Objective: To report a patient diagnosed with idiopathic Parkinson’s disease (PD) with an intact Swallow tail sign (STS) shown in his MR susceptibility-weighted imaging (SWI) sequences and explore the usefulness of this neuroimaging marker in the diagnosis of PD.

Background: PD has no pathognomonic nor specific neuroradiological markers that can be utilized to support its diagnosis, especially at an early stage of the disease, due to diagnostic uncertainty. However, with High-resolution SWI sequences of a 3T MRI of nigrosome-1 (N1), a neuroimaging marker called the swallow tail sign has been proposed as a potential pathognomonic marker (1). Several studies have reported that the STS is usually absent, at least unilaterally, in patients with PD or other types of neurodegenerative parkinsonism (2).

Method: Our patient was a 65-year-old gentleman with idiopathic PD for seven years who presented with typical signs and symptoms, such as unilateral predominant tremors, bradykinesia, and rigidity. During the initial workup, an MRI brain was requested that showed preserved and prominent bilateral STSs in his midbrain. The patient improved with oral dopaminergic medications and satisfied the MDS clinical diagnostic criteria of PD, with the absence of any red flags.

Results: Our case presented with typical signs and symptoms of PD with STS on both sides of his neuroimaging. Initially, it was reported that the absence of the STS, at least unilaterally, is both highly sensitive and specific for PD (2); however, recent controlled studies suggested otherwise. A study by Prasihn et al. concluded no difference between PD patients and matched healthy controls for the absence of the STS (3). Additionally, the STS may be absent due to anatomical aberrancy in the N1 microstructural organization hence reporting false absence of STS in otherwise healthy individuals (4). PD still remains purely a clinical diagnosis, and neuroimaging is primarily used to exclude and rule out other etiologies of Parkinsonism. The absence of swallow tail sign can also be appreciated in other confounding diseases such as atypical parkinsonism, Lewy body dementia, and normal aging (5).

Conclusion: STS can be an unreliable diagnostic marker for PD and has the potential of being observer bias. Large-scale-controlled studies are needed to fill the gaps in knowledge regarding the supportive diagnostic benefit of this neuroimaging marker with improvements in N1-targeted MRI sequences.

References: 1. Meijer, F. J. A., Steens, S. C., Van Rumund, A., Van Walsum, A. M. V. C., Küsters, B., Esselink, R. A. J., Verbeek, M. M., Bloem, B. R., & Goraj, B. (2016). Nigrosome-1 on susceptibility weighted imaging to differentiate parkinson’s disease from atypical parkinsonism: An in vivo and ex vivo pilot study. Polish Journal of Radiology, 81, 363–369. https://doi. org/10.12659/ PJR.897090
2. Mahlknecht, P., Krismer, F., Poewe, W., & Seppi, K. (2017). Meta-analysis of dorsolateral nigral hyperintensity on magnetic resonance imaging as a marker for Parkinson’s disease. Movement Disorders, 32, 619–623. https://doi.org/10.1002/mds.26932
3. Prasuhn, J., Neumann, A., Strautz, R., Dreischmeier, S., Lemmer, F., Hanssen, H., Heldmann, M., Schramm, P., & Brüggemann, N. (2021). Clinical MR imaging in Parkinson’s disease: How useful is the swallow tail sign?. Brain and behavior, 11(7), e02202. https://doi.org/10.1002/brb3.2202
4. Schmidt, M. A., Engelhorn, T., Marxreiter, F., Winkler, J., Lang, S., Kloska, S., Goelitz, P., & Doerfler, A. (2017). Ultra high-field SWI of the substantia nigra at 7T: reliability and consistency of the swallow-tail sign. BMC neurology, 17(1), 194. https://doi.org/10.1186/s12883-017-0975-2
5. Kim, D. S., Tung, G. A., Akbar, U., & Friedman, J. H. (2021). The evaluation of the swallow tail sign in patients with parkinsonism and gait disorders. Journal of the neurological sciences, 428, 117581. https://doi.org/10.1016/j.jns.2021.117581

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MDS Abstracts - https://www.mdsabstracts.org/abstract/how-helpful-is-the-swallow-tail-sign-in-parkinsons-disease/