Arachnoid cysts are benign congenital collections of cerebrospinal fluid. Arachnoid cysts are two types: (1) congenital cysts (true arachnoid cyst); (2) secondary arachnoid cysts [5–7]. Secondary (acquired) arachnoid cysts result from hemorrhage, trauma, and infection and usually communicate with the subarachnoid space . The common locations of arachnoid cysts are the surface of the brain at the level of main brain fissures, such as sylvian (49%), interhemispheric fissures (5%), cerebral convexity (4%), vermis (9%), cerebellopontine angle (11%), suprasellar and sella turcica (9%), and supracollicular area (10%)1. Intraparenchymal location in an adult is rare; only one case was reported in English literature till now .
The genesis of arachnoid cyst is controversial, and it is sporadic or familial. Different hypothesis are discussed like (a) it arise due to agenesis of part of the brain, or (b) due to minor aberration in the development of arachnoid that leads to splitting or duplication of the membrane (endomeninx) followed by collection of CSF (1), or (c) defect in condensation of the mesenchyme or from abnormalities of CSF flow [3, 8, 9]. Sylvian fissure arachnoid cyst and suprasellar arachnoid cyst  developed due to the splitting of membrane followed by one-way CSF flow (valve mechanism) due to vascular pulsation from surrounding vessels (sylvian: MCA, suprasellar: basilar) .
The cell of origin in primary intraparenchymal is still unknown. It was extrapolated that it originates from cellular remnants because of embryonic malformation as it was seen in intramedullary arachnoid cysts . Bayrakli et al.  reported genetic linkage of the intracranial arachnoid cyst and located chromosome 6q22.31-23.2.
Regarding cyst progression, three prevailing theories exist: (1) active secretion of fluid by cells in the cyst wall, (2) fluid influx due to an oncotic pressure gradient, and (3) trapping of fluid by a valve mechanism [6, 9]. Controversy exists about the content of cyst either CSF or different from CSF. Berle et al.  reported in the arachnoid cyst that Na-K-Cl co-transporter NKCC1 is upregulated. Hence, fluid content is slightly differing from normal CSF. Protein content is also high in cyst fluid compared to CSF . Helland et al.  also mentioned the increased expression of NKCC1 gene and suggested NKCC1 inhibitor for prevention of cyst expansion.
However, in our case, there was no communication seen with CSF space radiological image or during surgery. So unlikely ball valve mechanism is responsible for cyst expansion. Probably, the fluid influx is the mechanism for cyst expansion as we have not done the NKCC1 expression.
The clinical manifestations of arachnoid cysts are variable and depend on location . They cause neurological deficit through expansion that can compress normal neural tissue and obstruct cerebrospinal fluid flow. The most common presenting symptoms and signs are those of raised intracranial pressure, craniomegaly, and developmental delay in children . Hydrocephalus has been estimated to be present in 30–60% of patients with arachnoid cysts. In our case, the patient presented with features of the intrinsic mass lesion with compression of surrounding brain parenchyma at the age of 40 years with 2 years symptoms. Park et al.  reported a 65-year-old lady presented at 65 years of age with a headache. Computerized tomography (CT) with contrast studies, magnetic resonance imaging (MRI), radioisotope scintigraphy, and cine phase-contrast MRI imaging are the different tools of investigation for arachnoid cyst  to diagnose the cyst and differentiate from other primary intraparenchymal cyst and look for communication with CSF space. Radiologically, we considered ependymal cyst, neuroenteric cyst, and dermoid/epidermoid as a differential diagnosis in our case.
Management options of arachnoid cysts are craniotomy with resection of the cyst walls, a shunting procedure, stereotactic aspiration or fenestration of the cyst cavity, and neuroendoscopic fenestration [1, 3, 9]. Park et al.  reported craniotomy and excision of cyst wall, and 6 months follow-up showed no recurrence. El-Ghandour et al.  reported a series of endoscopic management of intraparenchymal arachnoid cyst in children. In our case, we managed with craniotomy and complete excision of the cyst. No communication was seen with ventricle or subarachnoid CSF space. However, for another parenchymal cyst like an ependymal/neuroenteric cyst, the chance of recurrence is high if not removed completely.
The primary intraparenchymal arachnoid cyst is to be considered as a differential of the intraparenchymal cystic lesion in adult population. The prognosis is favorable after complete surgical excision.