Arachnoid Cysts

Arachnoid cysts are fluid-filled sacs that occur on the arachnoid membrane that covers the brain (intracranial) and the spinal cord (spinal) majority of arachnoid cysts are in the anterior/middle cranial fossa or in a retrocerebellar location. It is cerebrospinal fluid covered by arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the arachnoid membrane, one of the three meningeal layers that cover the brain and the spinal cord. There are three membranes covering these parts of the central nervous system: the dura mater, arachnoid, and pia mater. Arachnoid cysts appear on the arachnoid membrane, and they may also expand into the space between the pia mater and arachnoid membranes (subarachnoid space). The most common locations for intracranial arachnoid cysts are the middle fossa (near the temporal lobe), the suprasellar region (near the third ventricle), and the posterior fossa, which contains the cerebellum, pons, and medulla oblongata.

Arachnoid cysts are cerebrospinal fluid-filled sacs that are located between the brain or spinal cord and the arachnoid membrane, one of the three membranes that cover the brain and spinal cord. The majority of arachnoid cysts form outside the temporal lobe of the brain in an area of the skull known as the middle cranial fossa. Arachnoid cysts involving the spinal cord are rarer. The location and size of the cyst determine the symptoms and when those symptoms begin. Most individuals with arachnoid cysts develop symptoms before the age of 20, and especially during the first year of life, but some people with arachnoid cysts never have symptoms. Males are four times more likely to have arachnoid cysts than females.

In many cases, arachnoid cysts do not cause symptoms (asymptomatic). In cases in which symptoms occur, headaches, seizures, and abnormal accumulation of excessive cerebrospinal fluid in the brain (hydrocephalus) are common. The exact cause of arachnoid cysts is unknown. Arachnoid cysts are classified according to their specific location.

Types

Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation.
Secondary arachnoid cysts are not as common as primary cysts and develop as a result of head injury, meningitis, or tumors, or as a complication of brain surgery.

Causes

The exact cause of arachnoid cysts is not known. Researchers believe that most cases of arachnoid cysts are developmental malformations that arise from the unexplained splitting or tearing of the arachnoid membrane. According to the medical literature, cases of arachnoid cysts have run in families (familial cases) suggesting that a genetic predisposition may play a role in the development of arachnoid cysts in some individuals.

The etiology of formation is uncertain, but arachnoid cysts, in theory, are predominantly formed due to abnormal splitting during embryogenesis of the arachnoid. If histopathology shows the presence of inflammatory cells, excess collagen, or hemosiderin staining, then an inflammatory or traumatic etiology is suggested. Underlying gliosis is not usually found in the adjacent brain secondary to mass effect.

In some cases, arachnoid cysts occurring in the middle fossa are accompanied by underdevelopment (hypoplasia) or compression of the temporal lobe. The exact role that temporal lobe abnormalities play in the development of middle fossa arachnoid cysts is unknown.

Some complications of arachnoid cysts can occur when a cyst is damaged because of minor head trauma. Trauma can cause the fluid within a cyst to leak into other areas (e.g., subarachnoid space). Blood vessels on the surface of a cyst may tear and bleed into the cyst (intracystic hemorrhage), increasing its size. If a blood vessel bleeds on the outside of a cyst, a collection of blood (hematoma) may result. In the cases of intracystic hemorrhage and hematoma, the individual may have symptoms of increased pressure within the cranium and signs of compression of nearby nerve (neural) tissue.

Arachnoid cysts can also occur secondary to other disorders such as Marfan’s syndrome, arachnoiditis, or agenesis of the corpus callosum. (For more information on these disorders, choose the specific disorder name as your search term in the Rare Disease Database).

Related Disorders

Symptoms of the following disorders can be similar to those of arachnoid cysts. Comparisons may be useful for a differential diagnosis:

Brain tumors are growths in the brain that can be either cancerous (malignant) or noncancerous (benign). The symptoms of malignant and benign brain tumors are similar and depend on the type of tumor and its location. Symptoms may include recurrent headaches, seizures, emotional instability, nausea, and/or facial pain or numbness. There are many different types of brain tumors. (For more information on these disorders, choose “Brain Tumor” as your search term in the Rare Disease Database).

Dandy-Walker malformation is a rare malformation of the brain that is present at birth (congenital). It is characterized by an abnormally enlarged space at the back of the brain (cystic 4th ventricle) that interferes with the normal flow of cerebrospinal fluid through the openings between the ventricle and other parts of the brain (foramina of Magendia and Luschka). Excessive amounts of fluid accumulate around the brain and cause abnormally high pressure within the skull, swelling of the head (congenital hydrocephalus), and neurological impairment. Motor delays and learning problems may also occur. Dandy-Walker malformation is a form of “obstructive” or “internal noncommunicating hydrocephalus”, meaning that the normal flow of cerebrospinal fluid is blocked resulting in the widening of the ventricles. (For more information on this disorder, choose “Dandy-Walker” as your search term in the Rare Disease Database).

Empty Sella syndrome is a rare disorder characterized by enlargement or malformation of a structure in the head known as the sella turcica. The sella turcica is a saddle-shaped depression located in the bone at the base of the skull (sphenoid bone), in which resides the pituitary gland. In empty sella syndrome, the malformed sella turcica is often either partially or filled with cerebrospinal fluid. As a result, the pituitary gland is often compressed and flattened so that the sella turcica appears empty. Most individuals with empty sella syndrome do not have any associated symptoms. Occasionally, headaches or pituitary dysfunction may occur. Empty Sella syndrome may occur as a primary disorder, for which the cause is unknown (idiopathic), or as a secondary disorder, in which it occurs due to an underlying condition or disorder such as a pituitary tumor or trauma in the pituitary region. (For more information on this disorder, choose “Empty Sella” as your search term in the Rare Disease Database).

Porencephaly is a general term for certain disorders that affect the central nervous system. Porencephaly may be classified as sporadic or familial. Sporadic porencephaly can have many different causes including infection just before or just after birth (perinatal infection), trauma, maternal disease or sickness, maternal diabetes, or maternal use of alcohol or drugs such as cocaine during pregnancy. Autosomal dominant porencephaly type I occurs due to mutations of the COL4A1 gene. In porencephaly, cysts or cavities form on the surface of the brain. These cysts or cavities may become filled with cerebrospinal fluid, a colorless fluid that normally surrounds the brain and spinal cord to provide protection and nourishment. The severity and associated symptoms of porencephaly vary dramatically from one person to another based upon the size and exact locations of the fluid-filled cavities or cysts. Some infants develop serious complications shortly after birth; other individuals may have mild symptoms that may go undetected. The signs and symptoms of porencephaly may include weakness or paralysis of one side of the body (hemiparesis or hemiplegia), seizures, varying degrees of cognitive impairment, and migraines. Individuals with autosomal dominant porencephaly type I are predisposed to damage to small blood vessels, including the small vessels within the brain.

Symptoms

In most cases, arachnoid cysts are present at birth (congenital), but usually do not cause any symptoms (asymptomatic) throughout an individual’s life. Less often, arachnoid cysts may develop because of head injury, the presence of a tumor, infection, or surgery on the brain. Some syndromes have been associated with arachnoid cysts, and these syndromes include Aicardi syndrome, mucopolysaccharidosis, acrocallosal syndrome, Marfan syndrome, a missense mutation (c.2576C>T) in the arginine-glutamic acid dipeptide repeats gene (RERE), and Chudley-Mullough syndrome.[rx][rx][rx]

Whether symptoms develop depends upon the size and the specific location of the cyst within the brain. Small cysts usually do not cause symptoms. However, cysts can increase in size causing symptoms to appear, especially if they press against a cranial nerve, the brain, or the spinal cord.

Most cases of arachnoid cysts that are associated with symptoms occur in childhood. The specific symptoms present vary from case to case. It is important to note that affected individuals will not have all of the symptoms listed below.

The most common symptoms associated with arachnoid cysts are usually nonspecific and include headaches, nausea, vomiting, dizziness, and the accumulation of excessive cerebrospinal fluid in the brain (hydrocephalus), resulting in increased intracranial pressure In rare cases, in some children, an arachnoid cyst can cause the malformation of certain cranial bones, resulting in an abnormally enlarged head (macrocephaly).

A variety of additional symptoms occur in some individuals with arachnoid cysts depending upon the size and location of the cyst. Most cysts occur near the middle fossa region of the brain. Such symptoms include lethargy, seizures, vision abnormalities, and hearing abnormalities. Neurological signs may be present because arachnoid cysts may cause increased pressure on structures of the brain. Such neurological findings may include developmental delays, behavioral changes, an inability to control voluntary movements (ataxia), difficulties with balance and walking, and cognitive impairment. Weakness or paralysis on one side of the body (hemiparesis) has also been reported.

In addition to hydrocephalus, cysts located in the suprasellar region may be associated with visual disturbances, continuous bobbing of the head, and abnormalities affecting certain hormone-producing glands that help to regulate the rate of growth, sexual development, and certain metabolic functions (endocrine system).

Although they occur much less often than those found within the skull (intracranial), arachnoid cysts may also arise near the spine (spinal arachnoid cysts). Spinal arachnoid cysts may be associated with progressive weakness of the legs, tingling or numbness in the hands or feet, abnormal side-to-side curvature of the spine (scoliosis), back pain, and involuntary muscle spasms (spasticity) that result in slow, stiff movements of the legs. In rare cases, these cysts may cause paralysis of the legs (paraplegia). Urinary tract infections may also occur in individuals with spinal arachnoid cysts.

Other symptoms and physical findings have been reported to be associated with arachnoid cysts including migraine headaches, attention-deficit disorder, and difficulties understanding or expressing language (aphasia). The exact cause and effect relationship between these findings and arachnoid cysts is not clear.

Symptoms vary by the size and location of the cyst(s), though small cysts usually have no symptoms and are discovered only incidentally.^[rx] On the other hand, a number of symptoms may result from large cysts:

Cranial deformation or macrocephaly (enlargement of the head), particularly in children^[rx]
Cysts in the suprasellar region in children have presented as bobbing and nodding of the head called bobble-head doll syndrome.^[rx]
Cysts in the left middle cranial fossa have been associated with ADHD in a study on affected children.^[rx]
Headaches.^[rx] A patient experiencing a headache does not necessarily have an arachnoid cyst.
- In a 2002 study involving 78 patients with a migraine or tension-type headache, CT scans showed abnormalities in over a third of the patients, though arachnoid cysts only accounted for 2.6% of patients in this study.^[rx]
- A study found 18% of patients with intracranial arachnoid cysts had non-specific headaches. The cyst was in the temporal location in 75% of these cases.^[rx]
Seizures^[rx]
Hydrocephalus (excessive accumulation of cerebrospinal fluid)^[rx]
Increased intracranial pressure^[rx]
Developmental delay^[rx]
Behavioral changes^[rx]
Nausea
Dysdiadokinesis
Hemiparesis (weakness or paralysis on one side of the body)^[rx]
Ataxia (lack of muscle control)^[rx]
Musical hallucination^[rx]
Pre-senile dementia,^[rx] a condition often associated with Alzheimer’s disease
In elderly patients (>80 years old) symptoms were similar to chronic subdural hematoma or normal pressure hydrocephalus:^[rx]
- Dementia
- Urinary incontinence
- Hemiparesis
- Headache
- Seizures

Location-specific symptoms

A supratentorial arachnoid cyst can mimic a Ménière’s disease attack.
Frontal arachnoid cysts have been associated with depression.
Cysts on the left temporal lobe have been associated with psychosis. A left frontotemporal cyst showed symptoms of alexithymia.^[rx]
Cyst on the right Sylvian fissure resulted in the new onset of schizophrenia-like symptoms at age 61.^[rx]
A patient with a cyst on the left middle cranial fossa had auditory hallucinations, migraine-like headaches, and periodic paranoia
Patients with left temporal lobe cysts had mood disturbances similar to manic depression (bipolar disorder) and were known to show outward aggression^[rx]

Diagnosis

A diagnosis of arachnoid cysts is often made incidentally, often during the examination of an individual with seizures. A diagnosis may be suspected based on detailed patient history, a thorough clinical examination, and a variety of specialized tests, especially advanced imaging studies such as computed tomography (CT scan) and magnetic resonance imaging (MRI). CT scans and MRIs can reveal or confirm the presence of arachnoid cysts. During CT scanning, a computer and x-rays are used to create a film showing cross-sectional images of the brain’s tissue structure. During MRI, a magnetic field and radio waves are used to create cross-sectional images of the brain.

History and Physical

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.[rx][rx][rx]

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.[rx][rx][rx]

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.[rx] 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.[rx][rx][rx]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.[rx] 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.[rx][rx] 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.[rx]

The rupture of an arachnoid cyst to cause a subdural hygroma is rare.[rx][rx][rx] 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.[rx][rx] 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.[rx] 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.[rx][rx]

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.[rx]

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.[rx]

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.

Treatment

Most arachnoid cysts are found incidentally and remain constant in size, leading many physicians to recommend conservative treatment. When no symptoms are present, no treatment may be necessary, and affected individuals may be periodically monitored. If symptoms arise, a cyst can be reevaluated.

Most arachnoid cysts are asymptomatic and do not require treatment. Treatment may be necessary when symptomatic.^[rx] A variety of procedures may be used to decompress (remove pressure from) the cyst.

Surgical placement of a cerebral shunt:^[rx]
- An internal shunt drains into the subdural compartment.^[rx]
- A cystoperitoneal shunt drains to the peritoneal cavity.^[rx]
Fenestration:
- Craniotomy with excision^[rx]
- Various endoscopic techniques are proving effective,^[rx] including laser-assisted techniques.^[rx]
Drainage by needle aspiration or burr hole.
Capsular resection^[rx]
Pharmacological treatments may address specific symptoms such as seizures or pain.

When treatment is necessary the specific therapy used depends upon whether symptoms are present, the size of the cyst, and the specific location of the cyst within the skull.

In cases where treatment is recommended, therapy traditionally consists of one of two procedures – an open craniotomy fenestration or ventriculoperitoneal shunting.

During a craniotomy fenestration, a portion of the skull is removed to give a surgeon access to the cyst, where multiple openings are made in the cyst wall, (fenestrations), to allow cerebrospinal fluid to drain into the subarachnoid space where the fluid is reabsorbed into the surrounding tissue. Alternatively, some cases may be treated by surgically inserting a device (shunt) into the cyst to provide drainage either into the ventricular system of the brain or into the abdominal cavity. This will drain the cyst and provide an adequate passageway for cerebrospinal fluid to circulate.

Surgical excision of the wall is performed with the creation of communication with the subarachnoid space when the lesion is readily amendable.[rx][rx] Also, surgical fenestration or endoscopic fenestration (opening of the cyst cavity into the closest part of the ventricular system) is utilized.[rx][rx]

Rarely, the placement of a cystoperitoneal shunt is done.[rx][rx] 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.[rx] Surgery is almost always curative, but recurrences have been reported.[rx] When significant hydrocephalus or mass effect is present, emergency treatment could be needed.

More recently, advancements in the minimally invasive brain and skull base surgery have evolved these traditional procedures into fully endoscopic techniques, distinguished by shorter operating times, fewer complications, excellent outcomes with faster recovery, and overall decreased patient morbidity. While the approach varies depending on the size and location of the arachnoid cyst, fully endoscopic surgical management has provided the surgeon with superior access for either fenestration or in other cases, resection of the cyst without the complications and risks associated with brain manipulation or retraction. Few facilities provide minimally invasive, endoscope-assisted fenestrations, endoscopic shunt placement, and endoscope-assisted or fully endoscopic resection of arachnoid cysts as treatment when indicated.

Spinal arachnoid cysts may be treated by the complete surgical removal (resection) of the cyst, if possible. Surgery generally leads to a resolution of symptoms. In some cases, complete surgical removal of a spinal cyst is not possible. In such cases, fenestration or shunting of the cyst to drain the fluid may be necessary.

Another treatment is symptomatic and supportive.