Mucocele of the anterior clinoid process causing acute monocular amaurosis

This is an unauthorised machine translation into English made using the DeepL Translate Pro translator. The editors do not guarantee that the content of the article corresponds fully to the original language version.

Dear Editorial Board,

Pneumatization of the anterior clinoid process is an anatomical variation present in 9.2% of patients and may communicate with the sphenoidal, ethmoidal, or both paranasal sinuses [1]. Obstruction of this communication may result in retention of secretions and formation of a mucocele, with the risk of subsequent inflammatory or compressive neuropathy of cranial nerves II, III, IV, and VI due to their intimate proximity to the optic canal and the superior orbital fissure. The consequence may be severe visual impairment or oculomotor dysfunction [2,3].

Hereby, we describe the case of a clinoidal mucocele with a report of a 54-year-old female patient referred to our department from a regional neurology ward. Approximately three weeks after an uncomplicated respiratory infection, she developed a left-sided vision loss. Upon initial neurological examination, functional amaurosis was present on the left side. Subjectively, the patient perceived only purple lights at the periphery of the visual field. Direct and consensual pupillary light reflexes were preserved, and ocular motility was intact. Ophthalmologic examination showed no signs of optic disc edema. Perimetry using the HFA method demonstrated a pronounced visual field defect on the left (PD 18.71*, OD −2.42, MD −4.16, PSD 13.06). Initial working diagnosis by the neurologist was optic neuritis.

Initial CT and MR imaging revealed bilateral pneumatization of the anterior clinoid processes, with enhancing, presumably inflammatory changes on the left, consistent with a possible mucocele (Fig. 1A–D). CT angiography excluded a vascular lesion. Intravenous corticosteroid therapy administered by the neurologist (1g of methylprednisolone a day intravenously to a cummulattive dose of 5g) had no effect; the consulted ENT department did not recommend surgery. The patient was therefore referred to us for further evaluation. Due to the persistent visual deficit, we indicated surgical treatment after detailed patient counselling.

Under general anesthesia, we performed a standard left pterional approach with drilling of the greater sphenoid wing followed by extradural dissection along the lesser wing of the sphenoid. After transecting the meningoorbital band (Fig. 2A), we proceeded with dural dissection toward the base of the anterior clinoid process, where inflammatory bone erosion was present (Fig. 2B). After further dissection, we resected the remaining bony roof of the optic canal and then removed the inflammatory pseudotumor —⁠ a cystic structure filled with mucoid material (Fig. 2C). Following the removal of the cyst lining and taking a specimen for histological evaluation, the course of the optic nerve through this pneumatized segment of the sphenoid bone was clearly identified (Fig. 2D). Based on the intraoperative findings, we concluded that the patient had compressive-inflammatory neuropathy of the optic nerve caused by the mucocele. Its removal achieved satisfactory decompression of the nerve. Communication between the pneumatized clinoid remnant and the sphenoid sinus was closed using a muscle graft harvested from the temporalis muscle and secured with fibrin glue. Complete anterior clinoidectomy was not required. Hemostasis was achieved, a bone flap was fixed with microplates, and the wound was closed in a standard fashion.

The postoperative course was uneventful; the patient was discharged on postoperative day 3 with mild subjective improvement. At the 3-month follow-up, she reported significant improvement of left-eye visual function and return of vision to normal. Histology confirmed an inflammatory pseudotumor consistent with a mucocele. Control perimetry showed marked improvement (PD 3.84*, OD −1.26, MD −4.09, PSD 4.29). The wound healed per primam, and the patient was satisfied with the treatment outcome. Follow-up MRI at 6 months demonstrated an asymptomatic mucocele of the right anterior clinoid process (also present initially in the setting of bilateral pneumatization) and regression of the left-sided clinoidal mucocele (Fig. 1E–F). The patient remains symptom-free.

The development of a mucocele of the anterior clinoid process results from obstruction of drainage from the variably pneumatized cavity within the clinoid process. The incidence of pneumatization of the anterior clinoid processes ranges from 9.2% to 9.6% [1,4] up to 25.5% in a CT study of 597 patients by da Costa et al. [1,5]. Based on the extent of pneumatization, Abuyazed et al. distinguish Type I—<50% pneumatization of the anterior clinoid with an incidence of 6.6% in the population, Type II—>50% pneumatization (3.5%), and Type III—complete pneumatization (2.5%) [4]. Communication of the pneumatized cavity in the anterior clinoid process is typically present with the sphenoid (81.8%) or ethmoid sinus (10.9%), or with Onodi’s cells (the posterior part of the ethmoid sinuses penetrating into the sphenoid bone) [1,6]. Both of these paranasal sinuses communicate with the anterior clinoid in 7.3% of cases [1]. Pneumatization of the anterior clinoids typically does not occur before the age of 10 [5]; it may develop alongside the pneumatization of other peripheral parts of the sphenoid bone (dorsum sellae, processus pterygoidei) after closure of the spheno-occipital suture and regresses with increasing age [7]. The development of a mucocele in this area is therefore highly unlikely in pediatric and adolescent patients. Pneumatization of the anterior clinoids occurs via the so-called “optic strut” (type I –⁠ 74.5% of cases), via the base of the clinoid in the region of the planum sphenoidale (the so-called “anterior root” –⁠ type II –⁠ 14.5% of cases), or again by a combination of both of the aforementioned pathways (type III –⁠ 10.9% of cases) [1]. In our patient, the assessment of the origin of pneumatization was distorted by the presence of a mucocele, which had secondarily eroded the surrounding bone; however, it was likely a combined type of pneumatization (Fig. 1 D).

Obstruction of the ostium occurs due to mucosal thickening from recurrent inflammation or by bony overgrowth leads to increased intraluminal pressure and progressive expansion of the mucocele. Expansion causes thinning, remodeling, and sometimes even dehiscence of the bony walls of the anterior clinoid process.  This leads to compression of the optic nerve within the optic canal and development of inflammatory neuropathy [8]. The clinical picture of a symptomatic clinoidal mucocele is often nonspecific and may result in misdiagnosis—most commonly as retrobulbar neuritis [2,3,8]. Typical symptoms include retrobulbar pain, visual field defects with scotomas, and diplopia or even ophthalmoplegia due to compression of extraocular motor nerves in the superior orbital fissure [2,3,8,9].

Imaging plays a crucial role in diagnosis, although interpretation may be challenging. CT typically reveals an expansile lesion with remodeling or thinning of the anterior clinoid bone. MRI findings vary depending on the water and protein content of the mucocele. Early mucoceles are usually T2-hyperintense; with increasing protein concentration and chronicity, the lesion may become hyperintense on T1-weighted images and hypointense on T2. Mild post-contrast enhancement of the cyst wall is typical. The imaging appearance may mimic a thrombosed aneurysm [3,6,10]; therefore, CT angiography is useful to exclude a vascular lesion [3,10].

Conservative management of symptomatic clinoidal mucocele may be effective when neurological symptoms accompany an acute sinusitis and includes targeted antibiotic therapy combined with corticosteroids [11]. Due to the risk of delaying surgical treatment and subsequent risk of irreversible deficit, conservative therapy is not generally accepted [12]. If conservative measures fail or if acute severe deficits occur, surgical treatment is indicated. Recommended optimal timing window is within one week of symptom onset [13]. The goals are evacuation of the mucocele contents and lining, histological verification, and decompression of neural structures [9,12]. This may be achieved via transnasal endoscopic marsupialization into the sphenoid sinus [6,8,9,12] or via microsurgical resection, typically through a pterional approach, allowing direct decompression of adjacent structures, clinoid resection, and closure of communication with the sphenoid sinus [12,13].

In our case, the patient was referred with a substantial delay and already presented with a severe visual field deficit. To achieve direct optic nerve decompression in an advanced disease stage, we have decided for a microsurgical approach, which resulted in an excellent clinical outcome. This case highlights the importance of a proactive therapeutic approach in patients with clinoidal mucoceles.

Abbreviations:

HFA –⁠ Humphrey field analyser

PD –⁠ Pattern defect

OD –⁠ Overall defect

MD –⁠ Mean deviation

PSD –⁠ Pattern standard deviation

Figures:

Fig. 1: Initial MR examination –⁠ T1 weighted native (A) and contrast-enhanced image (B) in coronal plane depicting hyperintense lesions in both anterior clinoid processes enhancing after administration of contrast medium. Initial CT in axial (C) and coronal (D) plane depicting pneumatisation of bilateral anterior clinoid processes with erosion of bony margins of left-sided optic canal. Postoperative MR in T1 native (E) and contrast-enhanced (F) sequence displays regression of left-sided mucocele.

Fig. 2: Intraoperative finding: A –⁠ disconnection of meningoorbital band, B –⁠ drilling of lesser wing on the sphenoid bone showing pneumatised cavity inside of the anterior clinoid process, C –⁠ mucoid contents of the cavity, D –⁠ decompressed optic nerve after exstirpation of the mucocele