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Silent sinus syndrome: systematic review and proposal of definition, diagnosis and management

SUMMARY Silent sinus syndrome (SSS) is a rare disease consisting of a collapse of maxillary sinus walls with concomitant orbital floor descent. Due to its rareness, the literature highlights some confusion on its definition, diagnosis and management. A PRISMA-compliant systematic review was performed on SSS with focus on definition, diagnosis and therapeutic management. Twenty-eight studies were selected, with 276 patients evaluated. The analysis revealed that the leading definition of SSS includes evidence of both enophtalmos and maxillary atelectasia. Although the definition of SSS accepts only spontaneous sinus collapse, the presence of sinonasal diseases and history of facial trauma are starting to be included in the criteria. Most studies (n = 21) considered CT scans satisfactory for diagnosis of SSS, while 7 also performed MR. The majority of SSS were successfully treated with isolated functional endoscopic sinus surgery (n = 17), sparing orbital reconstruction as a rescue procedure in case of non-satisfactory long-term resolution of signs. Although the literature is starting to coordinate on diagnosis of SSS, our review revealed the necessity of consensus on its definition and management.


Introduction
Silent sinus syndrome (SSS) or imploding antrum syndrome is a very rare condition, usually consisting of asymptomatic spontaneous collapse of the sinus walls and floor of the orbit 1 . Due to its rarity, the literature has often retrospective and prospective studies were included, while case reports and small case series were excluded because of their intrinsically lower level of evidence (the minimum number of patients was arbitrarily set at 3). Published reviews on SSS were similarly excluded, but their reference list was reviewed to identify possible additional studies. A manual search in the reference lists of these articles was performed to identify potentially relevant papers missed during the database search. Differing opinions were resolved by consensus between the two authors. Data extracted and analysed included study design, sample size, mean patient age, diagnostic criteria, associated factors, instrumental diagnosis, surgical approach and timeline, outcomes, complications and follow-up time.

Results
The systematic review retrieved 28 original articles 3,4,6,7,10-13, [16][17][18][19]21,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] . Twenty-one were retrospective case studies (RCS), while 7 were case reports reporting at least 3 cases. A total of 276 patients were evaluated with a mean of 9.8 cases per study (range 3-57). Mean age was 40.4 years, although 3 papers did not report the age of participants (Tab. I). There was a wide variety of diagnostic criteria among studies (Tab. II). The most frequently required findings for diagnosis were evidence of enophthalmos (En) (25 studies), maxillary atelectasia (MA) (23 studies) and hypoglobus (15 studies), while post-traumatic, facial asymmetry and diplopia evidence were less required (respectively, in 3, 3 and 1 studies) as seen in Table II. Moreover, 7 studies did not directly define diagnostic criteria, which were extrapolated from the manuscript. Nine studies included patients with sinonasal symptoms; 6 included post-traumatic cases, and 1 had post-surgical SSS (Tab. I). Twenty studies diagnosed SSS with CT alone, while 7 performed both CT and MR imaging. One study did not define what imaging was performed (Tab. I). Surgical approaches consisted in combined FESS and OR in 72 patients, mostly in a one-step surgery. Four studies performed both types of timing-choices (one step or two steps 6 months apart), deciding according to the individual case. FESS alone was performed in 116 patients; other less common therapies were a wait & see approach (57 patients), OR alone (8 patients) and antibiotic therapy (1 patient) (Tab. III). A study by Khon et al. did not define their therapeutical approach in the 22-patient case series 29 . All patients had clinical resolution (CR) or clinical improvement (CI). Only Lin and Brown reported 2 patients who had FESS surgery alone that did not resolve enophthalmos, who then underwent subsequent OR 13,16 . Complications are reported in Table I.

Discussion
SSS is a relatively rare disease that most ENT specialists and ophthalmologists know as an entity, but the sporadic evidence leads to ambiguity on many of its aspects, from definition, to diagnostic criteria to treatment.

Definition
Our systematic review revealed that among the 28 studies evaluated, the most required diagnostic criteria were evidence of enophthalmos (25 studies) and maxillary atelectasia (MA) (23 studies), with 21 requiring both signs. Hypoglobus observation follows as essential criteria in 15 studies, also with evidence of orbital floor remodeling at CT scans (11 studies) and the absence of sinonasal diseases (10 studies) (Tab. II). Therefore, even if there is much misunderstanding around the definition of SSS, it is possible to assume that most authors agree in having both enophthalmos and MA as necessary criteria. Only 2 studies required only MA in the absence of enophthalmos, considering CMA and SSS as a same entity 4,17 . Post-traumatic silent sinuses were originally excluded from the definition of SSS 18,19 , and some recent papers continue highlighted the confusion around its definition, diagnosis and proper management. The aetiology of SSS remains controversial. Patients typically deny preexisting sinus disease or orbitofacial trauma. Most authors postulate that collapse of the inferior orbital wall is induced by negative pressure generated by resorption of gas after a natural ostium occlusion that may occur during the first or second decade of life 2,3 . Clinical appearance usually consists in asymptomatic enophthalmos and altered facial appearance. Nevertheless, diplopia, sinusitis, rhinorrhoea, post-nasal drip, facial pressure, or pain may also be present 1,4 . SSS and Chronic Maxillary sinus Atelectasia (CMA) are terms which have been used interchangeably; the latter can be defined as a persistent and progressive decrease in maxillary sinus volume secondary to inward bowing of the antral walls 5 . Some authors define CMA as SSS, while others sustain that the two are separate clinical entities 6,7 . For example, according to Ende et al, it is possible to consider three stages of CMA: the last, group III, is defined by clinical facial deformity with the onset of ocular disturbances, and may be also defined as SSS 8 . Brandt et al. also argued that SSS should be considered a subtype of CMA 9 . The original definition of SSS dates to 1994 and included only spontaneous enophthalmos not associated with prior trauma or surgery 2 . Nevertheless, many authors have started accepting wider inclusion criteria involving more aetiologies (idiopathic, post traumatic, and iatrogenic) and sustain that they should be included in the definition of SSS 10 . Treatment is also a subject under continuous re-evaluation. At first, surgical treatment consisted in Caldwell-Luc sinus surgery with inferior meatotomy and concurrent transconjunctival repair of the orbital floor. In 1993, Blackwell et al. described endoscopic maxillary antrostomy (FESS: functional endoscopic sinus surgery) in conjunction with a transconjunctival orbital floor repair (OR) in three patients and reported resolution of maxillary disease on follow-up and no recurrence of enophthalmos 1 . Nowadays, the leading treatment of SSS, in order to restore the natural ventilation of the maxillary sinus, appears to be FESS 11 . Instead, the main controversy in literature remains the necessity and timing of the orbital floor reconstruction via a subconjunctival approach. Some authors prefer a one-stage approach, with endoscopic antrostomy and reconstruction of the orbital floor performed at the same time 10,12 . Others believe that a two-stage approach is more convenient, with a delay between antrostomy and the orbital reconstruction of 6 months 13 . Still others consider FESS alone to be the best choice of therapy with less invasiveness and satisfactory results, with no need of OR 6 .
To shed light on these controversies, we performed a systematic review of the literature on SSS, with particular focus on definition, diagnosis and surgical approach.

Materials and methods
A PRISMA-compliant systematic literature review 14 was carried out in December 2020 on the Web of Science, Pub-Med and Scopus databases, using a search strategy for "(Silent Sinus) AND (Maxillary)" and "(Silent Sinus Syndrome) AND (Maxillary)". We included studies focused on SSS with the following criteria. Data extraction and quality assessment Two of the authors (CR and CP) independently screened the retrieved studies based on title and abstract; when uncertainty existed in the abstract evaluation, we retrieved and assessed the full text. After completion of all searches, duplicates were removed. Evaluation through full-text screening was then carried out. Critical appraisal led to the selection of 28 studies (Fig. 1). Both continues ▶  of anatomical impairment and diplopia, if present. Among these, the majority (n = 9) performed both the procedures simultaneously, while 4 preferred to first carry out FESS surgery and to observe a possible progressive improvement of the enophtalmos to decide whether or not to perform additional OR. Eventually, 3 studies performed FESS and either simultaneous or delayed OR according to the criticality of the case (Tab. III). The reasons which guide each surgical team towards one choice or another are multiple. The main factor is undoubtedly the severity of enophthalmos and hypoglobus, although no study defined a quantitative cut-off which could help in the surgical decision. OR approach is supported by the fact that, although there have been reports of resolution of the progression of enophthalmos by antrostomy alone, it is unclear whether other aesthetic deformities caused by SSS as hypoglobus or superior sulcus deformity also respond. Behbehani et al. believe that delaying orbital implant placement in cases with significant enophthalmos and hypoglobus is unjustified since complications like diplopia or infection are rare with this procedure. Furthermore, simultaneous implant placement also obviates the need for additional anaesthesia and hospitalisation 25 . Cobb et al. maintain that FESS alone may stop the descent of the orbital wall, but there would be no reason to expect that the position of the orbital floor, and thus the globe, would be reversed 21 .

Inclusion criteria
On the contrary, Thomas et al. support delayed repair of the orbital floor as in some patients enophthalmos improves with antrostomy alone 22 . Moreover, OR has not been shown to provide any significant restoration in the orbital muscle functions, and because of that diplopia is not corrected 6 .
A total of 116 patients underwent FESS surgery alone, being considered as a necessary and sufficient procedure to resolve MA and enophthalmos (Tab. III). If we consider the entire case series of the review, FESS alone appears as the leading therapeutic choice, with only 7 reporting residual enophthalmos (6%), and 4 requiring subsequent OR (3.4%). Moreover, Numa et al. undertook a review of 84 cases and concluded that for patients with SSS diagnosis, uncinectomy alone may be sufficient 24 . FESS + OR follows with 72 cases treated with this management. Wait & see (57 patients) may be a valid alternative in asymptomatic cases or young population 13 . OR alone (8 patients) and antibiotic therapy (1 patient) remain marginal therapeutic options (Tab. III). This leads us to the conclusion that most of SSS may be successfully treated with isolated FESS surgery, sparing OR as a rescue procedure in case of non-satisfactory resolution of enophthalmos or diplopia.
Treatment proposal FESS is the leading treatment for SSS and orbital reconstruction should be performed only in selected and symptomatic cases or, if needed, in a second approach when no resolution is seen. The proposed timepoint for a potential postponed OR, according to the literature, is 6 months. Our review clearly shows the need to develop consensus regarding the definition of SSS and most of all its management. Even if the literature has started to find marginal consensus in recent years, it appears necessary to define shared diagnostic criteria, as well as a shared approach to the best treatment choices with the lowest rate of invasiveness and morbidity. The literature would also considering post-traumatic cases not definable as SSS due to their lack of spontaneous development 10,20 . Nevertheless, a retrospective analysis of 6 cases and literature review showed how traumatic SSS management follows the same principles as for spontaneous SSS 21 . In fact, our analysis revealed that recent papers started to include enophthalmos with MA secondary to traumatic events in the group of SSS, explaining that clinical and radiological presentations are comparable, as well as surgical treatment 13,21,22 . Moreover, the association of SSS with sinonasal symptoms is debated. Ten studies considered the evidence of sinonasal symptoms as an exclusion criterion because MA secondary to chronic sinusitis with sinus dysventilation and sinonasal symptoms is, by those authors, considered as CMA and not as SSS 9 . At the same time, the majority of studies do not clarify this point, and include among SSS subjects who have associated factors such as sinusitis, chronic nasal congestion and other sinonasal symptoms of obstruction, even though these are not considered diagnostic criteria (Tab. II). Finally, 7 studies did not specify diagnostic criteria, which were extrapolated from the clinical description of cases. This risks producing inaccurate studies given that the literature lacks in consensus in SSS diagnosis and the reader may find its definition confusing. From our analysis, it appears that the leading definition of SSS should include the evidence of enophthalmos and maxillary atelectasia. We believe that presence of sinonasal diseases and history of facial trauma may be reasonable factors to be included in anamnestic data because of the comparable clinical presentation and treatment.

Definition proposal
Our proposal for definition is: • criteria to diagnose SSS are contemporary presence of enophtalmos and maxillary atelectasia; • minor associated factors may be the presence of sinonasal diseases and history of facial trauma.

Diagnosis
Most studies only required CT for imaging investigation, since the relevance was to investigate maxillary bony walls atelectasia with possible resorption of the orbital floor. Nevertheless, 7 groups also considered it useful to add MR scans (Tab. I) to evaluate: • dislocation of extra and intra-conical fat, extrinsic ocular muscles and optic nerve; • differentiation between mixed signal central secretions and high signal peripheral; • thick edematous mucosal lining within the maxillary sinus 6,18,23 .

Diagnostic proposal
Only CT scan is required. MR imaging can be associated in selected cases, and specifically to evaluate a marked hypoglobus. We propose to add evaluation of extraocular muscle movement and potential diplopia at first ENT clinical evaluation, in order to select symptomatic and therefore the most critical cases 10,20 .

Treatment
The review also showed disagreement on management strategies. Seventeen authors proposed combined FESS and OR surgery as the leading therapy to obtain resolution