Arachnoid Cysts

Arachnoid cysts contain cerebrospinal fluid (CSF) and are most typically enclosed in arachnoid or arachnoid-like membranes and can be located anywhere within the craniospinal axis. With the advent and widespread use of magnetic resonance imaging (MRI), these lesions have been encountered with significant frequency. However, despite being well described throughout the literature, arachnoid cysts are actually rare, accounting for around 1% of intracranial space-occupying lesions. They exhibit a male predominance, at approximately a 3:1 ratio,^1,2 and a predilection for the sylvian region. Although these cysts are occasionally bilateral and multiloculated, they are usually unilateral and single.³

Diagnosis of arachnoid cyst can be made easily with currently available imaging modalities. Computed tomography (CT) and MRI are the gold standard for visualizing and making the diagnosis. Arachnoid cysts will appear markedly hypodense on CT scan (Fig. 7.1) and show low signal on T1 MRI and high signal intensity on T2 MRI (Fig. 7.2), as they are generally isointense with CSF. Diffusion MRI reveals a low signal secondary to high water diffusibility and high apparent diffusion coefficient (ADC). Arachnoid cysts display a smooth, well-demarcated surface and are heterogeneous and nonenhancing, distinguishing them from other cystic lesions such as epidermoids.⁴ In utero, arachnoid cysts may be visible via ultrasound.⁵

FIGURE 7.1 Computed tomography (CT) scan of a patent with a large suprasellar arachnoid cyst extending into the third ventricle and causing headaches, declining school performance, and progressively impaired memory. The patient underwent an endoscopic fenestration into the ventricles (Video 1) which led to complete symptom resolution.

FIGURE 7.2 T2-weighted magnetic resonance image showing an area of high signal intensity on the right temporal fossa and marked compression of the temporal lobe. Lesion is consistent with an arachnoid cyst.

The embryogenesis of arachnoid cysts remains controversial. Some authors report the arachnoid layer originating strictly from neural crest cells, but others propose the arachnoid originating from two layers: neural crest ectoderm and mesoderm. The primitive mesenchyme, or mesoderm, surrounding the neural tube separates into an endomeninx, which forms the pia-arachnoid membrane, and an ectomeninx.⁶ The subarachnoid space is then formed by the rupture of the rhombic roof and dissection of the ecto- and endomeninges by CSF pulse pressure from the choroid plexus. Any disruption of this separation is thought to be the initiating event in arachnoid cyst formation.^7,8 This disruption can occur anywhere along the neuraxis as evidenced by reports of spinal intradural and extradural arachnoid cysts.⁹ Congenital spinal arachnoid cysts have been mostly described in patients with neural tube defects.¹⁰ Posterior fossa arachnoid cysts have been specifically discussed in the literature along with Chiari malformations and Dandy-Walker syndrome as a possible result of embryonal atresia of the fourth ventricle.⁴ Two theories predominate in the literature with respect to the pathogenesis of middle fossa arachnoid cysts. Robinson’s theory proposes primary temporal lobe agenesis as the main factor in middle fossa arachnoid cyst formation. Starkman and co-workers propose the arachnoid cyst as the primary abnormality leading to eventual temporal lobe hypoplasia secondary to cyst expansion.¹¹ Both theories are supported equally well throughout the literature.

In 1831, Bright submitted what is considered the first pathological description of arachnoid cysts. He reported the cysts as malformations caused by a splitting of the arachnoid membrane.¹² More recently, Rengachary and Watanabe described the structural features of arachnoid cysts after review of several hundred cases: (1) splitting of the arachnoid membrane at the margin of the cyst; (2) thickened collagen layer in the cyst wall; (3) absence of normal arachnoid trabeculations within the cyst; and (4) hyperplastic arachnoid cells in the cyst wall.^6,13 On pathological review of optic nerve arachnoid cysts, authors report three common features of the cyst wall: meningothelial cell proliferation, thickened dura, and psammoma bodies.^14–16

Galassi and associates introduced a classification scheme for arachnoid cysts based on their communication with the adjacent cisterns. In this classification scheme, type I arachnoid cysts freely communicate with the cisterns. Type II cysts are intermediate and may or may not communicate with the cisterns but it is likely they communicated with the subarachnoid space at one time, before sealing off their communication.¹⁷ Type III cysts do not communicate with any region of the subarachnoid space and cause local mass effect.^2,18 This classification scheme also hints at a possible treatment guide, with type I cysts rarely needing surgical intervention, owing to the free communication with the subarachnoid space, and type III cysts more frequently requiring surgical intervention secondary to mass effect.

The most common location for arachnoid cysts is the middle fossa or sylvian fissure, usually behind the greater wing of the sphenoid bone¹⁹ accounting for nearly 50% of arachnoid cysts in one study. Posterior fossa cysts including cerebellopontine angle and the cerebellar vermis comprise 20% to 30% of lesions,²⁰ and supracellar cysts, 9%. Other documented locations include interventricular, optic nerve, cerebral convexity, and clival interpeduncular area arachnoid cysts.^12,21 Intraspinal arachnoid cysts are rare and mostly traumatic except in the cases of intramedullary cysts, which have been reported as truly congenital. Most intradural spinal arachnoid cysts occur in the thoracic region (80%) followed by the cervical (15%) and lumbar regions (5%).²²

Macrocephaly is one of the most common presenting signs of an arachnoid cyst in infants and can be diagnosed in utero. In older patients, cyst location correlates with presenting symptoms. Headache, secondary to increased intracranial pressure from mass effect, is usually the most common presenting symptom, frequently seen with middle fossa cysts. Middle fossa cysts are also often associated with post-traumatic subdural hemorrhages and may present with signs of mass effect from the hemorrhage.¹⁹ Suprasellar, pineal, and posterior fossa region arachnoid cysts present with signs of obstructive hydrocephalus. Suprasellar cysts in particular may present with precocious puberty, hyperinsulinism, and even visual loss.^23,24 These cysts are frequently symptomatic and rarely respond to surgical treatment, requiring initiation of long-term hormonal therapy regimens. In cases of optic nerve arachnoid cysts, patients may present with a childhood history of blindness in the affected eye with progressive complaints of proptosis, erythema, and pain.⁸ There are also several reports of patients presenting with cranial nerve palsies of the occulomotor, trigeminal, abducens, facial, vestibulocochlear, and hypoglossal nerves.^25–29 Patients with spinal arachnoid cysts may present with a constellation of symptoms if associated with a congenital syndrome. Others may present with back pain or, rarely, with progressive spastic paraparesis.³⁰

The natural history of arachnoid cysts is not yet clearly delineated given that most are found incidentally and remain static in size over time. Patients are frequently asymptomatic throughout their lives and many are found at autopsy. However, arachnoid cysts are often associated with other congenital disorders and in these cases the natural history may be related to the associated disorder. Glutaric aciduria type I (GAT1) is an inborn metabolic disorder that appears to have a strong association with bitemporal intracranial arachnoid cysts, the most consistent reported finding on all imaging modalities.⁷ Patients with these findings in combination with macrocephaly, psychomotor development, and dystonic cerebral palsy warrant a detailed metabolic workup. Recently, a case of bitemporal arachnoid cysts was also reported in a patient with tuberous sclerosis. Short-rib polydactyly syndromes and their variants, including Beemer-Langer syndrome, have been associated with arachnoid cysts, along with other genetic syndromes including cri-du-chat, autosomal dominant polycystic kidney disease, Aicardi syndrome, neurofibromatosis, and proteus syndrome. Further supporting a genetic basis of arachnoid cyst formation are reports of familial cysts. Familial arachnoid cysts usually occur in the same location in affected family members and have even been documented as mirror-image cerebellopontine-angle arachnoid cysts in monozygotic twins. An association with behavioral and cognitive disabilities has been recently documented and centers largely around attention-deficit hyperactivity disorder (ADHD), epileptic aphasia (Landau-Kleffner syndrome), and other developmental language disorders. Some authors report positron emission tomography (PET) studies revealing hypometabolism in cortical regions surrounding the cyst. After surgical decompression of the cysts, repeat PET studies demonstrated improvement in cortical metabolism and clinical performance on language testing.^19,31

Another area of controversy surrounds arachnoid cysts in middle fossa locations of epileptic patients. Many studies suggest only an incidental association, yet others report improvement in seizures following surgical treatment. There are several reports of epileptic foci over the region of the arachnoid cyst as confirmed by electroencephalography.³ However, other reports describe epileptic patients with temporal lobe arachnoid cysts and seizure onset localization far from the cyst,^32–34 varying treatment decisions on among cases. With regard to spinal arachnoid cysts, associations with congenital spinal malformations including caudal regression syndrome and acquired anomalies such as syringomyelia have been documented.³⁵

In incidental cases of arachnoid cysts, and those unrelated to other congenital anomalies, the size and dynamic status of the cyst appear to play a role in the natural history. Several theories have been proposed to explain the mechanism of arachnoid cyst expansion, the most common being the ball-valve theory. In this theory, an anatomical communication exists between the cyst and the subarachnoid space which acts as a unidirectional valve. Multiple reports of MRI studies have demonstrated this effect through cine-mode studies. However, this mechanism has not been observed in all arachnoid cysts and cannot explain the spontaneous resolution of cysts reported in the literature.^36–38 Another proposed mechanism is an osmotic gradient between the cyst and the surrounding CSF. This theory is not widely accepted in cases of congenital arachnoid cysts given that the cystic content is quite similar to the composition of CSF. This theory could be plausible in cases of traumatic arachnoid cyst formation, especially in the presence of hemorrhagic or inflammatory foci. A third theory, backed by clinical evidence, is that of fluid production by the cells of the cyst wall. There are many reports of isolated or closed compartment cysts that expand over time and, as discussed previously, the cyst wall is physiologically similar to the subdural and arachnoid granulation neurothelium.¹³

Another factor affecting the natural history of arachnoid cysts is related to location of the cyst and susceptibility to hemorrhage. Middle fossa arachnoid cysts can be complicated by post-traumatic subdural hemorrhages, regardless of the age of the patient.³⁹ The mechanism of the hemorrhage is likely secondary to the displacement at stretching of the bridging veins by the cyst as they extend from the cortical surface to the dura. As they stretch, and tear, hemorrhage accumulates mostly in the subdural space, and occasionally in the cyst itself.^19,40 Some authors cite annual increases in the risk of subdural hemorrhage by 20- to 40-fold in patients with arachnoid cysts.^19,41,42