Discussion

A review of the literature revealed a limited number of publications addressing FESS outcomes in chronic rhinosinusitis patients with a hypoplastic maxillary sinus. The majority were case reports, with a small number of case series.^{Reference Salib, Chaudri and Rockley7,Reference Ho, Wong, Gunaratne and Singh9,Reference Hanna, Levine, Sanford and Kotton10} In this retrospective study describing a single surgeon's experience, FESS was found to be a safe and effective approach in the management of hypoplastic maxillary sinus related sinusitis.

The aetiology of maxillary sinus hypoplasia remains unknown. Obstruction of the maxillary sinus ostium secondary to chronic rhinosinusitis is commonly proposed as the main pathogenesis.^{Reference Jang, Kim, Lee and Kim11} Maxillary sinus hypoplasia is usually associated with changes in the uncinate process configuration, which affects the sinus ventilation and physiological mucociliary drainage pathway.^{Reference Wang, Jiang and Gu12} The maxillary sinus is ventilated normally during the expiratory phase of respiration. The structure of the uncinate process may be essential to direct the expired air into the sinus cavity.^{Reference Sirikçi, Bayazit, Gümüsburun, Bayram and Kanlikana13}

Embryonic developmental abnormalities leading to the arrest of sinus pneumatisation and trauma are other possible aetiological factors.^{Reference Bolger, Woodruff, Morehead and Richardson3} Kosko et al. reported five children who developed maxillary sinus hypoplasia after undergoing FESS for chronic rhinosinusitis management.^{Reference Kosko, Hall and Tunkel14} The authors suggested that changes in maxillary sinus volume after endoscopic surgery may be due to the activation of bone formation within the maxilla and the removal of pneumatisation centres, caused by sinus inflammation and or surgical trauma.^{Reference Kosko, Hall and Tunkel14}

Maxillary sinus hypoplasia is a relatively uncommon condition. Using plain roentgenograms, Karmody et al. reported the prevalence of maxillary sinus hypoplasia as 8.9 per cent, with the cases being bilateral in 7.2 per cent and unilateral in 1.7 per cent.^{Reference Karmody, Carter and Vincent4} In another study using conventional radiography, the prevalence of maxillary sinus hypoplasia was 7.2 per cent, being bilateral in 50 per cent of cases.^{Reference Bassiouny, Newlands, Ali and Zaki5} Bolger et al. found the prevalence of unilateral hypoplastic maxillary sinus to be 10.4 per cent on coronal CT scans.^{Reference Bolger, Woodruff, Morehead and Richardson3} A retrospective study on 1293 patients who underwent CT for sinonasal complaints evaluation found that 6.4 per cent had maxillary sinus hypoplasia.^{Reference Jang, Kim, Lee and Kim11} In a series reported by Erdem et al., maxillary sinus hypoplasia was detected in 18 of 280 patients (6.4 per cent), with a total of 24 maxillary sinus hypoplasia cases in these 18 patients; the maxillary sinus hypoplasia presented unilaterally in 12 cases and bilaterally in 6 cases.^{Reference Erdem, Aktas, Erdem, Miman and Ozturan6} We found a hypoplastic maxillary sinus in 5 per cent of all patients who underwent FESS for chronic rhinosinusitis. Our results are consistent with the literature; however, interpretation of previous publications should consider the differences in inclusion and exclusion criteria, and the various diagnostic and analysis methods.

Bassiouny et al. were the first to attempt classification of maxillary sinus hypoplasia.^{Reference Bassiouny, Newlands, Ali and Zaki5} These authors used four standard plain X-ray views to divide maxillary sinus hypoplasia into the following categories: (1) isolated case caused by developmental problems, or secondary to infection or trauma; (2) associated with regional abnormalities involving the first branchial arch; and (3) related to systemic disorders.

Bolger et al. used coronal CT scans to classify maxillary sinus hypoplasia into three types, according to the degree of maxillary sinus pneumatisation and the status of uncincate process configuration.^{Reference Bolger, Woodruff, Morehead and Richardson3} Sirikçi et al. recommended adding the criteria of orbital size to Bolger type II and III maxillary sinus hypoplasia.^{Reference Sirikçi, Bayazit, Gümüsburun, Bayram and Kanlikana13} We adopted the Bolger classification, as it is the most widely accepted grading system, and it describes the peri-operative evaluation and surgical management for maxillary sinus hypoplasia patients with sinusitis.

Patients with a hypoplastic maxillary sinus present with non-specific sinonasal symptoms. Jang et al. found that these patients had more than one complaint that was not distinguishable from typical sinusitis symptoms.^{Reference Jang, Kim, Lee and Kim11} Post-nasal drip and nasal obstruction were the most common clinical presentations. Our results are consistent with the symptoms profile described by Jang et al. The majority of our patients had more than one sinonasal-related chief complaint. However, nasal obstruction and headache were the most common reported symptoms in our series. This difference may be explained by the high prevalence of anatomical variations reported in our cohort.

We found that the maxillary sinus hypoplasia patients, who underwent FESS, frequently had anatomical variations in the sinonasal region, with nasal septum deviation and ipsilateral middle turbinate concha bullosa being the commonest. Similar findings were observed by Sirikçi et al.^{Reference Sirikçi, Bayazit, Gümüsburun, Bayram and Kanlikana13} Other types of anatomical abnormalities in middle turbinate lamellas were also found to be high.^{Reference Al-Qudah15} The close relationship between the lateral nasal wall and the maxillary sinus in terms of embryological development and environmental factors can explain such associations. These anatomical variations can further limit endoscopic visualisation and the accurate identification of surgical landmarks, and should be carefully evaluated prior to FESS.

The medical management of sinusitis associated with maxillary sinus hypoplasia is usually ineffective, and FESS is advocated as a definitive therapy.^{Reference Salib, Chaudri and Rockley7–Reference Ho, Wong, Gunaratne and Singh9} Jang and colleagues reported their medical treatment outcomes of 23 hypoplastic maxillary sinus patients with sinusitis who had a patent osteomeatal complex.^{Reference Jang, Kim, Lee and Kim11} All patients were administered clarithromycin (250 mg, once per day), with a mean treatment time of two months, and antihistamine (10 mg, once per day). The symptoms completely resolved in only nine patients (24 per cent).

Thick, viscid mucopurulent secretion was aspirated in 10 of our patients; 5 of them grew MRSA resistant to many antibiotics. Methicillin-resistant S aureus isolated from intranasal cultures, which was essentially absent before the year 2000, has become significantly more common in the last two decades.^{Reference Rujanavej, Soudry, Banaei, Baron, Hwang and Nayak16} The prevalence of MRSA in refractory chronic rhinosinusitis is estimated to be less than 10 per cent.^{Reference Manarey, Anand and Huang17} The high rate in our series could be explained by the possible frequent and multiple different antibiotics that may have been prescribed prior to careful evaluation and correct management.

The primary surgical goals in managing chronic rhinosinusitis are to restore sinus aeration and a natural drainage pathway, with preservation of healthy mucosa. Endoscopic infundibulotomy and opening of the maxillary sinus natural ostium are the surgical methods of choice to achieve these objectives in hypoplastic maxillary sinus patients with chronic rhinosinusitis. However, proper identification of dissecting structures and avoidance of orbital injury are challenging because of the laterally displaced uncinate process and medial maxillary sinus wall, the obliterated infundibulum, and the close relationship with orbital contents. The classical FESS steps in these patients should be tailored to pre-operative CT scan and intra-operative findings, with the employment of additional helpful techniques to avoid potential serious complications. Although a posterior fontanelle antrostomy is not usually recommended, as it could lead to mucus circulation between the surgical antrostomy opening and natural ostium of the maxillary sinus, in type II and III maxillary sinus hypoplasia patients with associated sinusitis, there is no natural anterior ostium and this complication cannot occur.^{Reference Salib, Chaudri and Rockley7}

The difference between a hypoplastic maxillary sinus, chronic maxillary atelectasis and silent sinus syndrome is unclear and confusing in the literature. Maxillary sinus hypoplasia is an anatomical variant detected on CT. It is usually asymptomatic, but the sinus could become infected and chronic rhinosinusitis could develop.

Chronic maxillary atelectasis is a descriptive term that refers to a persistent decrease in the sinus volume of the maxilla associated with inward bowing of the antral walls. It is diagnosed based on clinical and radiological findings. Kass et al., in 1997, suggested two criteria for the diagnosis, based on the presence of: (1) persistent maxillary sinus opacification of at least three months, or an intra-operative finding of tenacious mucus secretion filling the sinus; and (2) lateral displacement of the medial infundibular wall and fontanelle, as documented on clinical examination, endoscopic exploration or CT scan, with or without inward bowing of the osseous walls.^{Reference Kass, Salman, Rubin, Weber and Montgomery18} These authors classified chronic maxillary atelectasis into three stages according to the degree of wall collapse. In stage I, the fontanelle and medial infundibular wall were laterally positioned. In stage II, one or more osseous walls of the maxillary sinus bowed inward. In stage III, marked deformation of the sinus walls manifested clinically as enophthalmos, hypoglobus and/or midfacial deformity. The different Bolger classification grades of hypoplastic maxillary sinus and chronic rhinosinusitis fit within stage II chronic maxillary atelectasis. Sinonasal symptom severity is inversely related to chronic maxillary atelectasis stage. Chronic maxillary atelectasis stages I and II are commonly associated with sinonasal symptoms, whereas stage III is often asymptomatic and presents with orbital deformity.

The clinical characteristics of silent sinus syndrome are almost identical to those of stage III chronic maxillary atelectasis.^{Reference Ende, Mah and Kass19–Reference Ho, Wong, Gunaratne and Singh21} It appears that stage III of chronic maxillary atelectasis may in fact be the final pathway that brings about the asymptomatic spontaneous enophthalmos observed with the entity referred to as silent sinus syndrome.^{Reference Kass, Salman, Rubin, Weber and Montgomery18}