Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by motor and non-motor impairment. Its diagnosis is quite challenging, based on the clinical evaluation of bradykinesia, tremor at rest, rigidity or postural instability. As often the clinical diagnosis can not be fully defined, there is a constant search for biomarkers capable of assisting in the identification of these patients.
Recently, it has been proposed the detection of nigrosome-1 as a biomarker in the diagnosis of the disease.

The nigrosome-1 constitutes the area with the highest concentration of dopaminergic neurons in the dorso-lateral mesencephalic substantia nigra. In normal individuals, the nigrosome is identified on magnetic resonance imaging as a SWI (susceptibility-weighted imaging) hyperintense area within the dorso-lateral substantia nigra, called swallow-tail sign.

As demonstrated by histopathological studies, patients with PD present depletion of nigrosome-1 dopaminergic neurons of substance nigra, which manifests as absence of the typical hypersignal of nigrosome-1 in dorsolateral substance nigra, in SWI-weighted sequences (loss of the swallow -tail sign). This finding has shown to be very promising in the diagnosis of PD, with high sensitivity, specificity and high positive and negative predictive values.

Depletion of nigrosome-1 dopaminergic neurons in PD may occur bilaterally or involve only one side. Some studies have shown that asymmetric depletion of nigrosome-1 neurons is also associated with more severe clinical manifestations on the contralateral side, showing high concordance of the laterality of the nigrosome-1 at 3T MRI and the clinical laterality.

In this way, the loss of swallow-tail sign seems not only to present good diagnostic properties, but also seems to determine clinical manifestations characteristics of PD.