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Antrochoanal polyp

contributed by Sandeep Bhuta on March 15, 2010

System: Head & Neck

An antrochoanal polyp (ACP) is a solitary polyp that arises within the maxillary sinus but passes through and enlarges the sinus ostium or more commonly an accessory ostium 6. The nasal cavity is therefore extended posteriorly into the nasopharynx through the posterior chonae. Similar polyps can arise in the sphenoid sinus and extend in to nasopharynx: these are termed sphenochoanal polyps.

Epidemiology

Antrochoanal polyps represent only approximately 3 - 6% of sinonasal polyps 3.  The exact aetiology is not known, but it is thought that infection may be a common causative association. Chronic sinusitis is found in approximately 25% of patients 7, but again, a causal relationship has not been firmly established. 

Unlike other sinonasal polyps, antrochoanal polyps are usually found in non-atopic patients 3. They are most commonly seen in yound adults and in 3rd to 5th decades. They are slightly more common in males compared to females 3.

Clinical presentation

Clinical presentation is usually with an obstructed nasal passage and/or sinus symptoms. Occasionally, larger masses may prolapse posteriorly enough that they may be visible through the mouth as they hang down from the nasopharynx 5. 

Pathology 

Pathologically, antrochoanal polyps are identical to other inflammatory polyps. However, unlike other polyps, they usually have a narrow stalk that arises from the maxillary sinus 5,7.  

Histologically, the polyp is lined by respiratory epithelium with increased inflammatory infiltrate. Due to the narrow pedicle, vascular compromise with secondary change may be seen including: haemorrhage, oraganising haematoma, neovascular changes, and papillary endothelial hyperplasia 5,7. 

Radiographic features

Plain film

Plain films are no longer considered adequate in assessment of sinus pathology. However, they continue to be performed in some cases. Features include 3:  

  • unilateral opacification of the maxillary sinus
  • nasopharyngeal mass is occasionally seen
  • frequently bilateral sinus involvement (23 - 42%) 3
CT

CT is a prefered method for diagnosis since it is able to give exquisite bony detail of the paranasal sinus anatomy. In general, a non-contrast scan suffices. Although classification system exists, detailed description is usually preferred (see classification of antrochoanal polyps). Typically, antrochoanal polyps have the following features: 

  • defined mass with mucin density is seen arising within the maxillary sinus
  • widening of maxillary ostium and extending in to nasopharynx
  • no associated bony destruction but rather smooth enlargement of sinus

These features are best appreciated on true coronal or coronal reformat scans. Although pathologically antrochoanal polyps have a narrow pedicle or stalk, this is usually not defined on CT. 

Occasionally, antrochoanal polyps may have a higher density and HU values if they are long standing and/or have an associated fungal infection (see fungal sinusitis) 1. A contrast scan is not necessary but may demonstrate peripheral enhancement.

MRI
  • T1 : intermediate to low signal
  • T2 : 
    • high homogeneous T2 signal 
    • signal may vary if they are chronic and/or if fungal infection is present 
  • T1 C+ (GAD) : peripheral enhancement is seen on post contrast images.

Treatment and prognosis

Classic treatment is intranasal snare polypectomy. However, if the base of the stalk is not excised, recurrence may occur. Ideally, minimally endoscopic surgery is performed; the attachement site of the stalk is identified and it is existed along with a small cuff of adjacent mucosa. Visualisation of the stalk base can be achieved in many cases via a medial meatotomy, or may require creation of an additional window through the inferior meatus 6. 

Recurrence rate if the stalk base is excised is low (~7%) 6. 

Etymology

They were first described by Killian in 1906. Killian expressed the opinion that it arose from the maxillary sinus and gained access to the nasal cavity through the accessory ostium, based on a detailed study of 22 cases in 1906 8.

Differential diagnoses