Arachnoid cyst

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Arachnoid cysts are cerebrospinal fluid cysts covered by arachnoidal cells and collagen. They develop between the arachnoid membrane and the surface of the brain. Arachnoid cysts are mostly a congenital disorder.

Arachnoid cysts were first described by Richard Bright in 1831.

The exact pathophysiology of arachnoid cysts is not known.

Arachnoid cysts may be classified according to cause into 2 subtypes/groups: primary or secondary, or according to symptoms into 3 groups.

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, being classified as primary. There are also arachnoid cysts that arise from secondary causes, such as trauma, surgery, intracranial hemorrhage and infection.

The main differential diagnosis of Arachnoid cysts are epidermoid cysts. They can be differentiated using MRIs.

Arachnoid cysts occur in about 1.2% of the general population and are more frequently seen in men than in women.

There is no doccumented risk factor for the development of arachnoid cysts.

There is insufficient evidence to recommend routine screening for arachnoid cysts.

Prognosis of arachnoid cyst is generally excellent and is usually associated with a benign disease course, despite being symptomatic in a few patients. Most are found incidentally, and a few may, rarely, enlarge and complicate with the symptoms previously described such as hydrocephalus, seizures, hearing loss, visual changes and cervical myelopathy.

Patients with arachnoid cysts will most likely never show symptoms, even in cases where the cyst is large. Therefore, while the presence of symptoms may provoke further clinical investigation. Symptoms independent of further data cannot, and should not be, interpreted as evidence of a cyst’s existence, size or location. These symptoms usually are:

• Headache;
• Nausea/Vomiting;
• Seizures;
• Skull/spine deformation;
• Developmental delays;
• Obstructive hydrocephalus;
• Hearing loss;
• Head bobbing;
• Visual changes.

The diagnostic study of choice are MRIs as they can differentiate arachnoid cysts from all other causes of cystic CNS lesions while also presenting details about the origins, and anatomy of the cyst.

Physical examination is generally not very useful as most cases of arachnoid cysts are diagnosed incidentally, but in larger cysts, depending of the affected region, there may be some findings such as:

• Ataxia;
• Hearing loss;
• Visual changes;
• Protrusions from the head or spine;
• Head bobbing;
• Endocrine issues, e.g. early onset of puberty.

There are no laboratory findings associated with arachnoid cysts.

There are no ECG findings associated with arachnoid cysts.

There are no x-ray findings associated with arachnoid cysts.

There are no ultrasonographic findings associated with arachnoid cysts.

On brain/spine CT scan, arachnoid cysts are characterized by cystic images with similar density to CSF and non-enhancing borders, mostly found in the middle cranial fossa while they only rarely occur in the spinal cord.

On brain/spine MRI, arachnoid cysts are characterized by cystic images with similar density to CSF and non-enhancing borders, mostly found in the middle cranial fossa while they only rarely occur in the spinal cord. MRIs are more adequate than CT scans for evaluating arachnoid cysts.

There are no other imaging findings associated with arachnoid cysts.

There are no other imaging findings associated with arachnoid cysts.

There is no medical treatment for arachnoid cysts. Some medications may be used for treating its symptoms, such as analgesics for headache, sleep aids for insomnia and other medications for the dizziness/nausea.

There are a number of approaches in treating arachnoid cysts, the most common are:

• Endoscopic fenestration and removal of the cyst wall (advances in neurosurgical techniques favor fenestration over shunt insertion as the preferred method for initial management).
• Microsurgical fenestration;
• Craniotomy followed by shunting.

There are no primary preventive measures available for arachnoid cysts.

There are no secondary preventive measures available for arachnoid cysts.

Most patients won’t demand treatment. The very few who do must be assessed regarding cyst size, location and symptoms. If symptomatic, it may be a significant cause of morbidity and the costs of neurosurgical procedures, which are mostly safe and effective, may be high.

Advances in neurosurgical techniques continue to improve the safeness and efficacy of the neurosurgical procedures in treating arachnoid cysts.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Arachnoid cysts were first described by Richard Bright in 1831.

• Arachnoid cysts were first described by Richard Bright in 1831, who called it “serous cyst in the arachnoid” during an autopsy.
• It has been mistakenly thought that it was due to a temporal lobe agenesis during the 1950s.
• Lewis in 1962 wrote a review article first hypothesizing about the causes of arachnoid cysts.
• The condition was only further investigated with the development of the CT scans.^[1]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Arachnoid cysts may be classified according to cause into 2 subtypes/groups: primary or secondary, or according to symptoms into 3 groups.

Arachnoid cysts can be classified by cause:

• Primary: developmental cysts, formed during the formation of arachnoid membranes in utero;
• Secondary: appear after trauma, infection, hemorrhage or surgery. Much less common.^[1]

They can also be further classified regarding the symptoms that they cause, as proposed by Choi et al.:

• First group: present with hydrocephalus and intracranial hypertension;
• Second group: present with dizziness, headaches, large head, seizures, strabismus, developmental delays and skull deformation;
• Third group: minimal or no symptoms.

This classification is closely related with prognosis. Patients from the first group had significant improvement after surgery, the third group had a much smaller benefit while patients from the second group had minimal benefit.^[1]

Nabors et al. suggested classifying arachnoid cysts of the spinal cord into three types:

• Type I – extradural cysts without nerve root involvement;
• Type II – extradural cysts with nerve root involvement;
• Type III – intradural cysts.^[2]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

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, being classified as primary. There are also arachnoid cysts that arise from secondary causes, such as trauma, surgery, intracranial hemorrhage and infection.^[1]

• Arachnoid cysts are not tumors. They are small sacs that develop between the brain and the arachnoid membrane;
• They are filled with intracranial cerebrospinal fluid and lined with arachnoid membranes;
• Arachnoid cysts can be primary, in which the cause is unknown and develops in a fetus during pregnancy;
• And it can also be secondary to CNS infections, trauma, brain tumors and may be a complication of brain surgeries.^[1]
• There have been cases of arachnoid cysts appearing in multiple members of a family (familial cases) which suggests that there might be a genetic component playing a role in the development of arachnoid cysts in some patients.^[2]
• They may also be related to Marfan’s syndrome, agenesis of the corpo callosum and other disorders.^[2]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

The main differential diagnosis of Arachnoid cysts are epidermoid cysts. They can be differentiated using MRIs.

• The main differential diagnosis of Arachnoid cysts are epidermoid cysts.
• They can be differentiated using MRIs
• Other cystic lesions of the CNS also must be differentiated.
• Eventually, arachnoid cysts may contain proteinaceous fluid or blood, which can cause diagnostic confusion.^[1]

Arachnoid cysts differential diagnosis^[2]

Intraventricularly:	Colloid cysts
Intraparenchymally:	Parasitic infections, cystic metastases
Porencephalic cysts
Craniopharyngiomas
Holoprosencephalies
Agenesis of corpus callosum
Defect in the hemispheral cleavage
Dandy-Walker complex (posterior fossa cysts)

As differential diagnosis, the following hypothesis must be considered:

1. Enlarged CSF space;
2. Epidermoid cyst;
3. Subdural hygroma/chronic subdural hemorrhage;
4. Cystic tumors;
5. Pilocytic astrocytoma;
6. Hemangioblastoma;
7. Neurenteric cyst;
8. Neuroglial cyst;
9. Porencephalic cyst;
10. Neurocysticercosis.^[3]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Arachnoid cysts occur in about 1.2% of the general population and are more frequently seen in men than in women.

• The prevalence of arachnoid cysts is estimated to be about 1200 in 100,000 of the general population;^[1]
• Arachnoid cysts are more prevalent in men;
• 50% occur in the middle cranial fossa;
• One third of the cases occur in the posterior fossa;
• Temporal cysts are 70% left-sided.^[1]

• Only 20% of these have symptoms, usually from secondary hydrocephalus.^[2]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

There is no doccumented risk factor for the development of arachnoid cysts.

• There is no documented risk factor for the development of arachnoid cysts.

• There are, however, risk factors that may cause a pre-existing arachnoid cyst to bleed and these are: larger arachnoid cyst size and recent head trauma.^[1]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

There is insufficient evidence to recommend routine screening for arachnoid cysts.

There is insufficient evidence to recommend routine screening for arachnoid cysts.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Prognosis of arachnoid cyst is generally excellent and is usually associated with a benign disease course, despite being symptomatic in a few patients.

• Arachnoid cyst usually develops asymptomatically and remains asymptomatic in most patients through their lifetime;
• Most are found incidentally;
• In a few patients they may have: headaches, hydrocephalus, ataxia/gait imbalance, seizures, dizziness, visual changes, hearing loss, vertigo, nausea/vomiting, speech abnormalities, cervical myelopathy with syrinx;
• In a paper from 2013, 203 patients with arachnoid cysts were followed up, with only 5 patients presenting with enlargement of the cyst and only 2 developing symptoms that could be due to the arachnoid cyst. ^[1]

• Arachnoid cysts may, rarely, enlarge and complicate with the symptoms previously described such as hydrocephalus, seizures, hearing loss, visual changes and cervical myelopathy.

• In a very restricted selection of cases they may be surgically treatment, but surgery may occasionally result in complications including:
  • Subdural hygroma;
  • Subdural hematoma;
  • Pseudomeningomyelocele;
  • Subdural empyema;
  • Wound complications;
  • Meningitis;
  • Hydrocephalus;
  • Seizure;
  • Need for more surgical procedures after a failed initial surgical attempt.^[1]

• The prognosis of arachnoid cyst is often good, and in the majority of cases won’t demand treatment.
• Without treatment, in a few cases the symptoms may worsen over time.
• Surgical decompression of the arachnoid cyst yielded a reduction in 66% of the patients of less than 50% of the preoperative volume. 82% were asymptomatic after the procedure, while 12% reported no relief and 6% experienced worsening of the symptoms. ^[2]