Overview

Anosmia, the loss of smell, can significantly impact quality of life and may result from various causes, including inflammation, trauma, congenital conditions, infections, and neurodegenerative diseases. Anosmia is often underdiagnosed as it relies on patient self-report and is not part of routine health check-ups.

Diagnostic imaging plays a crucial role in identifying the underlying cause. MRI (Magnetic Resonance Imaging) is particularly effective in evaluating central or neurological causes, such as congenital anosmia or post-traumatic olfactory damage. It provides detailed images of soft tissues, such as the olfactory bulb and tract, without requiring exposure to ionising radiation. 

On the other hand, CT (Computed Tomography) scans are more suitable for assessing sinonasal causes such as chronic rhinosinusitis, nasal polyps, or structural obstructions. CT provides rapid results and excellent visualisation of bone structures. Alongside imaging, other diagnostic tools include endoscopy, smell identification tests, and skull X-rays. Treatment approaches vary based on cause, with corticosteroids and theophylline being commonly used options. 

In summary, MRI and CT scans are both valuable, but they are used in distinct contexts, depending on the suspected aetiology of anosmia, with MRI being favoured for neurological evaluation and CT for structural or sinonasal assessment.

Introduction

Anosmia is the loss of smell. It can disrupt the quality of life, as it affects various social interactions and experiences. It can occur temporarily, be permanently acquired or be congenital. 

The ability of the olfactory sensation to accurately label odours can be categorised into three

 types: normosmic, hyposmic, and anosmic. The sense of smell is very crucial as it is directly connected with the generation of appetite. The sense of smell is one of the five senses. The major issue with anosmia is that it cannot be detected until the patient self-reports the symptoms, as it is not included in the general health check-up tests.¹ 

Olfactory loss is observed in  50% of people aged 65 years and 80% of people aged 80 years.²

What is an MRI?

MRI is a non-invasive method of mapping the internal structure of the body. It works on the principle of Nuclear Magnetic Resonance (NMR). During the MRI scan, the patient is placed inside a large magnet, which induces a relatively strong external magnetic field. It is generally used to measure diseases and injuries in the brain, ligaments, and soft tissues, among other conditions.³ Additionally, it is also applicable for measuring anosmia. 

MRI can diagnose morphological changes in the olfactory bulb, olfactory tract, olfactory

sulcus, and rectus gyrus. A recent case study also suggested that MRI can be useful to differentiate congenital anosmia from acquired sensorineural anosmia. The presence of hypoplasia and aplasia can be used to determine congenital anosmia from syndromes such as Kallmann syndrome. Thus, MRI can be used in patients with a suspicion of a tumour

or to verify the absence of olfactory bulbs in Kallmann syndrome.^4,5

What is a CT scan?

CT scan stands for Computed Tomography. It utilises X-rays and advanced computer algorithms to create detailed cross-sectional images of the human body. The CT scan is essentially an X-ray study, where a series of rays are rotated around a specified body part, and computer-generated cross-sectional images are produced. CT scans are used to help physicians diagnose, narrow the differential diagnosis, and confirm their suspicions. This modality can also be used for cancer screening, staging, and follow-up; its use helps to perform biopsies accurately and to assist during surgical procedures. ^6,7

When to use an MRI and when to use a CT scan?

MRI is the most commonly used imaging modality for evaluating anosmia, particularly when a central or post-traumatic cause is suspected. It is useful for detecting abnormalities in the olfactory bulb, tract, or orbitofrontal cortex, especially in patients with head trauma, congenital anosmia, or suspected frontal lobe lesions. MRI is also preferred for evaluating changes in the olfactory bulb volume. 

CT scans, on the other hand, are better suited for identifying structural obstructions in the nasal cavity or paranasal sinuses, such as in chronic rhinosinusitis or nasal polyps. They provide excellent visualisation of bony structures and are useful in detecting skull base fractures or anatomical causes of anosmia following trauma. 

Causes of anosmia^7,8

• Inflammatory and Obstructive Disorders: These are the most common causes of anosmia, including the common cold, nasal polyps, hay fever, COVID-19, and a deviated septum
• Head Trauma: Head trauma is another common cause of anosmia, as trauma to the head can cause damage to the nose or sinuses, leading to mechanical blockage and obstruction. The central nervous system (CNS) trauma leading to anosmia can be temporary or permanent, depending on the area and extent of the injury
• Ageing and Neurodegenerative Processes: Normal ageing is associated with decreased sensitivity to smell. Neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Lewy Body dementia are associated with anosmia
• Congenital Conditions: Congenital conditions that are associated with anosmia include Kallmann syndrome and Turner syndrome
• Infective Conditions: Anosmia is said to be one of the early symptoms of COVID-19 infection.
• Other Traumatic or Obstructive Conditions: Other causes of anosmia include toxic agents such as tobacco, drugs, and vapours that can cause olfactory dysfunction

Other diagnostic tests⁵

• Endoscopy
• Alcohol Screening Test
• Sniffin’ Sticks Test
• X-ray of the skull, etc.

Treatments⁵

• Corticosteroids -  Systemic corticosteroids are used in treating anosmia as they modulate the sodium-potassium adenosine triphosphate receptor
• Theophylline- It is a phosphodiesterase inhibitor that improves smell and taste functions

Conclusion

In the diagnosis of anosmia, both MRI and CT scans serve as valuable imaging tools, each suited to specific clinical scenarios. MRI is particularly advantageous for evaluating central or neurological causes, such as congenital anosmia or post-traumatic cases, as it provides high-resolution soft tissue images without exposing the patient to radiation. It also allows for multiple imaging planes—coronal, sagittal, axial, and oblique, offering detailed visualisation of the olfactory structures. In contrast, CT scans are preferred for assessing sinonasal conditions, such as chronic rhinosinusitis or nasal polyps. They deliver quicker results and provide superior visualisation of bony anatomy, making them ideal for identifying structural obstructions or fractures. Ultimately, the choice between MRI and CT depends on the suspected cause of anosmia, with MRI favoured for neurological assessment and CT for sinonasal evaluation.