Hypo and anosmia: causes and imaging aspects H. ZAGHOUANI BEN ALAYA, Z. ACHOUR, W. BEN AFIA, W. KARMANI, S. MAJDOUB, H. AMARA, D. BAKIR, CH. KRAIEM HN26

Purpose: The aim of this work is to highlight imaging aspects of the common causes of smelling loss or weakness especially on MRI.

Patients and method we conducted a retrospective review of 7 patients who had come with a chief complaint of anosmia. All patients had undergone an imaging evaluation by either MRI or CT. the causes were: nasal polyposis (3 cases), head injuries (2 cases), kallmann syndrome (2 cases), and olfactory meningioma (1 case)

RESULTS: The etiology of olfactory loss is varied.

NASAL POLYPOSIS: Axial and corona CT images: polypoid masses associated with partial pansinus opacification and infundibular widening.

OLFACTORY MENINGIOMA:

A well circumscribed extra axial dura based lobulated mass at the floor of anterior cranial fossa in the mid line overlying cribriform ethmoid. Lesion is soft tissue signal intensity slightly hyperintense to cortical grey matter, no cystic component. (a, b and c) T2 hyperintense perilesional odema in adjacent brain parenchyma. Homogenous enhancement on post contrast T1. (d) a a b c d

The gyrus rectus and medial orbital gyrus are normal. kALLMANN SYNDROME: coronal T2 images through the frontal lobes demonstrate abnormal anatomy with absence of the olfactory bulbs and sulcus. The gyrus rectus and medial orbital gyrus are normal. 

Coronal T2-weighted MR image through the anterior fossa. olfactory bulbs are absent and the left olfactory sulcus is hypoplastic.

DISCUSSION:

Smell a disorders are common in the general population Smell a disorders are common in the general population. Although these disorders can have a substantial impact on quality of life and may represent significant underlying disease Based on the site of the pathology, clinical olfactory deficits are classified as one of three types: transport, sensory, or neural. Imaging has an important role in diagnosis.

Simplified diagram of cortical regions thought to be involved in the processing of olfactory information as it passes from the olfactory epithelium to the brain.

SINO NASAL POLYPOSYS: nasal polyposis does not appear to be a single disease entity but may correspond to a uniform reaction of the nasal mucosa to a variety of stimuli. Since polyps result from fluid accumulation and have an hypocellular nature, they demonstrate a fluid density on CT. Symptomatic polyposis occurs in 1% of the population Histogically, they differ from mucoperiosteal thickening of chronic sinusitis in that they have fewermucus secreting glands and a disordered vascular bed but these differences cannot be revealed by CT imaging.

CT scan is the preferred imaging technique for the diagnosis of polyps and the evaluation of the extent of polyposis. In clinical practice, sinonasal polyposis diagnosis is usually assessed by endoscopy but CT scan is frequently performed to help evaluate the disease.

IMAGING: The major CT features are polypoid masses associated with partial or complete pansinus opacification and infundibular widening. Less frequent CT characteristics include polyps within the individual paranasal sinuses, attenuation of the bony ethmoid trabeculae Truncation of the bony middle turbinate is also a feature of polyposis which may be present iny60% of patients. When polyposis is more severe, bony changes take on a destructive appearance.

CT is important to confidently diagnose sinonasal polyposis, to appreciate its extent and severity, and to properly evaluate deeper pathology which is not visualized by endoscopy. In this respect, coronal CT should be performed when endoscopy fails to explain symptoms reported by the patients. Isolated, unilateral polyposis should be regarded with suspicion and also requires CT because it suggests malignancy. When medical therapy has failed or when steroid treatment is contraindicated, surgery can be elected and pre-operative assessment includes CT examination.

OLFACTORY MENINGIOMA:

By virtue of their subfrontal location, GM may become very large prior to producing symptoms. Personality changes, such as apathy and akinesia, can be common when the tumors grow to large sizes . Onset of these symptoms is gradual, and they may not be observed early in their course. Other common symptoms include headache and visual deficits. Interestingly, anosmia is noted in hindsight by a significant number of patients, although it is not a common primary complaint.

IMAGING: CT: Computerized tomography scanning is particularly useful for defining the osseous anatomy, including areas of hyperostosis or erosion that may assist in the diagnosis or planning of a surgical approach to these lesions. Meningiomas typically appear slightly hyperdense relative to the brain parenchyma on non contrast CT scans and enhance homogeneously and brightly after administration of contrast. Paranasal sinus extension through the floor of the anterior cranial fossa is well demonstrated on CT scans, particularly on coronal views.

MRI: Both MR imaging and MR angiography will define the relationship of the tumor to the optic nerves and chiasm as well as the anterior cerebral arteries and communicating complex. Meningiomas commonly appear isointense to gray matter on T1-weighted sequences and iso- or hyperintense on T2-weighted sequences. Dense enhancement after administration of Gadolinium is also seen.

Posttraumatic olfactory dysfunction: The prevalence of posttraumatic anosmia ranges from 24% to 30% among patients who have sustained severe TBI (traumatic brain injury). In overall, about 5% of all patients admitted to hospital with a TBI is known to have anosmia. The precise cause and mechanism is not clearly uncovered yet. However, shearing injuries at the cribriform plate that lacerate the primary olfactory nerves extending from the nasal cavity to the olfactory bulb seem to be the most common mechanism involved in posttraumatic smell loss.

On MRI in patients with posttraumatic olfactory dysfunction, the injury sites reported were: olfactory bulb and tract (88%), subfrontal region (60%), temporal lobes (32%). Decrease in volume and size of the olfactory bulb was also reported.

CONCLUSION: Loss or weakness of smell is a common but hidden problem. CT and MRI of the brain and sinus can be useful to establish the etiology.