Definition

A mucocele is an epithelium-lined sac completely filling the paranasal sinus and capable of expansion,1 as opposed to an obstructed sinus which simply contains mucus.

Etiology

Prevailing theories of mucocele formation include obstruction and inflammation, but these states occur in many more patients than those who develop a mucocele. The normal metabolic process of bone resorption and regeneration alters in favor of resorption, leading to expansion;2,3 others have suggested that it is pressure erosion that results in the bone erosion,4 but the histological appearances are against this (see below). In either case it is an active process that can be accelerated by additional infection, or exacerbation of preexisting infection, when a “pyocele” forms. Cystic degeneration of glandular tissue has also been proposed, but if this occurs one would expect to see early stages before the cyst completely fills the sinus and a double wall to the mucocele. The only circumstance when this might occur is in the “mucoceles” described quite frequently 16 to 19 years after Caldwell-Luc procedures in the Japanese.5 In the Western literature, cysts are found as incidental and asymptomatic phenomena in the maxillary sinus, the rarest site of all for a mucocele,6 in over 30% of normal individuals, so this proposition seems highly unlikely.

Some bone-resorbing factors have been shown in mucocele mucosa, including PGE2, leukotrienes, and a range of cytokines.2 Compared with normal controls, chronic rhinosinusitis, and mucosa from obstructed sinuses, IL-1α, IL-1β, and tumor necrosis factor-α (TNF-α) are increased and there is upregulation of the vascular adhesion molecules e-selectin, and I-CAM.3 This suggests that more than just obstruction of sinus outflow is needed for a mucocele to form. It is therefore proposed that following obstruction, superadded infection leads to chronic inflammation mediated by bacterial antigens, which in turn leads to an acceleration of bone resorption outstripping bone formation; but it is still unclear what predisposes an individual to this process.

Thus, while it is not clear why mucoceles form when sinus inflammation in general is so common, in ~2/3 of patients a possible initiating factor can be determined (Table 18.2).7 This includes various forms of trauma to the sinus outflow, often related to road traffic accidents and other pathology such as osteomas and Paget′s disease.8,9 However, the largest group comprises chronic rhinosinusitis, nasal polyps, and allergic/eosinophilic fungal disease, and the contents can be composed of a variety of secretions including the “peanut-butter” of fungal material and polypoid tissue itself. Many of these patients have also undergone previous sinus surgery, often multiple, and one-quarter have had external frontoethmoidectomy. There are also patients who have undergone surgery for nonsinus conditions such as orbital decompression and oncological surgery. There is often a significant time lag between the event and presentation with the mucocele. In the case of surgery or trauma, a mean time of 23 years elapses as compared with 22 months after an acute infection.7

Synonyms

The first description of a pyocele possibly relates to the case of Francis I, king of France (1494–1547).10 Lagenbeck11 described a “hydatid” in the sinuses in 1819 and in 1896 Rollet used the term “mucocele.”12

Incidence

Mucoceles are relatively rare given the proposed etiology and there is no predisposing factor in at least one-third. Around 10% are bilateral and 8% are multiloculated, usually associated with nasal polyps. Our personal cohort comprises 266 cases which is largely the result of referrals from the United Kingdom′s postgraduate ophthalmic hospital, Moorfields, with which we have had a longstanding relationship. Other large series are shown in Table 18.3.

Site

(See Table 18.3.)

Mucoceles most often affect the frontal sinus and frontoethmoidal region, but individual ethmoidal cells (mainly anterior), the sphenoid, and occasionally the maxillary sinus can be affected. A concha bullosa can also house a mucocele.

In our series the majority occur in the frontal and frontoethmoidal region (86%), followed by the ethmoid (8.6%). As in other series, the maxilla is the rarest site (2%), but the sphenoid may be commoner than our 3.4% as they may also present to neurology. This distribution may relate to the complexity of drainage from the respective sinuses but is not otherwise easy to explain. The right and left sides are affected equally.

Diagnostic Features

Histological Features and Differential Diagnosis

The mucosal lining is generally a pseudostratified columnar epithelium with some squamous metaplasia, goblet cell hyperplasia, and a cellular infiltration dependent on the degree of chronic or acute-on-chronic inflammation. This includes neutrophils, eosinophils, macrophages, monocytes, and plasma cells. Fibroblastic activity is increased, as is vascularity of the submucosa.27 With time, the epithelium becomes more flattened or cuboidal but not atrophic as might be expected with high intramucocele pressure or keratinization, which if present in the maxilla would suggest an odontogenic keratocyst. The underlying bone shows woven and lamellar bone, with frequent osteoclasts and osteoblasts, indicative of the active bone turnover and remodeling.

The differential diagnosis includes fungal disease, cholesterol granuloma, benign and malignant neoplasms (schwannoma, inverted papilloma, mucinous adenocarcinoma), and, in the maxilla, dental or odontogenic cysts. Sphenoidal mucoceles must be distinguished from nasopharyngeal and pituitary lesions.

Natural History

The natural progression of the mucocele is slow expansion over many years, often unrecognized by the patient but brought to light by someone who has not seen them for some time and who notes the cosmetic change. An acute infection can result in rapid change, which can precipitate presentation. Up to 17 mm of axial proptosis has been observed in some elderly individuals, with minimal visual symptoms.

Treatment

In the vast majority of cases mucoceles in all the sinuses can be adequately marsupialized using an endoscopic approach, providing the contents are liquid enough to drain, as it is not necessary to strip away the mucosal lining.28 Exceptions to this can therefore be a frontal sinus where there may be other pathologies such as a large osteoma or a “fungocele” compartmentalized in the most lateral part of the frontal sinus or severe polyposis, although both of these latter conditions may be managed by surgery combined with aggressive medical management. Furthermore, the mucocele itself may be in a compartment of the frontal sinus that lies lateral to the midpoint of the orbit. In these cases, a small external incision may assist drainage in the rare circumstances when a binasal Draf III approach is not adequate. Occasionally, in the case of multiloculated “mucopolypoceles” often associated with aspirin-sensitive asthma (Samter′s triad), an osteoplastic flap may be required (Fig. 18.2). Previous surgery and sclerotic bone may also make an endoscopic approach technically impossible (Table 18.4). If an external approach is used, the lateral support of the frontonasal recess must not be undermined by removal of bone as this will result in medialization of the orbital contents and is the main reason why the Lynch-Howarth type procedures fell into disrepute.

The standard endoscopic approach under general anesthesia is utilized with wide marsupialization of the mucocele.17 Image-guidance may assist for some frontal mucoceles but is rarely necessary. Once the mucocele is opened a large quantity of mucus can be aspirated; this may comprise various components depending on the precipitation of the contents into clear or proteinaceous fluid. In frontal mucoceles, this, combined with transmitted pulsation from the dura if bone is missing posterosuperiorly, can be quite alarming when first encountered as it immediately suggests a CSF leak. However, the situation rapidly becomes clear as the flocculating, colored, and thicker contents appear!

Once the mucocele sac has been opened, usually with a sickle knife or cutting instrument, the opening should be made as large as the anatomy will allow. This can be done with circumferential cutting mushroom punches, through-biting instruments, or powered instrumentation. Specimens for microbiology should be taken but this is positive in only <30% and, if fungus is suspected, histopathology of the secretion should also be requested.

In most cases there will be areas where erosion of bone exposes orbital periosteum or dura covered only by mucosa. Gentle pressure on the eye will readily demonstrate this. There is absolutely no necessity to repair these areas as there is no risk of CSF leak or meningitis unless mucosa is stripped away. In younger patients, evidence of recalcification has been observed at these areas of interface.

As the mucocele is drained, the position of the eye will normalize. The degree to which this happens will depend on how much bone is missing. Usually there is significant improvement immediately, but there will be continued improvement as the bone remodels over subsequent months. The degree of diplopia following endoscopic surgery is much less than that after external procedures, due in part to the more progressive nature of improvement but also to the lack of disturbance of the trochlea region, which frequently occurs with conventional external frontoethmoidectomy. Lund and Rolfe29 reported diplopia in 7/22 (32%) of a prospective study of mucocele patients undergoing Lynch-Howarth procedures. Consequently, patients should be warned that diplopia may occur, or may worsen if preexisting. This can be improved by wearing a prism on spectacles postoperatively and occasionally strabismus surgery is required. This has never been needed after endoscopic drainage but was undertaken in one of our patients having external operations in a prospective study of 22 patients.

No packing is required and virtually all patients treated endoscopically are day cases.

Routine postoperative medications such as saline douching and intranasal steroids can be given, but it is important to see the patient within the first few weeks to check that the opening into the mucocele remains widely patent.