Role of imaging in nasal polyposis
Imaging plays a crucial role in the diagnosis and management of nasal polypi. The following are some of the important contributions imaging is supposed to make:
It clinches the diagnosis
It helps in evaluation of progression of disease
Helps in surgical planning
Helps in monitoring for recurrence
CT scan is the primary imaging modality used to evaluate patients with chronic sinusitis with nasal polyposis. MRI has only a limited role to play and is used only sparingly. Imaging may be really vital and could even replace diagnostic nasal endoscopy in patients whose nasal cavities are completely filled with polypi and is virtually impossible to perform diagnostic evaluation using a nasal endoscope. This scenario is tailor made for imaging.
CT appearance of nasal polypi:
They appear as rounded bodies of soft tissue arising from the mucosal surfaces of nose and paranasal sinuses. They can be clearly differentiated from the surrounding inflammed mucosal lining and nasal secretion as they are more radio dense and hence appear brighter. Rarely a pedicle attaching the polypoidal mass to the nasal mucosal lining can be seen clearly in the CT scan (pedicle sign). If present it is virtually diagnostic of nasal polypi.
One important point that should be borne in mind while evaluating CT images from a patient with nasal polypi is that they never cause bone erosion. If soft tissue mass arising from the nasal mucosa is associated with bone erosion then it is a definite pointer towards the diagnosis of malignancy. Pressure effects of nasal polyp can be evidently seen in imaging. These effects include local bone remodelling causing a scalloping effect. This scalloping effect should not be confused with that of the scalloping of margins produced by the mucocele since it is always associated with enlargement of the sinus cavity. Rarely this bone remodelling may occasionally cause thinning of the bony septa of the ethmoidal sinus. This thinning could be so extreme that it could go even below that of the resolution of the CT scan. This creates a picture of bone erosion which is not a true one.
Appearance of nasal polyp when contrast CT is taken:
Nasal polypi do not show enhancement on injection of contrast media. The mucosa surrounding the nasal polyi may show enhancement causing an impression of rim enhancement around the nasal polypi.
Types of nasal polyp:
Ethmoidal polypi – arising from ethmoidal sinus and are multiple. They can be visualised in the CT scan of paranasal sinuses as multiple polypoid lesions. Polypi arising close to the cribriform plate area can cause olfactory disturbances.
Polyp arising from the maxillary sinus – is usually solitary. It exits the antrum via the natural / accessory ostium. This causes an enlargement of ostia. Radiologically it appears like a dumbbell because of the constriction present in the midline (ostial exit point). In these patients the medial wall of the maxillary sinus bows into the nasal cavity. This can be clearly visualised in the CT scan images. Obstruction caused by this polyp to the drainage channels of ethmoidal and frontal sinuses (middle meatus) can cause opacification of those sinuses also there by making it difficult to identify the exact origin of the nasal polyp. In this scenario the bone remodelling that takes place in the medial wall of maxillary sinus could be the clincher. If these polyp passes posteriorly to exit via the choana it could be clearly visualized in the axial cuts taken at the choanal level.
Fungal disease may coexist with nasal polypi. If present they could be visualized as hyperdense areas between the nasal polypi shadows.
CT differences between acute sinusitis & nasal polypi:
Acute sinusitis causes a near uniform opacification of the paranasal sinuses whereas nasal polypi inaddition to the opacification show multiple convexities.
Mucous retention cyst can be safely eliminated if the polypo shows a pedicle radiologically. If there is associated bone remodelling then in all probability it could be nasal polyp rather than mucous retention cyst. In case of diagnostic dilemma MRI will clinch the diagnosis.
In cystic fibrosis in addition to the radio densities seen in the CT scan images there is also associated thickening of the maxillary sinus walls due to osteoneogenesis.