Swallow tail sign (substantia nigra)

The swallow tail sign describes the normal axial imaging appearance of a hyperintensity within the substantia nigra on high-resolution T2*/SWI weighted MRI. It was previously thought that this sign was caused by nigrosome-1, but this has not been found to be correct in more recent studies, and thus, presence or absence of the swallow tail sign and nigrosome-1 should not be used synonymously.

Terminology

This is one of those radiological signs where its presence is normal. A normal-appearing substantia nigra is reminiscent of a swallow's tail. If the normal hyperintensity is lost on SWI, the split-tail appearance is lost and this should, therefore, be reported as an "absent swallow tail sign". 

Pathology

The cause for the swallow tail sign is unclear. It was initially postulated that nigrosome-1, which is present within the posterior third of the substantia nigra, had a high signal on axial SWI in a linear or comma shape. This, when surrounded anteriorly, laterally and medially by low SWI signal intensity, was thought to lead to the resemblance of the distinctive split tail of a swallow ⁵. However, since that initial hypothesis, multiple studies have shown that the hyperintensity on SWI only partially corresponds to nigrosome-1 in autopsy analyzes, and thus there are likely to be other contributors to this sign ⁵.

It has been described that in Parkinson disease there is loss of nigrosome-1 ⁵, and it on this basis that an absence of the swallow tail sign has been proposed as a useful radiographic sign in Parkinson disease. While the absent swallow tail sign does seem to be potentially useful in Parkinson disease, as aforementioned, nigrosome-1 does not fully account for the hyperintensity of the swallow tail sign ⁵, and thus loss of nigrosome-1 may not fully account for the absent swallow tail sign either.

Radiographic features

MRI

It is important to note that adequate visualization of the swallow tail sign requires targeted 3D high-resolution SWI. In the original publication the parameters were:

• FEEPI, TR/TE 60/30, echo train length 5, flip angle 19°

• number of slices: 70

• voxel size 0.55 x 0.55 x 0.7 mm

Interpretation

Absence of the sign (absent swallow tail sign) is reported to have a diagnostic accuracy of greater than 90% for Parkinson disease and dementia with Lewy bodies ^1,2,4. In the original description, the negative predictive value was 100% ¹ and for dementia with Lewy bodies 89% ².