Giant intradiploic arachnoid cyst for 13 years.

A case of intradiploic arachnoid cyst is reported. The patient presented with a progressively enlarging swelling situated over left frontal region for approximately 13-years following blunt trauma to head. Magnetic resonance imaging showed an intradiploic fluid containing cyst having intensity like cerebrospinal fluid (CSF). He underwent craniotomy and successful surgical repair. Intraoperatively CSF cyst was located in the frontal pole with a large defect over inner table and large rent in the dura. It was lined with arachnoid membrane. Pertinent literature is reviewed in brief.

Introduction

Intracranial arachnoid cysts are considered pathologically developmental benign anomalies of arachnoidal membrane. Expansive cerebrospinal fluid (CSF) accumulation within the diploic space is an exceedingly rare occurrence. Intradiploic CSF accumulations can be categorized into two distinct categories. First, when no traumatic origin can be identified and second as a result of traumatic skull fracture or related to surgical intervention - these are commonly labeled as intradiploic CSF fistula.

Intradiploic CSF cysts are rare lesions of the skull. Different nomenclature used to describe this lesion in the literature includes intradiploic CSF fistula,[1] intraosseous leptomeningeal cyst[23] traumatic and posttraumatic arachnoid cyst,[45] intradiploic arachnoid cyst (IAC).[6] However, these nomenclatures are used for both categories either traumatic or non-traumatic. We are reporting an interesting case of IAC associated with trauma. The common features of both types are presence of intact thinned out outer table of skull vault, CSF-containing cyst lined by arachnoid membrane, and defect in the inner table and associated rent in the dura causing continuity of CSF flow.

Case Report

This 14-year adolescent sustained a fall at the age of 1-year, which was managed conservatively. Gradually the parents noticed progressive swelling over the left side of forehead associated with occasional mild to moderate headaches. However, the swelling showed rapid progression over last 2 years and caused a huge bulge over left frontal region resulting in a gross cosmetic deformity.

On examination, approximately 10 × 8 cm hard globular swelling was noticed over the left frontal-temporal region extending inferiorly to superior orbital margin and medially toward midline and superiorly just 2 cm behind the hair line. Rest of examination including neurological examination was normal. Computerized axial tomography (CT) scan of head revealed expansion of both table of left frontal bone pole with intact thinned out outer table and huge expansion of the intradiploic space. However, a gliotic hypodense area was observed in medial frontal lobe [Figure 1]. Magnetic resonance imaging (MRI) brain findings corroborated including intradiploic space expansion with fluid filled lesion, the contents having similar signal intensity to CSF signal in T1-weighted (T1-W) and T2-W MRI sequences and fluid attenuation inversion recovery sequence [Figure 2]. Medial frontal lobe also showed presence of gliosis. Provisional diagnosis of posttraumatic IAC was made and planned for surgical repair.

Figure 1

Preoperative computed tomography brain showing deformity of frontal bone caused by large intradiploic arachnoid cyst with gliosis of medial frontal lobe

Figure 2

Preoperative magnetic resonance imaging brain T1-weighted image axial showing deformity of frontal bone caused by large intradiploic arachnoid cyst with content signal is similar to cerebrospinal fluid

Under general anesthesia, bicoronal scalp incision given and subgaleal flap raised. Pericranium elevated separately. Left frontal craniotomy made using drill and the bone flap separated. The outer table was found to be papery thinned out with porous architecture. An irregular defect of about 3 × 4 cm size was present in the inner table with an underlying dural defect of about the same size and herniation of arachnoid mater and gliotic brain tissue into it. The herniating leptomeninges and gliotic brain tissue were repositioned and the dural defect was closed in a water-tight fashion using pericranium graft, reinforced with fibrin glue. Cranioplasty was carried out with autologus calvarial split-bone graft and scalp closure done [Figure 3]. He had uneventful recovery.

Figure 3

Postoperative noncontrast computerized tomography scan head showing acceptable skull reconstruction using autologous split calvarial graft

Histopathological examination of the specimen revealed arachnoid membrane as covering of the cyst wall. He was doing well at last follow-up at 1 year following surgery with better cosmetic appearance.

Discussion

Intradiploic arachnoid cysts are characterized by accumulation of CSF in a sac, whose covering is lined by arachnoid membrane situated within the diplopic space, and have exceedingly rare occurrence. IAC was first reported by Weinand et al. in 1989.[7] Posttraumatic IAC was also called as CSF-diploic fistula,[1] intraosseous leptomeningeal cysts[2] and posttraumatic arachnoidal cyst.[458] Exact mechanism is not clear. Many postulates are put forward. It is postulated to be caused by calvarial injury in early childhood causing only fracture of inner table and dura matter, however sparing outer table of calvaria. With passage of time, small arachnoid diverticula pass through defect in inner table which due to repeated pulsation of CSF causes enlargement of herniated diverticulum and associated defect in inner table with expansion and thinning of both table of clavaria causing cosmetic defect and occasional headache.[5910] Our case had history of trauma at age 1 year. Patil and Etemadrezaie proposed a recoil theory-as the trauma was caused by a blunt object at low velocity, it led only to fracture inner table, which is relatively thinner. The fractured edges protruded inward, leading to tear of dura and arachnoid, and then recoiled back creating negative pressure at the fracture site, which causes sucking of the arachnoid mater through dura rent into the intradiploic compartment.[11] Over time, CSF pulsation can be mechanism of enlargement of size of the cyst.[612]

Growing fracture of the skull is a well-known clinical entity.[1213] It occurs in infancy and early childhood. Basic underlying pathology includes traumatic fracture of the skull vault with rent in underlying dura extending to arachnoid membrane. The defect in the bone involves both tables of the skull and is filled with a leptomeningeal cyst or herniated brain tissue.[101213]

Pathologically IAC can be differentiated from growing skull fracture as outer table is intact in former, while complete fracture of both table of calvaria with dural tear are necessary for growing skull fractures development. However, growing skull fractures are more commonly observed in young children.[10] However, IAC has been frequently reported in adult, and due to long time interval, patient might not remember initial injury.[17]

Commonly IAC is located in occipital bone, thick bone and associated large musculature might protect fracture of outer wall of calvaria, helping in formation of IAC.[710] However it can occur in the parietal or occipital bone.[45] Dunkser and McCreary hypothesized thicker occipital bone prevented fracture of the outer table, resulting in only fracture of the inner table with sequential evolution of herniation of arachnoid membrane through the defect in the inner table.[2] Lyle Hillman et al. attributed thick muscles overlying the injured bone cushion the outer table so spared.[3]

However, our case is unique in view of location in the frontal region and causing gross deformity of forehead.

Regarding size of defect in inner table through which arachnoid outpouching occurs can vary in number and size. Weinand, et al., reported presence of multiple lesions with very small dural openings, however, it was not associated with trauma, and size of lesions was not progressive. However, our case had sustained trauma, there were solitary dural rent and inner table defect was relatively larger, and the lesion progressively increased in size. Regarding content it is usually CSF with arachnoid lining, however in many cases arachnoid membrane gets hypertrophied due to infection. However content may very rarely even contain herniated gliosed brain.[711]

Common neuroimaging include X-ray of skull showing egg shell expansion of diploic space with intact outer table. CT scan head can delineate extent of bony defect, intactness of the outer table and three-dimensional reconstruction aids in surgical planning and associated brain parenchymal pathology is visualized. MRI is neuroimaging of choice and helps in diagnosis of IAC by excluding dermoid cyst, epidermoid cyst and rarely hemangioma, or eosinophilic granuloma. In IAC, signal intensities are similar to CSF in both T1-W image and T2-WI.

Surgical management is usually considered treatment of choice and indicated for large disfiguring swelling and persistent headache.[914] Main aim is watertight dural closure using duraplasty and cranioplasty using autologous split calvarial graft can be considered. Surgical management is successful in management of cosmetic disfigurement with good outcome.