Arachnoid cyst

What is Arachnoid cyst?

It is a sac-like structure filled with cerebrospinal fluid that may develop between the surface of the brain and the base of the skull, or on the arachnoid membrane, a delicate fibrous layer forming the middle of the three coverings of the central nervous system.

Arachnoid cyst Types

According to the Galassi classification system, the cyst can be of the following types:

Type 1

In this case, the cyst is small, spindle-shaped, and normally restricted to the anterior middle cranial fossa, a small depression or cavity in the cranium.

Type 2

The lesion extends superiorly along sylvian fissure, a deep, lateral indentation that divides the lobes of the brain, with displacement of the temporal lobe.

Type 3

In this type, the closed sac is large that covers the entire middle cranial fossa, with displacement of the temporal, frontal and parietal lobes. It is slightly close to the subarachnoid space.

Type 4

The posterior fossa, which contains the cerebellum, pons, and medulla oblongata, is a common location for this cyst.

Type 5

Cysts found in the supracollicular or retrocerebellar regions of the brain tend to be composed of either arachnoid connective tissues, or glioependymal tissue.

Spinal cysts could be classified into the following forms:

  • Sacral extradural: The abnormal structure lies outside the dura mater near the sacrum, the bottom of the spine.
  • Intradural: In this case, the cyst is situated beneath the dura mater that covers the cervical and thoracic regions.
  • Perineural: The anomalous growth is present near a nerve or nerves.
  • Intraventricular: The cyst arises from vascular mesenchyme or extends from the subarachnoid region through the choroidal fissure and into the lateral ventricle.
  • Fetal: The unusual mass is often diagnosed in the fetal brain due to a congenital disorder.

Arachnoid cyst Symptoms

Affected patients in some cases do not exhibit any symptoms irrespective of the size of the cyst. Unlike a small cyst, presence of a large cyst could produce symptoms like:

  • Involuntary bobbing of head and shoulders forward and back, or sometimes side-to-side
  • Abnormally large head
  • Behavioral and learning disorders
  • Convulsions
  • Headaches
  • Abnormal accumulation of cerebrospinal fluid in the brain
  • Developmental disabilities in children
  • Increased pressure inside the skull
  • Lack of voluntary coordination of muscle movements
  • Nausea
  • Partial paralysis
  • Dementia
  • Dizziness
  • Hallucinations
  • Urinary incontinence
  • Tingling or numbness in the legs or arms
  • Back pain

Health experts believe that the clinical manifestations of the condition vary according to the location of the cyst. Patients with Ménière’s disease, a condition of the inner ear causing hearing and balance problems, are usually present with a supratentorial arachnoid cyst. On the other hand, the benign lump develops in the frontal region of the arachnoid. Cysts occupying the left temporal lobe may give rise to a series of mental health problems. A left fronto-temporal cyst causes alexithymia in which the patients are unable to describe emotions in a verbal manner. Geriatric patients with a cyst on the lateral fissure or sulcus of the right hemisphere often display few signs of schizophrenia. Frequent episodes of hallucinations, migraine as well as paranoia are normally linked to an unusual growth on the left middle cranial fossa. Many ailing patients with a left temporal lobe cyst suffer from mood disruption and bipolar disorder.

Arachnoid cyst Causes

Medical investigators consider developmental malformations in the brain to be the predominant cause of the condition. In such cases, the arachnoid membrane unknowingly tears or splits. Cysts involving the middle fossa are normally a manifestation of an underdeveloped or compressed temporal lobe. Genetic factors also play significant roles in the formation of these cysts. Cysts in the suprasellar region are responsible for bobbing and nodding of the head. Non-congenital cases in both adults and children may have a myriad of causes. In all likelihood, these abnormal masses are sometimes related to conditions like:

  • Arachnoiditis: This is a neuropathic disorder caused by inflammation of the arachnoid.
  • Agenesis of the corpus callosuma: It is a congenital defect marked by complete or partial absence of the corpus callosum.
  • Marfan syndrome: In this disorder, the connective tissues get affected due to a genetic mutation.

Arachnoid cyst Diagnosis

Asymptomatic conditions normally fail to receive timely diagnosis and manifest into severe problems. Imaging techniques such as CT scan or MRI can easily identify and spot the cyst. In most cases, doctors accidently discover the sac-like growth while carrying out the diagnosis for some other disorder. Patients with mental disabilities may have to take up the mini–mental state examination or Folstein test, which comprises of a series of questions and tests, for screening cognitive function.

Arachnoid cyst Differential diagnosis

Arachnoid cyst in brain needs to be segregated from the following ailments to enable proper diagnosis:

  • Epidermoid cyst
  • Subdural hygroma
  • Cystic tumors
  • Pilocytic astrocytoma
  • Haemangioblastoma
  • Neurenteric cyst

Mega cisterna magna v/s Arachnoid cyst

The cyst resembles mega cisterna magna, an abnormal and asymmetric mass, which is not associated with cerebellar abnormalities. In order to distinguish this non-malignant lesion from spinal meningeal cyst, ventriculography or cisternography is typically conducted to demonstrate communication of the cystic mass with the subarachnoid space.

Arachnoid cyst Treatment

Healthcare givers can initiate various therapeutic procedures based on the symptoms of the condition. The main aim of the treatment is to reduce the pressure exerted by the cyst on the surrounding regions of the brain and spinal cord. Some of these include:

Drug therapy

Constant bouts of headaches, seizures as well as other symptoms can be ameliorated with the help of certain medications recommended by physicians.


In the medical field, the term generally refers to an opening in the surface of a membrane. For example, craniotomy is commonly performed to remove the cyst. In this technique, a section of bone from the skull is carefully excised to access the affected region of the brain for surgical repair. Endoscopic interventions, including laser treatment, help the cystic secretion to drain into the cerebrospinal fluid pathway and subsequently decrease the pressure inside the brain. Surgical removal of cysts is, however, performed in very severe cases.

Placement of cerebral shunts

Generally, a brain shunt is a device that diverts fluid from the brain into the abdominal cavity where it gets safely absorbed into the blood stream. In this way, the method relieves pressure in the brain that has swollen due to excess buildup of cerebrospinal fluid. Depending on the location of the cyst, either of the following two shunts could be used:

  • Internal shunt: It aids in the draining of the fluid into the subdural compartment.
  • Cystoperitoneal shunt: Insertion of the device drains the fluid into the peritoneal cavity.

Fluid drainage

Presence of large cysts may require fine-needle aspiration in which a thin, hollow needle is inserted into the abnormal mass to drain some amount of cystic fluid. In other cases, trephination is performed to drill a hole into the skull and exposing the dura mater for surgical correction.

Capsular resection

In few cases, the cyst severely compresses the surrounding tissues and causes microscopic capsular invasion of the brain. Therefore, many specialists excise this particular section of the brain as soon as possible.

Arachnoid cyst Complications

Although asymptomatic cases do not require any treatment, the cyst may suddenly grow and lead to severe complications. A minor head trauma can be detrimental if the cystic fluid leaks into the subarachnoid space. Bigger cysts can cause the surrounding blood vessels to tear and bleed, resulting in intracystic hemorrhage. Persistent bleeding can also lead to accumulation of blood outside the lesion, a common condition called hematoma.

Arachnoid cyst in children

The condition can lead to many health complications such as ADHD in young children.

Arachnoid cyst Prognosis

Appropriate treatment and management can improve the outlook for patients with benign cysts. The cognitive abilities of the sufferers have drastically improved post decompression of the cyst. In fact, the various behavioral and psychological symptoms associated with the condition have alleviated after surgery.

Arachnoid cyst Incidence

The cyst has been reported in about 1.1% of the population and occurs in about twice as many males as females. It has been estimated that up to 80% of individuals are diagnosed with this problem before the age of 16. These cysts account for 1% of intracranial masses and are present in 2.5% of patients with chronic subdural hematomas.

Progressive expansion of an Arachnoid cyst may cause permanent neurological damage if left untreated. Prompt diagnosis and correct treatment can always help the patients to lead a normal life.

Arachnoid cyst Pictures

Image of Arachnoid cyst


Arachnoid cyst photo


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