Objective: To evaluate the swallow tail sign accuracy to distinguish between essential tremor (ET) and Parkinson’s disease (PD) in clinical practice. 

Background: The swallow tail sign (STS), a hyperintensity within the substantia nigra, mostly corresponding to the nigrosome-1, on high-resolution T2*/SWI brain MRI, is absent in PD subjects from early stages, and has been suggested as a diagnostic biomarker because of its high accuracy when evaluated by experienced neuroradiologists and with MRI slice thickness equal or under 1.5mm[1]. 

In ET, the STS is preserved, so its evaluation could help differentiate ET from PD[2].  

We performed a retrospective study to evaluate the accuracy of STS detection in clinical practice by untrained movement disorders neurologists using non-optimal MRI quality.

Method: Seventy-two 3T brain MRIs from patients who were clinically diagnosed with PD (n=43) or ET (n=29) were retrospectively assessed by two movement disorders neurologists. In the SWAN sequence (2mm slice thickness), five slices caudal to the upper limit of the red nucleus were analyzed. The STS was assessed blindly on each side to define PD diagnosis.

Results: The overall accuracy was 80.6%, with 76.7% sensitivity and 86.2% specificity for the first rater and 69.4%, 69.8% and 69.0% respectively for the second rater. The diagnostic match between both neurologists was 77.8%, with a Cohen’s kappa of 0.55 (moderate/good inter-rater agreement).  When the diagnosis made by both raters matched, the specificity was 83.3% and the sensitivity was 81.3%, with an overall accuracy of 82.1%. 

Conclusion: The visual inspection of the STS performed by movement disorders neurologists in clinical practice is an inexpensive and fast assessment that yields good diagnostic accuracy to discriminate between ET and PD even with suboptimal image quality (slice thickness >1.5mm). This supports the utility of this radiological sign in real life scenarios, despite the results being inferior than in ideal conditions.

References: 1. Noh Y, Sung YH, Lee J, Kim EY. Nigrosome 1 detection at 3T MRI for the diagnosis of early-stage idiopathic Parkinson disease: assessment of diagnostic accuracy and agreement on imaging asymmetry and clinical laterality. American Journal of Neuroradiology. 2015 Nov 1;36(11):2010-6.

2. Perez Akly MS, Stefani CV, Ciancaglini L, Bestoso JS, Funes JA, Bauso DJ, Besada CH. Accuracy of nigrosome-1 detection to discriminate patients with Parkinson’s disease and essential tremor. The Neuroradiology Journal. 2019 Dec;32(6):395-400.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/assessing-the-swallow-tail-sign-in-clinical-practice/