Necrotizing Myopathy is a Unique Form of Myositis Save

Muscle involvement in immune-mediated necrotizing myopathy (IMNM) was more extensive compared with other inflammatory myopathies, according to a retrospective chart review.

Compared with polymyositis (PM) and dermatomyositis (DM), muscle involvement in IMNM was characterized by a higher proportion of thigh muscles with edema, atrophy and fatty replacement, reported Iago Pinal-Fernandez, MD, PhD, from the National Institute of Arthritis and Musculoskeletal and Skin Diseases in Bethesda, Md., and colleagues.

In addition, IMNM patients with anti-signal recognition particle (SRP) autoantibodies were found to have more severe muscle involvement than those with antibodies to HMG-CoA reductase (HMGCR), they wrote online in the Annals of the Rheumatic Diseases.

Patients with IMNM with anti-SRP had more atrophy (19%, P=0.003) and fatty replacement (18%, P=0.04) than those with anti-HMGCR, they noted.

"Taken together, these findings reinforce the idea that IMNM represents a unique form of myositis that can be distinguished from PM based on autoantibodies and muscle biopsy findings, and on the extent and pattern of muscle involvement," the authors explained.

They studied 666 patients enrolled in the Johns Hopkins Myositis Center longitudinal cohort from May 2008 to April 2015 who fulfilled criteria for IMNM (n=101), DM (n=219), PM (n=176), inclusion body myositis (n=153), or clinically amyopathic DM (n=17) with an available thigh MRI scan. The presence of edema, fatty replacement, atrophy, and fascial edema were evaluated in 15 muscles of both thighs.

Fifty (50%) of the 101 patients with IMNM were positive for anti-HMGCR and 22 (22%) for anti-SRP autoantibodies.

On MRI, 56% of patients with IMNM had edema, compared with 29% of patients with PM and 30% with DM (all P<0.0001).

More patients with IMNM also had atrophy (23.2%) than those with PM or DM (12.7% and 5.7%, respectively, P<0.02 for both). As well, 38% with IMNM had fatty replacement compared with 28.3% of patients with PM and 17.5% with DM (P<0.02 for both). As expected, patients with CADM had the least extensive muscle involvement by thigh MRI.

By muscle group, IMNM patients had atrophy and fatty replacement preferentially in the lateral rotators, glutei, medial compartment, and posterior compartment. Patients with IBM had edema, fatty replacement, and atrophy predominantly in the anterior, medial, and posterior compartments. Patients with DM had more prevalent fascial edema in the anterior, medial, and posterior compartments compared with the other myositis patients.

IMNM patients with anti-SRP had more extensive edema, atrophy, and fatty replacement in the lateral rotator group, more atrophy and fatty replacement in the anterior compartment, and more atrophy in the medial compartment, compared with those with anti-HMGCR (all P-values between 0.001 and 0.05).

On multivariate analysis, patients with IMNM had significantly more extensive edema, atrophy, and fatty replacement than patients with DM, PM or CADM. Compared with anti-HMGCR-positive IMNM patients, anti-SRP-positive IMNM patients had more extensive atrophy and fatty replacement. This latter finding reinforces "that autoantibodies define distinct groups and serve as important prognostic factors in patients with myositis," according to the authors.

Adductor brevis edema and obturator externus atrophy were common in IMNM while fascial edema in the semitendinosus was rare in this group.

Fatty replacement of the vastus lateralis and atrophy of the vastus medialis were more prevalent in IBM than in other myositis subgroups, whereas edema in the obturator internus was rare.

Patients with PM had no defining pattern of muscle involvement. Fascial edema was the hallmark thigh MRI feature of DM. "Indeed, fascial edema surrounding the rectus femoris and the semimembranosus were the most supportive features for DM, while the presence of atrophy in the vastus medialis and edema in the biceps femoris were the two features most unlikely to be found in this subgroup of patients," the authors wrote.

The extent of both atrophy and fatty replacement in all forms of myositis increased faster immediately after disease onset than later on. The early occurrence of fatty replacement and its early spread to additional muscle groups "is consistent with the importance of early therapeutic intervention with immunosuppressive agents so as to maximize the chances of limiting the spread of disease in patients with PM, DM and IMNM. Unfortunately, to date, no therapies have been shown to affect disease progression in IBM," they noted.

Study limitations include the lack of longitudinal data because only one muscle MRI was obtained, the inclusion of only the presence or absence of muscles features rather than semiquantitative data, and the availability of thigh MRI in only about half of myositis patients at Johns Hopkins.