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CC BY-NC-ND 4.0 · Asian J Neurosurg
DOI: 10.1055/s-0046-1815950
Case Report

Extracranial Frontal Arachnoid Cyst in an Adult Patient: Case Report

Authors

  • Shradha Maheshwari

    1   Department of Neurosurgery, HBT Medical College and Dr. R N Cooper Hospital, Mumbai, Maharashtra, India

  • Yash Aneesh Gangal 

    1   Department of Neurosurgery, HBT Medical College and Dr. R N Cooper Hospital, Mumbai, Maharashtra, India
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Abstract

Arachnoid cysts are a relatively common finding in the general population. Primary arachnoid cysts are congenital malformations and are commonly intracranial cysts. Secondary arachnoid cysts develop as a result of an osseous defect and inadequate dural closure. Intradiploic arachnoid cysts are rare lesions of the skull that are typically seen in posttraumatic cases. Extracranial arachnoid cysts, when suspected, should be studied thoroughly on imaging, and they should be explored beyond the extracranial compartment to provide permanent treatment of this problem. The dura should be explored even if the underlying bone looks intact because if not explored and a possible rent is not treated, the extracranial cyst has the potential to recur. To the best of our knowledge, this type of unusual pathology has not been reported earlier. We hereby report a case of successfully treated recurrent extracranial arachnoid cyst in an adult patient.


Keywords

arachnoid cyst - leptomeningeal cyst - extracranial - de novo arachnoid cyst - case report

Introduction

Arachnoid cysts are benign collections of cerebrospinal fluid (CSF) within the arachnoid layer.[1] They are often asymptomatic and discovered incidentally.[1] [2] The pathogenesis of these cysts is not well understood.[2] They are commonly thought to be a congenital anomaly or a result of trauma.[1] The location of arachnoid cysts is intracranial and only rarely intradiploic. De novo formation of arachnoid cysts is exceedingly rare, with very few cases reported in the literature, most of which have occurred in the pediatric age group.[1] [3] Although congenital arachnoid cysts are most often intracranial in nature, extracranial component, when present, often shows communication with the intracranial component through some defect. We report, to the best of our knowledge, the first case of a nontraumatic extracranial arachnoid cyst in an adult patient.

Extracranial arachnoid cysts, when suspected, should be studied thoroughly on imaging, and they should be explored beyond the extracranial compartment to provide permanent treatment of this problem. The dura should be explored even if the underlying bone looks intact because, if not explored and a possible rent treated, the extracranial cyst has the potential to recur.


Case Report

Initial presentation: Our patient, a 24-year-old female, presented with the complaint of a frontal, midline scalp swelling which was noticed when she was 4 years old. The swelling had gradually increased in size since then. She had no complaints of headache or any neurological symptoms. There was no preceding history of trauma. Local examination showed the presence of a well-defined, non-tender, fluctuating swelling. Neurological examination was unremarkable. CT imaging showed an extracranial cystic swelling in the frontal region with uneven underlying bone but no clear breach in the bone. She was operated on for cosmetic complaints.

First surgery: During her first surgery, the swelling was found to be an extracranial cyst filled with clear fluid. The underlying bony surface was found to be uneven, but no obvious defects in the bone were identified. The extracranial cyst was completely excised and sent for histopathological examination. Histopathology demonstrated the cyst to be walled by an intact arachnoid membrane, and this confirmed that the swelling was an arachnoid cyst. Following this surgery, the swelling had completely resolved.

Recurrence: The patient started noticing a small swelling at the operative site approximately 14 months following her first surgery. The swelling had gradually increased in size over the next 10 months. She had no other complaints or any neurological deficit. Local examination revealed the presence of a well-defined, nontender, fluctuating swelling below the previous surgical scar.

Imaging findings on recurrence: CT scan showed an extracranial cystic swelling in the frontal region ([Fig. 1A, B]) with uneven bone as seen on 3D recon images ([Fig. 1C]), but no clear breach in the bone. The undersurface of the bone looked eroded partially with a small fluid collection that displaced the superior sagittal sinus inferiorly ([Fig. 1D, E]).

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Fig. 1 (A, B) CT images showing eroded inner table and intact outer table of the skull. (C, D) CT images showing the extracranial frontal cyst and an intracranial FFC. (E) Uneven bone underlying the cyst. FFC, fluid-filled cavity; EC, extracranial cyst; UB, uneven bone.

Second surgery: The patient was planned for reexploration. An extracranial translucent cyst filled with clear fluid was seen after opening the skin with uneven underlying bone ([Fig. 2A]). The extracranial cyst was completely excised. Although no obvious defects in the bone were identified, clear fluid was visibly oozing out of the uneven bone. After removal of bone, a small dural rent was identified just lateral to the superior sagittal sinus through which CSF leaked under high pressure ([Fig. 2B]), and the inner table of the bone was found to be eroded ([Fig. 2C]). The dural defect was closed using a graft from the galea aponeurotica, followed by cranioplasty using titanium mesh. The excised cyst was sent for histopathological examination. Histopathology demonstrated the cyst to be walled by an intact arachnoid membrane, and this confirmed that the swelling was an arachnoid cyst. Radiological imaging was thought to be unnecessary following this surgery.

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Fig. 2 (A) Intraoperative images showing a thin membranous cyst containing clear fluid. (B) Parasagittal dural rent after bone removal. (C) Excised bone with eroded inner table erosion and an excised cyst. DR, dural rent; Eit, eroded inner table; EC, excised cyst.

Follow-up: Following her second surgery, the swelling had completely resolved—no new neurological deficits, no recurrence.


Discussion and Conclusion

Arachnoid cysts are a relatively common finding in the general population, yet their pathogenesis remains poorly understood.[2] Primary arachnoid cysts are congenital malformations and are commonly intracranial cysts.[4] According to Rengachary and Watanabe, these cysts arise from the congenital splitting of the arachnoid mater, resulting in subsequent CSF entrapment.[5] However, some theories suggest arachnoid cysts may develop as a result of impairment of CSF drainage generated by venous dysplasia or agenesis.[4] An increased prevalence of arachnoid cysts has been noted in several syndromes, including Chudley–McCullough syndrome, acrocallosal syndrome, and autosomal recessive primary ciliary dyskinesia.[2] Secondary arachnoid cysts have a traumatic etiology and have been reported after head injury, tumor surgery, or as a complication of skull base surgery, and meningitis.[5] [6] [7] [8] They develop as a result of an osseous defect and inadequate dural closure.[6] These are rare as compared with primary arachnoid cysts. Intradiploic arachnoid cysts are rare osteolytic lesions of the skull, which occur because of the extension of arachnoid villi through the bony defects and are typically seen in posttraumatic cases, likely associated with growing skull fractures.[9] [10] [11] The nontraumatic variety of intradiploic cyst is extremely rare.[12] [13] The characteristic finding in these lesions is an intact outer table of the skull, and the cyst communicates with the subarachnoid space through a defect in the inner table.[14] [15] These cysts develop as a diverticulum of the arachnoid membrane through small defects in the dura, and continuous pulsations of the herniated arachnoid erode the inner table and expand the intradiploic space.[16] Several reports of de novo arachnoid cysts have been described, most developed following trauma and/or intraventricular hemorrhage of prematurity.[17] [18] [19]

In our literature search, we could not find any case report documenting an extracranial arachnoid cyst without an underlying obvious bony defect. The case most similar to ours was reported by Anbazhagan et al.[20] They reported a rare case of a tri-compartmental arachnoid cyst presenting as posterior parietal meningocele.[20] This case reports a clear extracranial extension of the cyst with an externally visible swelling.[20]

Our case differs from previously reported intradiploic or traumatic cysts for the following reasons: our patient had no history of head trauma. Also, late presentation and the presence of an arachnoid layer covering the extracranial cyst without any bony defect suggest de novo formation of the cyst. Intradiploic cysts are intracranial pathologies, while our case describes an extracranial cyst. The inner table of the skull was eroded with minimal thinning of the entire thickness in spite of the chronic nature of the lesion. The patient had a pinpoint dural defect just adjacent to the superior sagittal sinus, just below the cyst, which could suggest a traumatic etiology, but the absence of a clear bony defect, preserved thickness of the underlying skull bone, and no intracranial expansion of the cyst eliminates trauma as the possible etiology of the cyst in our case. We hypothesize that the CSF most likely oozed out through minute channels of emissary veins in the vicinity of the dural rent, forming an arachnoid cyst extracranially. To the best of our knowledge, this type of unusual pathology has not been reported earlier. We hereby report a case of successfully treated recurrent extracranial arachnoid cyst in an adult patient.

Extracranial arachnoid cysts, when suspected, should be studied thoroughly on imaging, and they should be explored beyond the extracranial compartment to provide permanent treatment of this problem. The dura should be explored even if the underlying bone looks intact because if not explored and a possible rent treated, the extracranial cyst has the potential to recur.



Conflict of Interest

None declared.

Patients' Consent

Informed consent was obtained from all the participants of the study.



Address for correspondence

Yash Aneesh Gangal, MBBS
Department of Neurosurgery, HBT Medical College and Dr. R N Cooper Hospital
Mumbai 400056, Maharashtra
India   

Publication History

Article published online:
06 February 2026

© 2026. Asian Congress of Neurological Surgeons. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/)

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Zoom
Fig. 1 (A, B) CT images showing eroded inner table and intact outer table of the skull. (C, D) CT images showing the extracranial frontal cyst and an intracranial FFC. (E) Uneven bone underlying the cyst. FFC, fluid-filled cavity; EC, extracranial cyst; UB, uneven bone.
Zoom
Fig. 2 (A) Intraoperative images showing a thin membranous cyst containing clear fluid. (B) Parasagittal dural rent after bone removal. (C) Excised bone with eroded inner table erosion and an excised cyst. DR, dural rent; Eit, eroded inner table; EC, excised cyst.