Paranasal sinus mucoceles are benign cystic tumors characterized by a surrounding layer of nonneoplastic epithelium [1]. These mucoceles typically contain fluid material, and if an infection is present, they may develop into a pyocele [2]. The causes of sinus mucoceles are still debated. They may originate from chronic inflammation in obstructed sinuses [2], be linked to other benign tumors, or occur due to postoperative complications such as synechia formation. There are two main theories regarding the etiology of mucoceles. The primary mucocele theory posits that obstruction of the mucous gland ducts in the sinus mucosa leads to the formation of retention cysts, which then evolve into mucoceles. Conversely, the secondary mucocele formation theory suggests that thickening of the sinus ostium mucosa due to factors such as traumatic injury, tumors, or surgery leads to obstruction of the nasal sinus outlets, resulting in mucocele formation [2]. Secondary mucoceles are generally more common than primary mucoceles [3]. The accumulation of mucus within obstructed sinus spaces, due to various etiologies, leads to the formation of mucoceles [4].

Mucoceles typically expand over time, filling the available space and often causing destruction of the surrounding bone tissue. They most frequently affect the frontal and ethmoid sinuses, and less commonly, the sphenoid and maxillary sinuses. Although the destruction of surrounding bone tissue is a common consequence of mucocele growth, instances where the growth leads to invasion of the skull base and subsequent cerebrospinal fluid (CSF) leakage are rare. In this report, we describe a case of CSF leakage that occurred during the endoscopic marsupialization of a frontal ethmoidal mucocele that had invaded the skull base.

A 24-year-old male patient presented with right proptosis and right periorbital pain that began 5 months ago. He was diagnosed with chronic sinusitis and nasal polyps 12 years prior, for which he underwent bilateral endoscopic sinus surgery and nasal polypectomy under general anesthesia at a local hospital. Following the surgery, he experienced intermittent episodes of purulent rhinorrhea. An ophthalmic evaluation was prioritized and revealed right proptosis and developing diplopia on left gaze. The patient was subsequently referred to our tertiary hospital for further assessment.

A nasal endoscopic examination was conducted at the outpatient clinic, revealing adhesions between the right inferior turbinate and the nasal septum (Fig. 1A), as well as swelling of the right middle turbinate (Fig. 1B), both presumed to be consequences of previous surgery. Additionally, purulent discharge was noted around the left middle turbinate (Fig. 1D). No other significant findings were observed during the nasal examination (Fig. 1).

Preoperative paranasal sinus computed tomography (CT) scans were performed (Fig. 2). The scans revealed findings consistent with chronic rhinosinusitis in the bilateral frontal and ethmoid sinuses (Fig. 2A and C), as well as a mucocele centered on the right ethmoid sinus (Fig. 2B). The swelling around the right middle turbinate was presumed to be induced by the mucocele (Fig. 1B). This mucocele was displacing the right orbit and invading it, causing lateral displacement of the right eyeball (Fig. 2C). Consequently, the right medial rectus muscle was also displaced laterally (Fig. 2D). Although there was suspicion of a partial meningocele in the right sphenoid sinus, no clear evidence of a basal skull defect was observed (Fig. 2C).

Endoscopic sinus surgery was performed under general anesthesia to treat a mucocele in the right ethmoid sinus and chronic rhinosinusitis in both sinuses. The mucocele extended from the right ethmoid sinus to the right frontal sinus (Fig. 3A). Using a curette, the anterior wall of the right mucocele was opened, creating a small hole through which the purulent contents were drained using a suction device. The upper part of the mucocele, which had formed a calcified wall obstructing the right frontal sinus, was also removed with a curette, allowing for drainage of the frontal sinus. Following this, continuous purulent secretion was aspirated from the frontal sinus ostium, succeeded by the flow of clear fluid from the defect on the posteromedial side of the frontal sinus ostium. Navigation tools confirmed the presence of a basal skull defect measuring 1×0.5 cm (Fig. 4).

To facilitate proper attachment of the grafted nasal mucosa, the surrounding mucosa at the leakage site was cleared, and the exposed bone was smoothed to a flat surface. Cartilage from the septum was then harvested, trimmed to the appropriate size, and used to reinforce the base of the skull with a two-layer graft (underlay and overlay). Mucosa from the nasal floor was also harvested in sufficient quantity to completely cover the lesion. This mucosa was placed over the septal cartilage and sealed with tissue adhesive (fibrin glue). The area was then compressed using absorbable gelatin sponge packing material (Gelfoam^®) and nonabsorbable packing material (Merocel^®). A mucocele was also discovered in the left ethmoid sinus, extending to the left frontal sinus. It was successfully treated with endoscopic fenestration.

Postoperatively, the patient was kept on strict bed rest for 5 days to stabilize the repaired basal skull area, during which no CSF leak was detected. After this period, no complications or discomfort were noted, and the patient was subsequently discharged. A follow-up examination at the outpatient clinic 2 months later revealed complete resolution of the right eye swelling and pain, with no recurrence of symptoms. The endoscopic evaluation confirmed stabilization (Fig. 5).

In this case, the etiology of mucocele formation was likely secondary to adhesions from previous surgery. Several studies have reported the occurrence of mucoceles following sinus surgery. Busaba and Salman [5] noted that mucocele formation can manifest as early as 1 year and as late as 13 years postsurgery. The development of mucoceles after surgery is believed to be caused by adhesions that obstruct mucous drainage. Additionally, the invasion of mucosa into surrounding tissues during surgery may contribute to the formation of mucoceles.

Preoperative CT scans showed a slight elevation of the basal skull but did not clearly reveal any bone defects (Fig. 2C). However, during the surgery, a bone defect measuring 1×0.5 cm was discovered. Preoperative CT scans are commonly used to evaluate anatomical structures and are usually interpreted in the coronal plane. In this instance, the bone defect was situated between the image intervals, which may have led to it being missed preoperatively. Consequently, when there is any suspicion of basal skull erosion due to a mucocele, it is crucial to meticulously examine the axial thin section planes in CT images used for navigation to accurately assess bone defects.

The presence of CSF leak associated with mucoceles is rare. Although mucoceles can cause bone erosion due to chronic pressure, a dural defect is not commonly observed. However, CSF leakage can still occur during the removal of a mucocele adjacent to the dura, even without direct compression, due to minor manipulation of the nearby mucosa [6]. In this case, the CSF leakage likely resulted from a similar mechanism, where high negative pressure was used to extract the content of the mucocele through a small hole. Consequently, when a mucocele invades surrounding structures, clinicians should always consider the potential for basal skull erosion. During endoscopic fenestration, it is crucial to avoid rapid pressure changes by carefully using a suction device. Furthermore, when clearing secretions around the skull base, creating a larger opening for mucus drainage and reducing suction pressure can help prevent iatrogenic CSF leakage.

While mucoceles can occasionally lead to bone defects in surrounding tissues, severe cases may extend these defects to the basal skull. It is crucial to consider the possibility of tissue defects during surgery, particularly when assessing the size of a mucocele. This case highlights the importance of a comprehensive physical examination, radiological evaluation, and consideration of potential tissue defects during surgical management of mucoceles.