Sniffing out FTD: can a test of smell differentiate FTD from mood disorders?

In a study published in the journal Applied Neuropsychology, Heyanka and colleagues studied the sense of smell in patients with frontotemporal dementia and elderly patients with major depression, to determine if this simple test can differentiate between the two groups.

The early stages of FTD can sometimes present in a similar fashion to depressive disorders and therefore it is essential to differentiate between these conditions. Management of these conditions is very different and an accurate diagnosis allows doctors to inform patients and their families of what they can expect in the future to help them plan ahead.

Smell has been already been studied extensively in some brain aging conditions such as Parkinson’s disease and Alzheimer’s disease, but less so in others such as FTD. We know that sense of smell is affected early on in Parkinson’s disease and often occurs before the onset of motor symptoms. Patients with Alzheimer’s disease often have problems in the differential identification of odours but not in their ability to detect odour strength. In contrast smell in FTD has been explored less thoroughly and has not been compared to patients with major depression.

Deficits in our ability to identify odours can occur as a result of local damage to the nerve cells within the nasal cavity, or to brain structures involved in carrying the messages to the sensory smell centres. Brain areas known to be involved include the frontal and temporal regions that are also affected in FTD. In addition, the part of the brain known as the olfactory bulb, which carries smell messages from the nose to the brain, has been shown to contain the pathology of FTD. Furthermore, studies have shown that sense of smell in people with FTD is similar to those with Alzheimer’s disease and poorer than healthy controls, although the numbers have been small in these studies. Smell has also been shown to be abnormal in patients with a major depressive disorder.

In this study when Heyanka and colleagues compared odour identification in 21 people with FTD to 38 with a major depressive disorder they found a significant difference between the groups with the former scoring poorer than the latter. Using sophisticated statistical methods they were able to detect the accuracy of the smell test in identifying people with FTD. For example when a cut-off score of 4 or less (correctly identified odours from 20) was applied, the accuracy of diagnosis was reported at 82%.

This study used the Alberta smell test (AST) to measure the ability to correctly identify odours. This is a brief test consisting of 10 different smells to each nostril and has not been previously utilised in patients with dementia but has been shown to have similar efficacy to other well-established tests. The advantages of using the AST instead of other well-established tests include financial and time benefits. The AST is substantially cheaper, can be re-used and is quick to administer. It also measures olfaction using both nostrils, which may be of benefit if there is asymmetrical involvement, which is often the case in FTD.

The study did have some limitations however which must be considered. It was not possible to measure the effort made by each patient which could affect results. The authors also noted that their numbers were relatively small although larger than most other studies on this topic. Another potential issue was the lack of pathological confirmation of the diagnosis in the study participants. Unfortunately we were not told how long the patients had the illness before they were tested. As such it is not possible to determine if this test is useful in early disease.

Overall, this was a novel study which suggested that measurement of odour identification may aid in the differentiation of patients with FTD from those with a major depressive disorder.

Reference

Heyanka DJ, Golden CJ, McCue RB 2nd, Scarisbrick DM, Linck JF, Zlatkin NI. Olfactory deficits in frontotemporal dementia as measured by the alberta smell test. Appl Neuropsychol Adult. 2014;21(3):176-82.

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