Arachnoid Cysts

Etiology

The etiology of formation is uncertain, but arachnoid cysts, in theory, are predominantly formed due to abnormal splitting during embryogenesis of the arachnoid. Furthermore, histopathology demonstrating the presence of inflammatory cells, excess collagen, or hemosiderin staining suggests an inflammatory or traumatic etiology. Underlying gliosis is not usually found in the adjacent brain secondary to mass effect. Underlying syndromes associated with arachnoid cysts include:

• Aicardi syndrome [9]
• Mucopolysaccharidosis [10][11]
• Acrocallosal syndrome [12]
• Marfan syndrome [13]
• Missense mutation (c.2576C>T) of the arginine-glutamic acid dipeptide repeats gene (RERE) [14]
• Chudley-Mullough syndrome [15]
• Glutaric aciduria type 1 associated with bilateral arachnoid cysts

Intracranial Arachnoid Cyst Classification

Arachnoid cysts can be classified according to location.[16]

• Supratentorial
  • Sylvian
  • Suprasellar
  • Cerebral convexity
  • Interhemispheric
  • Intraventricular
• Infratentorial
  • Cerebellopontine angle
  • Retrocerebellar
  • Intraventricular
• Supratentorial or infratentorial
  • Quadrigeminal

The Galassi classification is another system that categorizes arachnoid cysts primarily based on location and size.[17]

• Type I cysts
  • Small
  • Usually asymptomatic
  • Located in the anterior, middle cranial fossa
• Type II cysts
  • Located superiorly along the Sylvian fissure
  • Displace the temporal lobe
• Type III cysts
  • Large
  • Take up the entire middle cranial fossa
  • Displaces the temporal, parietal, and frontal lobes

Epidemiology

The age range of the presentation of arachnoid cysts is extensive and spans from newborns to older individuals, though most arachnoid cysts are found in the first decade of life. Males tend to be affected more than females, an approximately 2 to 1 ratio, but the rate varies.[2][3][4] The prevalence is estimated at 2.6% in children and .2% to 1.7% in adults.[18]

Pathophysiology

Several theories have been proposed on the pathogenesis of arachnoid cysts. Starkman et al suggested a cerebral spinal fluid (CSF) flow abnormality as an underlying cause. An arachnoid cyst is hypothesized to be formed due to an irregular flow of CSF in between the embryonic meninges when the CSF pulse pressure dissects the tissue to generate the subarachnoid space, pia, and arachnoid. As these layers close abnormally, CSF is trapped.[6] Robinson et al proposed primary agenesis of the temporal lobe; however, this is contradicted by the fact that the temporal lobe often expands in size after adequate decompression of the arachnoid cyst implication lobar compression rather than agenesis.[19] Other authors theorized an in-utero insult may cause arachnoid cysts to occur. Features suggestive of an insult are evidenced by the electron microscopic features, including hemosiderin and inflammatory cells, thickened collagen layer, and hyperplastic arachnoid cells.[20]

The location of arachnoid cysts may also be an etiologic factor. Suprasellar arachnoid cysts, for instance, may originate from the splitting of the arachnoid and herniation via sella, while intrasellar arachnoid cysts may occur due to the herniation of the arachnoid through a defect in the diaphragma sella which subsequently seals off. Intraventricular arachnoid cysts may arise from arachnoid cells resting within the inward growth of the choroidal fissure. Additionally, most arachnoid cysts are sporadic, and significant genetic abnormalities have not been identified in these cases. However, some syndromes can be associated with the development of these cysts. (Refer to the Etiologies section for more information on syndromes associated with arachnoid cysts).

Causes of Arachnoid Cyst Expansion

Several hypotheses have been proposed to explain the growth of arachnoid cysts. One proposed cause is the presence of a one-way valve through which CSF can enter from the adjacent subarachnoid space into the cyst during systole, which results in cyst expansion. Other possible explanations include the difference in the osmotic gradient between the CSF and the arachnoid cyst fluid, which is a driving factor for the cyst expansion, or that there is secretion of CSF within the arachnoid cyst.[21]

Histopathology

The arachnoid cyst consists of a thin membrane. Histologically, the wall is variable but predominantly composed of arachnoid. There has also been found within arachnoid cysts fibrous tissue with a simple epithelial lining and nonarachnoid luminal epithelia with plentiful microvilli or cilia.[22][23]

History and Physical

A wide range in the percentages of symptomatic versus asymptomatic cysts has been reported, and this reported rate depends greatly on the population used. The location of arachnoid cysts is quite variable, with a clear dominance of lesions consistently found in the middle cranial fossa and the retrocerebellar locations. A smaller number of cysts are present at the convexity and in the suprasellar region. The lowest frequency of lesions is found in the cerebellopontine angle, intraventricular space, supracerebellar cistern, quadrigeminal cistern, ambient cistern, anterior fossa, and brainstem. A significant variation is reported in the incidence of the less frequent arachnoid cyst locations. For instance, Al-Holou et al reported an incidence of 0.3% intraventricular cysts in children and no intraventricular cysts in adults, while Hall et al reported 12% of cysts as intraventricular in children.[2][3][4]

Most commonly, arachnoid cysts are detected incidentally. Occasionally, arachnoid cysts are identified in patients referred for imaging due to symptoms. Fortunately, the most common locations for cysts in the middle cranial fossa and retrocerebellar region are not associated with symptoms. Cysts in less common locations are more likely to be symptomatic. In unusual cases, arachnoid cysts may cause symptoms secondary to mass effect or due to cyst rupture. Arachnoid cysts are discovered related to many different presentations since they are usually incidental and not associated with the reason the patient is being imaged. Arachnoid cysts have been found during imaging work-up for headache, seizure, increased intracranial pressure, hydrocephalus, suspected stroke, hemiparesis, nausea and vomiting, dizziness, cranial nerve dysfunctions, tinnitus, vertigo, developmental delay or regression, behavior concerns, visual symptoms, and trauma.[2][3][4] Even in patients who present with a headache, a detected arachnoid cyst is most likely an unrelated incidental finding. However, careful evaluation of each case is still required as arachnoid cysts can be very large and cause headaches and seizures in some patients.[24] Larger cysts are more likely to be symptomatic than small cysts, and larger cysts are more likely to undergo an operation.

Arachnoid cysts are rarely the cause of significant neurological symptoms such as hydrocephalus, ataxia, or cranial nerve impingement. However, given the relatively common incidence and the many locations where arachnoid cysts may occur, an extensive array of potential symptoms, including vision loss, nausea or vomiting, macrocephaly, paralysis of the oculomotor, trochlear, abducens nerves, trigeminal neuropathy, hemifacial spasm, sensory neuronal hearing loss, facial palsy, vagus nerve palsy, vertigo, and eighth cranial neuropathy, depending on the cyst location.[25][26][27][28][29] A quadrigeminal plate cistern arachnoid cyst could compress the trochlear nerves and quadrigeminal plate and result in trochlear nerve dysfunction.[30] An arachnoid cyst impinging the optic nerve could cause a deficit in the ipsilateral visual field in the eye of origin, a cyst in the suprasellar cistern could cause bitemporal hemianopsia, and a cyst affecting the occipital cortex would cause homonymous hemianopsia. Rarely, temporal arachnoid cysts can present with learning difficulties and attention deficit hyperactivity disorder.[31]

Arachnoid cysts may cause atypical symptoms also. Bobble-head doll syndrome, a rhythmic movement of the head, has been reported due to third ventricular dilation and potentially cerebellar involvement.[32][33] Depression that resolves with the marsupialization of an arachnoid has also occurred in some patients. The presentation of an arachnoid cyst causing depression is quite rare but plausible given the location and marked mass effect the cyst had on the brain.[34] The rupture of an arachnoid cyst resulting in a subdural hygroma has rarely been observed. The rupture can be due to trauma or can be spontaneous. Usually, the subdural hygroma can be followed without treatment, but surgical intervention is occasionally required. Furthermore, the traumatic rupture of an arachnoid cyst rarely results in hemorrhage or a subdural hematoma.[35][36][37]

Spinal arachnoid cysts are rare, occurring throughout the spinal column but most commonly in the thoracic region.[38] The most common symptoms are backache and fluctuating leg weakness. Correlations with monoparesis, radicular pain, spastic quadriparesis, neurogenic claudication, sensory disturbance, monoplegia, incontinence, pain, paresthesia, and neurogenic bladder have been reported. More commonly, the cysts will be located posteriorly but can also be located anteriorly.[38][39]

Evaluation

Computed tomography (CT) imaging is often enough to diagnose most fluid-filled, thin-walled arachnoid cysts. (see Image. Arachnoid Cyst). When additional information is needed, magnetic resonance imaging (MRI) is the modality of choice for evaluating the anatomical location, size, and adjacent structures. MRI can also help exclude differential diagnoses. The arachnoid cyst follows cerebrospinal fluid (CSF) on all pulse sequences unless impacted by a traumatic injury, and blood products are present within, although hemorrhage within an arachnoid cyst is rare.

Because arachnoid cyst walls are thin, special pulse sequences can be employed to visualize the wall. On magnetic resonance imaging (MRI), the arachnoid cyst follows the signal intensity of CSF on T1, T2 fast spin-echo (FSE), or T2 spin-echo (SE), T2-fluid attenuated inversion recovery (FLAIR), and diffusion. The apparent diffusion coefficient (ADC) maps created from the diffusion data demonstrate that the cyst has high ADC values similar to CSF. Due to CSF pulsations, mild signal differences between the fluid in the arachnoid cyst and the normal CSF on MRI sequences can exist. Arachnoid cysts do not enhance.

Additional diagnostic studies, such as CT and MRI cisternography, can be performed to confirm a diagnosis and further evaluate an arachnoid cyst. In CT cisternography, a neuro-safe nonionic iodinated contrast is injected intrathecally, and then dynamic CT images of the arachnoid cyst are obtained. The amount of filling and the time it takes to fill the cyst can be utilized to guide management. Cysts that fill completely and early are considered to be freely communicating and may not need to be treated.[7] MRI cisternography can be performed without contrast or with contrast. Intrathecal contrast-enhanced MRI cisternography has classically been thought to offer the advantage of demonstrating the connection of the arachnoid cyst with the surrounding CSF space.

MRI noncontrast techniques, constructive interference in steady-state (CISS), fast imaging employing steady-state acquisition (FIESTA), and 3D T2-weighted sampling perfection with application-optimized contrast with different flip-angle evolutions (3D SPACE), demonstrate the arachnoid cyst wall and surrounding structures (eg, cranial nerves and vessels). A clear demonstration of the surrounding structures can help decrease surgical morbidities. The 3D SPACE technique helps demonstrate a connection to the CSF space. The 3D SPACE shows signal loss in areas of CSF flow, and these areas of signal loss from flow highlight the connection between the cyst and the CSF space. Similarly, phase-contrast MRI can evaluate a CSF connection to an arachnoid cyst. The flowing CSF is specifically demonstrated as a signal change on phase contrast. However, secondary to false positives, a connection must be confirmed with contrast MR cisternography.[40]

Radiologists need to evaluate the brain structures for mass effects secondary to the arachnoid cyst, which ranges from a subtle mass effect on a cranial nerve to a brain herniation. The final analysis of the arachnoid cyst should only be made after obtaining an accurate history from the clinician. An accurate history will ensure that the radiology report properly reflects information about pertinent cranial nerve or vascular structures that are clinically affected by the arachnoid cyst. Radiologists must also be proactive for critically located cysts causing hydrocephalus or large cysts causing mass effect and brain herniation. In these situations, the ordering clinician should be informed, and a neurosurgeon should be called immediately.

Treatment / Management

The vast majority of cases of arachnoid cysts need no treatment. However, in patients with symptomatic arachnoid cysts, surgery should be considered based on the clinical factors present, including:

• Cyst location
• Presence of mass effects
• Impaired CSF flow dynamics (eg, hydrocephalus)
• Focal neurological deficits
• Headaches
• Seizures
• Developmental or cognitive deficits [8]

In cases where treatment is indicated, numerous surgical therapies exist. The surgical approach is often dictated by the location of the cyst and the involvement or lack of involvement of surrounding neurological structures. Surgical excision of the wall is performed with the creation of communication with the subarachnoid space when the lesion is readily amendable.[41][7] Also, surgical fenestration or endoscopic fenestration is utilized in which the cyst cavity is opened into the closest part of the ventricular system.[32][33][38][39] In perisylvian arachnoid cysts, endoscopic fenestration is widely used as a safe and efficient technique with the highest event-free survival compared to microsurgical fenestration.[42](A1)

Rarely is the placement of a cystoperitoneal shunt done.[33][8] Shunting is avoided when possible due to the complications that can arise with long-term shunt placement. Unusual treatments are undertaken with unusual cyst locations, such as percutaneous aspiration through the foramen ovale.[27] Surgery is almost always curative, but recurrences have been reported.[43] Emergency treatment could be needed when significant hydrocephalus or mass effect is present.(B3)

Differential Diagnosis

The differential diagnoses for an arachnoid cyst include epidermoid cysts, dermoid cysts, abscesses, and other cysts such as cystic neurocysticercosis. The differential is defined predominantly based on the radiological evaluation.

Epidermoid cysts have a similar appearance to CSF on T2 FSE and T2 SE. On other sequences, epidermoid cysts will have variations in the signal, will not enhance, and have markedly elevated signals on diffusion-weighted imaging, which helps to characterize these cysts further. Also, epidermoid cysts tend to be located anteriorly and laterally to the brainstem. A dermoid or lipoma can be differentiated on CT or MRI with signal intensities that follow fat. A T1 sequence with and without fat saturation to demonstrate the presence of fat on MRI is very useful to confirm a dermoid cyst or lipoma.

An abscess can appear cystic, but the internal signal is not similar to CSF due to inflammatory and cellular debris. Very likely, the central fluid of an abscess will have a markedly elevated diffusion-weighted image (DWI) signal, similar to an epidermoid, and rim enhancement that is most often thin. However, thick rim enhancement is seen with some bacterial and fungal infections.

Neurocysticercosis cysts in the CSF (ie, racemose neurocysticercosis) can have individual cysts that appear very similar to an arachnoid cyst, but these cysts are typically multiloculated. In cases of neurocysticercosis, there are very likely other findings indicating neurocysticercosis on brain imaging, in history, and by immunology workup. Usually, with neurocysticercosis, there are brain lesions with calcifications.

Prognosis

In most cases, no treatment is necessary. Surgery is almost always curative when needed, but recurrences have been reported.[43] There have been reports of spontaneous resolution of arachnoid cysts, especially following a trauma.[44]

Complications

Complications from arachnoid cysts are rare but may occur depending on the cyst's location and whether it causes distortions of adjacent anatomical structures. A ruptured arachnoid cyst can also cause subdural hygromas or subdural hematomas that need to be treated. Other complications, including vision loss, nausea or vomiting, macrocephaly, paralysis of the oculomotor, trochlear, abducens nerves, trigeminal neuropathy, hemifacial spasm, sensory neuronal hearing loss, facial palsy, vagus nerve palsy, and vertigo may also occur.[45][25][26][27][28][29]