 Raffel presented a series of 29 arachnoid cysts in children in the middle and posterior fossae. Fenestration alone was successful in 22, but 7 needed additional cystoperitoneal shunting.

Samii and colleagues reported a series of 12 patients who had CPA arachnoid cysts treated using cyst resection or maximal fenestration through a suboccipital approach. One patient had palsies of cranial nerves VII and VIII, but all 12 patients improved symptomatically with this approach, with no mortality by 3-year follow-up.208 Jallo used a similar approach in five pediatric patients who had CPA arachnoid cysts, including one who had failed previous cystoperitoneal shunting. All five patients were successfully treated using this approach, with a mean follow-up of 5 years, leading the authors to recommend microsurgical treatment for CPA arachnoid cysts as the optimal initial treatment.145