Disease Characteristics

Majority of arachnoid cysts occur during embryonic development. Arachnoid cyst articulates an estimated 1% of intracranial lesions. A male predominance is observed with a male to female proportion of ~2:1 [1,2]. Arachnoid cyst can be discerned within neonates and or the elderly population. Intraventricular arachnoid cyst is exceptional in children and absent in adults [1,2]. Of obscure aetiology, it is posited that arachnoid cyst is predominantly configured due to anomalous disruption of the arachnoid during embryogenesis [1,2]. Alternatively, an inflammatory or traumatic aetiology is postulated with the occurrence of an inflammatory cell exudate or deposition of excessive collagen or hemosiderin pigment [1,2]. Majority of arachnoid cysts emerge within anterior or middle cranial fossa adjacent to temporal lobe or within retro-cerebellar region [1,2]. Although locales are divergent, arachnoid cyst is predominantly and consistently exemplified within middle cranial fossa and retro-cerebellar sites [1,2]. Few arachnoid cysts appear within cerebral convexity and supra-sellar region. The cyst is exceptionally discovered within the cerebellopontine angle, intraventricular space, supra-cerebellar cistern, quadrigeminal cistern, ambient cistern, anterior fossa or brainstem [1,2]. Arachnoid cyst arising within the spinal canal is exceptional. No segment of vertebral column is exempt although thoracic spinal cord is commonly implicated [1,2]. Arachnoid cyst is usually situated within posterior segment of spinal canal although anterior lesions may be observed. Aforesaid cysts are associated with backache and fluctuating hemiparesis. Besides, mono-paresis, radicular pain, spastic quadriparesis, neurogenic claudication, sensory disturbances, monoplegia, incontinence, pain, paraesthesia and neurogenic bladder can concur [1,2].

Clinical Elucidation

Generally, arachnoid cyst is detected incidentally and may be symptomatic. Although predominantly miniature, enlarged arachnoid cysts can be incidental and asymptomatic [3,4]. Arachnoid cyst arising within middle cranial fossa and retro-cerebellar region are usually asymptomatic. Notwithstanding, arachnoid cysts confined to uncommon locations occur as symptomatic lesions [3,4]. Clinical features pertain to cyst magnitude and location and commonly appear as headache or seizures, especially in cysts confined to middle cranial fossa [3,4]. Arachnoid cyst engenders cogent clinical symptoms on account of concurrent mass effect cyst rupture [3,4]. Asymptomatic and symptomatic arachnoid cysts exhibit variable clinical symptoms. However, pertinent symptoms can be engendered with mass effect of an arachnoid cyst. Subjacent gliosis within adjacent brain tissue as a consequence of mass effect is usually absent [3,4]. Incidental arachnoid cysts can be discovered upon imaging of central nervous system adopted for headache, seizures, elevated intracranial pressure, hydrocephalus, possible stroke, hemiparesis, nausea, vomiting, dizziness, cranial nerve dysfunction, tinnitus, vertigo, delayed or regressed development, behavioural disorders, visual symptoms or trauma [3,4]. Exceptionally, arachnoid cyst engenders significant neurological symptoms as hydrocephalus, ataxia or impingement of cranial nerves [3,4]. Additionally, variously located arachnoid cyst may be associated with loss of vision, nausea, vomiting, macrocephaly, palsy of oculomotor (third) nerve, facial nerve or vagus nerve, trochlear dysfunction, trigeminal neuropathy, hemifacial spasms, sensory neuronal hearing loss, vertigo or neuropathy of vestibulocochlear cranial nerve [3,4]. Depression may occur due to significant mass effect pertaining to cyst location, a symptom which resolves with marsupialization of the cyst [3,4]. The symptomatic, enlarged, arachnoid cyst definitively engenders headache and seizures and can possibly be treated with cogent surgical manoeuvers [3,4]. Clinical symptoms are contingent to site of arachnoid cyst wherein a lesion arising within quadrigeminal plate cistern may compress trochlear nerve to induce trochlear nerve dysfunction. Arachnoid cyst impinging upon optic nerve may generate a deficit within ipsilateral visual field. A cyst confined to suprasellar cistern can induce bi-temporal hemianopsia. Arachnoid cyst confined to the occipital cortex activates homonymous hemianopsia [3,4]. Symptoms such as bobble-head doll syndrome, constituted of rhythmic motion of the head due to third ventricular dilatation and potential incrimination of cerebellum is extremely exceptional [3,4]. Arachnoid cyst is associated with specific syndromes such as Aicardi syndrome, mucopolysaccharidosis, acrocallosal syndrome, Marfan’s syndrome, a missense mutation (c.2576C>T) with arginine-glutamic acid dipeptide repeats gene (RERE) and Chudley-Mullough syndrome [3,4].

Histological Elucidation

Upon gross examination, arachnoid cyst can be significantly enlarged and cyst magnitude is variable. Cyst wall is attenuated, transparent and the cyst is permeated with clear or colourless fluid. Arachnoid cyst appears distinct from leptomeninges and dura mater [5,6]. Arachnoid cyst is encased within an attenuated membrane wherein cyst wall is predominantly and variably composed of arachnoid. Besides, fibrous tissue layered with simple epithelium or non-arachnoid, luminal epithelium with innumerable microvilli or cilia may be delineated [5,6]. Upon microscopy, cyst wall is configured of delicate, fibrous connective tissue superimposed with diffuse or focal layer of meningothelial cells [5,6].

Immunohistochemistry

Meningothelial cells are immune reactive to epithelial membrane antigen (EMA). Meningothelial cells are immune non-reactive to cytokeratin (CK), glial fibrillary acidic protein (GFAP), transthyretin or synaptophysin [5,6].

Figure 1: Arachnoid cyst exhibiting a cystic cavity lined with meningothelial cells and surrounding fibrous connective tissue(13).

Figure 2: Arachnoid cyst depicting a cavity layered with meningothelial cells encompassed within fibrous connective tissue(14).

Figure 3: Arachnoid cyst displaying a cavity coated with simple, non-ciliated epithelium and an encasing fibrous connective tissue(15).

Figure 4: Arachnoid cyst exemplifying a layer of arachnoid and surrounding loose fibrous connective tissue(16).

Figure 5: Arachnoid cyst enunciating a layer of arachnoid with subjacent fibrous connective tissue(17).

Figure 6: Arachnoid cyst delineating a layer of simple, non-ciliated epithelium with subjacent fibrous connective tissue(18).

Figure 7: Arachnoid cyst depicting a layer of arachnoid situated upon fibrous tissue whereas epithelial cells are immune reactive to glial fibrillary acidic protein(19).

Figure 8: Arachnoid cyst exhibiting a cavity permeated with cerebrospinal fluid and circumscribing fibrous connective tissue(20).

Differential Diagnosis

Arachnoid cyst requires segregation from lesions of central nervous system such as an epidermoid cyst, dermoid cyst, abscess, cystic neuro-cysticercosis or a lipoma. Demarcation may be possible with features encountered with pertinent imaging techniques [7,8].

Investigative Assay

Upon ultrasonography, arachnoid cyst can be discovered in utero, during childhood or adulthood. Computerized tomography (CT) is an optimal technique for discerning arachnoid cyst, especially fluid-filled cysts with an attenuated cyst wall which are confined to specified sites [7,8]. Magnetic resonance imaging (MRI) is recommended for appropriately evaluating anatomical location, cyst magnitude and anatomical structures abutting an arachnoid cyst. MRI is optimal in differentiating arachnoid cyst from specified lesions of the central nervous system. Haemorrhage within an arachnoid cyst is infrequent [7, 8]. Upon magnetic resonance imaging (MRI), arachnoid cyst exemplifies an attenuated cyst wall and is devoid of image enhancement. Besides, MRI is optimal in distinguishing cerebrospinal fluid from lipid- rich contents pervading epidermoid cyst and dermoid cyst [7, 8]. Computerized tomographic (CT) cisternography and magnetic resonance (MR) cisternography can be adequately performed for evaluation and confirmation of an arachnoid cyst along with assessment of contiguous spaces pervaded with cerebrospinal fluid [9,10]. Besides, phase-contrast magnetic resonance imaging (MRI) can be employed to evaluate an arachnoid cyst for contiguity with tracts permeated with cerebrospinal fluid [9,10]. Mass effect upon the brain engendered by arachnoid cyst and pertinent, implicated cranial nerves or vascular articulations necessitate evaluation. Arachnoid cyst confined to critical sites with accompanying hydrocephalus or an enlarged cyst contributing to mass effect and herniation of brain mandates investigation [9,10].

Therapeutic Options

Cogent therapy for an arachnoid cyst generating anatomical distortion with accompanying neurological symptoms may be unnecessary. Besides, majority of arachnoid cysts do not necessitate therapeutic intervention [11,12]. Surgical treatment is recommended for managing an arachnoid cyst engendering cogent clinical symptoms. Optimal surgical approach pertains to cyst localization and incrimination of circumscribing neurological structures. Surgical excision of cyst wall is accompanied by contiguity with subarachnoid space [11,12]. Surgical fenestration or endoscopic fenestration of cyst cavity abutting the ventricular system can be adopted [11,12]. Occasionally, a cysto-peritoneal shunt can be incorporated. Singular treatment modalities are employed for aberrant cyst locations such as percutaneous cyst aspiration through foramen ovale [11,12]. Surgical eradication of arachnoid cyst is usually curative. Nevertheless, cyst reoccurrence may ensue [11,12]. Emergency medical procedures are necessitated for treating significant hydrocephalus or cyst induced mass effect [11,12]. Complications of an arachnoid cyst are exceptional and contingent to cyst location and distortion of abutting anatomical structures [11,12].

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