ASAS classification criteria - active sacroiliitis on MRI

The Assessment in SpondyloArthritis International Society (ASAS) classification criteria, published in 2009 ¹ with the revised 2016 consensus definition of a positive MRI (i.e. active sacroiliitis) still in use in 2019 ⁶, utilizes imaging features of the sacroiliac joints on x-ray and MRI as one of many criteria for the classification of axial spondyloarthritis with the aim to create homogeneous populations for research ^1,3,4. Its use in clinical practice has been discouraged by some authors ⁴. 

Usage

The ASAS classification criteria were developed for population selection for research with clinical, biochemical, genetic, and imaging criteria ^1-4. The sensitivity and specificity of the ASAS classification criteria are around 80% ^4,5, which limits its use as a diagnostic tool ⁴. 

Classification

The utility of MR in sacroiliitis assessment lies in its ability to characterize active inflammation, rather than just the consequences of it, as CR and CT do, and is thus particularly useful for the assessment of axial spondyloarthritis ^ref.

There is no grading system per se, however, the phrase "active sacroiliitis on MRI" can be used if there are features highly suggestive of inflammation. These features can be broken down into two categories ²:

• required MRI features

  • bone marrow edema (BMO) / osteitis on a T2-weighted sequence sensitive for free water (such as short tau inversion recovery (STIR) or T2FS) or bone marrow contrast enhancement on a T1-weighted sequence (such as T1FS post-Gd)*

  • inflammation must be clearly present and located in a typical anatomical area (subchondral bone)

  • MRI appearance must be highly suggestive of a spondyloarthropathy

• not required MRI features - other findings related to sacroiliitis may be observed on MRI but are not required to fulfill the imaging criterion of "active sacroiliitis on MRI":

  • the sole presence of other inflammatory lesions such as synovitis, enthesitis or capsulitis without concomitant BMO is not sufficient for the definition of "active sacroiliitis on MRI"

  • in the absence of MRI signs of BMO, the presence of structural lesions such as fat metaplasia, sclerosis, erosion or ankylosis does not meet the definition of "active sacroiliitis on MRI", but can be considered chronic lesions

*the 2016 consensus definition removed the quantitative requirement (i.e. two slices or two locations) from the required MRI features ^2,4