A wide range in the percentages of symptomatic versus asymptomatic cysts has been reported, and this reported rate will depend greatly on the population in which it is being calculated. The location of the arachnoid cyst 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 cysts locations. For instance, Al-Holou et al. report very few intraventricular cysts in children (0.3%), no intraventricular cysts in adults, and Hall et al. reported 12% of cysts to be intraventricular in children.[1][2][3]

Most commonly, the arachnoid cyst will be detected incidentally unless a patient is sent to a referral center due to being symptomatic. 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 found to be more likely symptomatic. In unusual cases, when arachnoid cysts cause symptoms, they present mainly by two mechanisms. One is secondary to mass effect, and the other is due to 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/vomiting, dizziness, cranial nerve dysfunctions, tinnitus, vertigo, developmental delay/regression, behavior concerns, visual symptoms, and trauma.[1][2][3] 

Even in patients who present with a headache, if an arachnoid cyst is detected, it most likely is not related to the headache and is an incidental finding. However, careful evaluation of each case is required. The arachnoid cyst can be very large and can be a definitive cause of headaches and seizures.[13] Larger cysts are more likely to be symptomatic than small cysts, and larger cysts are more likely to undergo an operation.

It is rare for the arachnoid cyst to be a cause of a significant neurological symptom such as hydrocephalus, ataxia, or cranial nerve impingement. However, given the relatively common incidence and the many locations that arachnoid cysts occur in, an extensive array of symptoms are caused by arachnoid cysts. These include vision loss, nausea/vomiting, macrocephaly, third nerve palsy, trochlear dysfunction, trigeminal neuropathy, hemifacial spasm, sensory neuronal hearing loss, facial palsy, vagus nerve palsy, vertigo, and eighth cranial neuropathy.[3][14][15][16][17][18][19][20][21] The symptoms depend on the cyst location. A quadrigeminal plate cistern arachnoid cyst could compress the trochlear nerves and quadrigeminal plate and result in trochlear nerve dysfunction.[22] 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.

Very unusual symptomatology has resulted from arachnoid cysts. Bobble-head doll syndrome, a rhythmic movement of the head, has been reported related to third ventricular dilation and potentially cerebellar involvement.[23][24] Depression that resolves with marsupialization of an arachnoid has occurred. 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.[25]

The rupture of an arachnoid cyst to cause a subdural hygroma is rare.[26][27][28] The rupture can be due to trauma, or the arachnoid cyst can rupture spontaneously. Usually, the resulting subdural hygroma can be followed without treatment, but surgical intervention might be needed.[26][28] Also, the traumatic rupture of an arachnoid cyst rarely results in hemorrhage in the arachnoid cyst or a subdural hematoma.        

The spinal arachnoid cyst is rare.[29] Arachnoid cysts are found throughout the spinal column but most commonly in the thoracic region. The most common symptoms relate to backache and fluctuating leg weakness. There have shown correlations with monoparesis, radicular pain, spastic quadriparesis, neurogenic claudication, sensory disturbance, monoplegia, incontinence, pain, paresthesia, and neurogenic bladder reported. More commonly, the cysts will be located posteriorly but can be located anteriorly.[29][30]

Computed tomogram (CT) imaging is often enough to make a diagnosis of an arachnoid cyst when the lesion is in an expected location with the appearance of a fluid-filled thin-walled cyst. When additional information is needed, an MRI is the imaging modality of choice for evaluating the anatomical location, size, and structures involved by an arachnoid cyst. MRI also can help refine the differential diagnosis when needed. The arachnoid cyst follows cerebrospinal fluid (CSF) on all pulse sequences unless it has experienced traumatic injury and blood products are present. Hemorrhage within an arachnoid cyst is rare.

The wall of the cyst is thin, and special pulse sequences can be employed to visualize the wall. The differential diagnosis for an arachnoid cyst includes epidermoid cyst, dermoid cyst, abscess, and other cysts such as cystic neurocysticercosis. On magnetic resonance imaging (MRI), the arachnoid cyst follows the signal intensity of CSF on T1 (dark signal), T2 fast spin-echo (FSE), or T2 spin-echo (SE) (high signal), T2-FLAIR (dark signal), and diffusion (dark signal). The apparent diffusion coefficient (ADC) maps created from the diffusion data will demonstrate the cyst with similar high ADC values as the CSF. Due to CSF pulsations, there can be mild signal differences between the fluid in the arachnoid cyst and the normal CSF on MRI sequences. Arachnoid cysts do not enhance.

Epidermoid cysts will be similar to CSF on T2 FSE and T2 SE, but on other sequences, will have variations in the signal, will not enhance, and have markedly elevated signal on diffusion-weighted imaging. This is a useful characteristic. Also, epidermoid cysts tend to be located anteriorly and laterally from the brainstem. A dermoid/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/lipoma.

An abscess can be cystic in appearance, but due to inflammatory and cellular debris, the internal signal will not be similar to CSF. 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. Although thick rim enhancement is seen with some bacterial and fungal infections.

Neurocysticercosis cysts in the CSF (racemose neurocysticercosis) can have individual cysts that are very similar in appearance to an arachnoid cyst, but these cysts are very likely 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.

CT cisternography and MRI cisternography can be performed to confirm a diagnosis and to further evaluate an arachnoid cyst. In CT cisternography, a neuro safe non-ionic 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 freely communicating and may not need to be treated.[31] 

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. The MRI non-contrast 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 demonstrate surrounding structures including cranial nerves and vessels. Clear demonstration of the surrounding structures can help decrease surgical morbidities. The 3D SPACE technique is useful for demonstrating a connection to the CSF space. The 3D SPACE demonstrates signal loss in areas of CSF flow, and these areas of signal loss from flow highlight the connection of the cyst and the CSF space.

In a similar fashion, phase-contrast MRI can be utilized to evaluate for 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 needs to be confirmed with contrast MR cisternography.[32]

The radiologist needs to evaluate the brain structures for mass effect secondary to the arachnoid cyst. This might be for a subtle mass effect on a cranial nerve or a prominent mass effect resulting in 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 assure that the radiology report properly reflects information about pertinent cranial nerve or vascular structures that are being clinically affected by the arachnoid cyst. The radiologist also needs to be proactive when there is a cyst in a critical location that is causing hydrocephalus or if a large cyst is present that is resulting in mass effect and brain herniation. In these situations, the ordering clinician needs to be called, or a neurosurgeon needs to be called directly.

The vast majority of cases of arachnoid cysts need no treatment. In cases when it has been determined that an arachnoid cyst is causing or is very likely causing symptoms, numerous surgical treatments 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.[21][31] Also, surgical fenestration or endoscopic fenestration (opening of the cyst cavity into the closest part of the ventricular system) is utilized.[23][24][29][30] 

Rarely, the placement of a cystoperitoneal shunt is done.[24][33] Shunting is avoided when possible due to the complications that can arise with long-term shunt placement. Unusual treatments are undertaken with unusual locations of the cyst, such as percutaneous aspiration through the foramen ovale.[16] Surgery is almost always curative, but recurrences have been reported.[34] When significant hydrocephalus or mass effect is present, emergency treatment could be needed.

The differential diagnosis for an arachnoid cyst includes epidermoid cyst, dermoid cyst, abscess, and other cysts such as cystic neurocysticercosis. The differential is defined predominantly on the basis of the radiological evaluation.

In the vast majority of cases, no treatment is necessary. When needed, surgery is almost always curative, but recurrences have been reported.[34]

There are usually no complications of an arachnoid cyst. If there is a rare complication from the arachnoid cyst, it is dependent on the location and what anatomical structure is distorted by the arachnoid cyst. A ruptured arachnoid cyst can also cause subdural hygromas or subdural hematomas that need to be treated.

Patients need to be informed that the vast majority of arachnoid cysts are incidental and no treatment is necessary. Only in rare situations does the arachnoid cyst result in the need for treatment.

• Arachnoid cysts are relatively common, and most are incidental.
• The most common locations for arachnoid cysts are in the anterior, middle cranial fossa, and in the retrocerebellar cistern.
• The arachnoid cyst should have a thin wall and no enhancement.
• The fluid in an arachnoid cyst should follow fluid density on CT and fluid intensities on MRI.

The vast majority of arachnoid cysts are incidental, and no special communication needs to take place outside of the radiology report. In very rare cases when the arachnoid cyst is causing significant mass effect, hydrocephalus, or has ruptured and resulted in a significant subdural hygroma or hematoma, the radiologist will need to emergently call the referring clinician or a neurosurgeon. The patient may need to go to an emergency department and be evaluated. [Level 5]