IMACS

What the International Myositis Assessment and Clinical Studies Group (IMACS) “tool” is

In practice, “the IMACS tool” means a standardized suite of outcome measures that IMACS recommends to assess myositis disease activity, damage, and patient impact in a consistent, reproducible way—originally to harmonize clinical trials, but increasingly used to structure clinic follow-up.

1) IMACS Core Set Measures for disease activity (the “6 CSM”)

IMACS recommends that studies (and often clinics that want structured monitoring) capture a core set spanning muscle strength, labs, function, and global disease activity.

The commonly referenced IMACS disease activity core set measures include:

1. Physician Global Disease Activity (typically VAS 0–10 cm)
2. Patient (or Parent) Global Disease Activity (VAS)
3. Muscle strength – usually MMT-8 (Manual Muscle Testing of 8 muscle groups)
4. Physical function/disability – typically HAQ-DI (or CHAQ in JDM)
5. Muscle enzyme activity – a panel (commonly CK ± aldolase, AST/ALT, LDH)
6. Extramuscular disease activity assessment – commonly via the MDAAT system (below), which captures skin, lung, joints, GI, cardiac, etc.

Why it matters: this combination prevents “CK-only” or “skin-only” monitoring and forces explicit tracking of strength + function + global activity + extramuscular organs.

2) The MDAAT: IMACS’s structured organ-system disease activity instrument

The Myositis Disease Activity Assessment Tool (MDAAT) is the umbrella instrument IMACS developed for comprehensive disease activity assessment across organ systems.

It has two main parts:

A) MITAX (Myositis Intention-to-Treat Activity Index)

• Organ-system based (e.g., constitutional, cutaneous, skeletal, GI, pulmonary, cardiac, etc.).
• “Intention-to-treat” concept: it grades activity in a way that reflects how aggressively you’d treat that domain (i.e., clinically meaningful activity, not just presence/absence).

B) MYOACT (Myositis Disease Activity Visual Analogue Scale)

• Uses VAS scoring by organ system (and a global score), giving a continuous measure of activity burden across domains.

Practical take-home:

• MITAX is strong for “activity that would change management.”
• MYOACT is strong for “how much activity is there, by domain, on a continuous scale.”
• Most trial frameworks accept either/combined within the MDAAT family depending on protocol.

3) IMACS disease damage measurement (separate from activity)

IMACS explicitly separates reversible inflammatory activity from persistent/irreversible damage (including therapy complications).

The major IMACS damage instrument is the Myositis Damage Index (MDI), intended to capture cumulative damage across organs and function.

Why this is critical in clinic: weakness, dyspnea, dysphagia, or skin change may reflect active myositis (treatable) vs damage (rehab/supportive focus), and management diverges.

4) IMACS response criteria: the Total Improvement Score (TIS)

IMACS core set measures are also the backbone for the ACR/EULAR myositis response criteria, which compute a Total Improvement Score (0–100) from weighted changes across the 6 core measures.

• In adults (DM/PM), commonly used thresholds: ≥20 minimal, ≥40 moderate, ≥60 major improvement.
• There is an online calculator hosted by National Institute of Environmental Health Sciences (NIEHS) resources used widely for trials/teaching.

How to use IMACS tools in a real DM/IIM follow-up (simple clinic workflow)

• Every visit (quick): Physician global + Patient global + MMT-8 + HAQ-DI/functional status + enzymes.
• Every 1–3 visits (or if flare/organ concern): Structured extramuscular scoring with MDAAT (MITAX/MYOACT).
• Every ~6–12 months (and after major events): MDI to document damage trajectory separate from activity.
• If you want an objective “response” number (especially for trials/teaching): calculate TIS.