Mucoceles of The Paranasal Sinuses
Carrie A. Roller, M.D.
May 25, 2000

Langenbeck first described mucoceles in the early 19th century, but their history certainly goes further back. Canalis described a 3rd century AD skull with changes in the frontal sinus that indicated that the specimen probably had a mucocele. Until Rollet coined the term "mucocele" in 1886, these lesions were known as "hydatid cysts." from the Greek word for "a drop of water."

Mucoceles are defined as chronic, cystic lesions in the paranasal sinuses. They are lined by respiratory epithelium, the normal lining of the paranasal sinus, but are the result of obstruction of the sinus ostium, causing the accumulation of secretions into an expanding mass. Expansion and inflammation lead to remodeling and erosion of bone, which changes the bony architecture significantly. The contents of the mucocele are sterile, and if it does become infected, the lesion is then known as mucopyocele.

Related, but still distinct, lesions include sinonasal polyps and mucous retention cysts. A polyp is an inflammatory swelling of the sinonasal mucosa, and a mucus retention cyst is a mucinous gland whose duct has become obstructed, causing the accumulation of mucus in the gland.

Mucoceles are the most common expansile lesions of the paranasal sinuses. In contradistinction, sinonasal polyps are the most common expansile lesions of the nasal cavity. As I mentioned before, a mucocele develops once the drainage of a sinus is compromised. The sinus fills with mucus and as more mucus accumulates, the sinus may expand from the pressure. Sinus walls may be remodeled or completely de-ossified and eroded. Various factors appear to be involved in the bone remodeling process. Lund et al implicated various cytokines and inflammatory mediators. A group at UC Davis published an interesting study in which frontal sinus mucoceles were induced in cats through mucosal trauma. They then measured the pressure increase on the sinus wall from the expansion of the mucocele.

Mucoceles can affect any age group, and there is no gender bias. Patient history may or may not be revealing of a cause of the initial obstruction. Suspicious historical elements include prior facial trauma, surgery, allergies or inflammatory disease. Less commonly, there may be a sinonasal malignancy. Very frequently, no specific etiologic factor is identified. Very often, over a year will elapse between a presumed initiating event and development of a subsequent symptomatic mucocele.

Patients with a paranasal sinus mucocele can present with a variety of signs and symptoms. These can be as dramatic as facial deformity, proptosis or enophthalmos, loss of vision or diplopia. They may also be non-specific and subtle, such as facial pain, headache or nasal obstruction.

Clinically, histologically and radiographically, mucoceles can resemble other expansile processes of the paranasal sinuses, such as neoplasia or nasal polyposis. It s therefore necessary to view the process with all of these features in mind to make a diagnosis of a mucocele.

Ninety percent of mucoceles involve the frontal or the ethmoid sinuses, with the majority being in the frontal sinus. The remaining 10% involve the maxillary sinuses and, far less commonly, the sphenoid.

Mucoceles have a number of consistent radiographic features; however, none of these features is unique to the mucocele. Complete sinus opacification occurs with the sinus filled with a mucoid, low-density material. The bone of the sinus is generally remodeled from the processes of inflammation, which cause thickening and expansion, which tends to thin the sinus walls. Thickening and thinning may even be seen in the different areas of the same sinus. Often the mucocele will be seen radiographically to have expanded or herniated into adjacent structures, particularly into the orbit or the cranial cavity.

On plain films, the mucocele may appear as a clouding of the involved sinus. The normal thin mucoperiosteal white line becomes attenuated. Dense reactive bone may surround an involved sinus. The sinus may be expanded, and the normal scalloping of the larger sinus may be effaced.

CT is the preferred imaging modality for mucoceles. On CT, a mucocele appears as an expanded, airless sinus filled with homogeneous material. The walls of the sinus may be normal or may be remodeled, with thickening, thinning and erosion to various degrees, often within the same sinus.

On MRI, the mucoid contents of the mucocele have a distinctive diffusely high signal intensity.

It is worth mentioning mucous retention cysts again, as they differ significantly from mucoceles both clinically and radiographically. Mucous retention cysts develop from the obstruction of a submucosal mucinous gland and are found incidentally in about 10% of patients who have sinus films. By and large mucous retention cysts are of little to no clinical significance, except in the very rare instance in which it swells to fill an entire sinus. In a CT, the distinction between a mucocele and a mucous retention cyst can be made on the basis of the presence of air outlining the upper surface of the mass. The distinction becomes less important when the lesion is large, as the treatment is similar for such a large mucous retention cyst as it is for a mucocele.

Histologically, a mucocele is typically described as a cyst lined by flattened, pseudostratified, ciliated columnar epithelium. There is reactive bone formation lying in proximity to the cyst epithelium. Reactive changes, such as fibrosis, granulation tissue, hemorrhage and cholesterol granuloma may be seen, and in long-standing cases squamous metaplasia may even be seen.

Mucoceles may extend into adjacent structures by herniation. They may herniate internally, staying within the confines of their sinus of origin or they may herniate externally, extending outside of the sinus into surrounding structures such as the orbit or cranial cavity.

Histopathologic features of the mucoceles are similar in many respects to the appearance of non-specific sinusitis, mucus retention cysts and nasal polyposis

The treatment for mucoceles may be roughly divided into two categories: radical surgery and conservative surgery.

Radical surgery entails the complete extirpation of the mucus membrane with obliteration or cranialization of the sinus cavity. Conservative surgery involves marsupialization of the mucocele with creation or preservation of adequate sinus drainage in order to minimize risk of recurrence. Adequate long-term follow-up is made difficult in patients with mucoceles, as recurrences often occur many years later, after the patient has been lost to regular follow up.

The traditional treatment of frontal and maxillary sinus mucoceles has been the radical approach, particularly in cases complicated by drainage fistulae, osteomyelitis or intracranial infection. The rationale stems from fear of mucocele recurrence due to remnants of traumatized mucosa. Total extirpation of the mucosa theoretically virtually eliminates this concern; however, the obliteration of the sinus with fat makes monitoring for recurrent disease very difficult.

Radical approaches to the frontal sinus include the classic Riedel approach, which entails the removal of the anterior and inferior walls of frontal sinus and collapsing forehead soft tissue down onto the posterior table of the sinus. This leaves the patient with a significant cosmetic deformity in the form of a large indentation in the forehead.

In 1920, Lynch introduced an orbital approach to the frontal sinus through an external ethmoidectomy. The floor of the frontal sinus is removed along with the ethmoids and the middle turbinate. The Lynch operation is relatively simple and may be performed rapidly in patients with suboptimal medical status. It does, however, leave a cosmetic deformity, particularly with resection of the supra-orbital rim.

The first documented use of an osteoplastic flap was by Schonbern in the late 19th century. Since then there have been many refinements of this technique and in the 1950s Bergara added obliteration of the sinus with fat. This approach significantly improves cosmesis, but sacrifices the ability to monitor for recurrent mucocele. Imaging studies must be performed to evaluate the sinus cavity, and the presence of fat within the sinus makes reading the CT a challenge.

Conservative surgery for paranasal sinus mucoceles is based on the principle that the underlying disturbance in a mucocele is one of blocked osteal drainage and not of diseased mucous membrane.

Marsupialization of the mucocele with establishment of osteal drainage will acutely relieve the symptoms of the mucocele and in the long term prevent re-accumulation of the mucus.

Minimally invasive procedures have been used for those rare cases of sphenoid sinus mucoceles, due to the morbidity of performing a radical procedure in that anatomic area. The advent of endoscopic sinus surgery has increased the safety and efficacy of intranasal marsupialization, and has been gaining favor for over a decade as a viable option for the treatment of mucoceles in all the paranasal sinuses, including those with intracranial or intraorbital extension.

In 1989 Kennedy came out in favor of endoscopic management of mucoceles. He published a series of 18 frontal mucoceles, most of which were complicated in that they eroded into the cranial cavity, into the orbit or had developed a Pott's puffy tumor. Sixteen of these mucoceles he treated with endoscopic marsupialization and re-establishment of osteal drainage. After 42 months of follow up, he noted no recurrences. The two cases that required open procedures were extensive in nature and anatomically complicated. One of these cases was associated with diffuse nasal polyposis and massive intracranial extension and the other wrapped around the optic nerve.

Postoperative care recommendations vary; however, most authors recommend a few basics. Nasal endoscopy in the office is recommended for periodic reassessment for osteal patency and removal of debris. The patient may do saline irrigation at home, particularly if disease was extensive.

Recurrences appear to be few with establishment of adequate drainage; however, it must be remembered that recurrences may not occur until many years later.

In summary, mucoceles are accumulations of trapped mucus, which form cystic expansile lesions that often extend into surrounding structures. The traditional treatment has involved primarily radical removal of the mucosa and obliteration of the sinus. However, in recent years, advances in endoscopic sinus surgery have lead to an acceptance of simple drainage procedures, even for some seemingly very complicated mucoceles.

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