Multi-Parametric MRI Approach at 3 T and 7 T for Assessing Skeletal Muscle Pathology in Myofibrillar Myopathies: A...

Myofibrillar myopathies (MFM) form a large group of clinically and genetically heterogeneous protein aggregate diseases. We investigated whether a novel quantitative MRI protocol can reveal new aspects of structural and biochemical muscle pathology in three classic MFM subtypes.

MRI of the lower legs was performed in nine MFM patients with filamin-C (FLNC; n = 5), desmin (DES, n = 2) and LIM domain binding 3 (LDB3; n = 2) gene mutations, one patient with non-MFM, filamin-C related distal myopathy (4 males, 6 females, 51.0 ± 7.7 years) and 10 age-matched healthy control subjects (5 males, 5 females, 50.0 ± 11.0 years). ¹H MRI at 3 T addressed fatty replacement and edema-like changes as well as quantitative measurements of proton density fat fraction (PDFF) and water T₂ relaxation times. ³⁹K/²³Na MRI at 7 T was employed to determine apparent tissue potassium and tissue sodium concentrations (aTPC/aTSC).

T₁-weighted and T₂-weighted STIR imaging showed the highest degree of fat replacement in the soleus and gastrocnemius medialis muscle regions and the highest degree of edema-like changes in the extensor regions in all 10 myopathy patients. The lowest degree of fat replacement and edema-like changes was present in the gastrocnemius lateralis muscles. Marked fatty replacement of peroneus muscles was also present in DES-related MFM and FLNC-related distal myopathy. Muscular PDFF values were significantly increased in all MFM patients (p = 0.003 - < 0.001) with 60 and 35 of 63 muscles analysed showing increased mean PDFF (> 10% and > 50%). When excluding the muscles with PDFF > 50%, the median water T₂ was significantly increased in all muscle regions of MFM patients with the exception of the tibialis anterior and posterior muscles. Fat-corrected aTSC values in MFM patients were significantly increased compared to healthy controls (55.6 ± 16.3 mM vs. 23.2 ± 5.5 mM, p < 0.001) in all muscles but peroneus muscles, whereas fat-corrected aTPC values were reduced in all muscles except for gastrocnemius lateralis, tibialis posterior and peroneus muscles (75.4 ± 13.3 mM vs. 108.9 ± 9.9 mM, p < 0.001).

Quantitative PDFF measurements and water T₂ mapping serve as valuable tools to objectively quantify fat and edema-like changes in MFM. Furthermore, changes in potassium/sodium ion balance in the lower leg muscles of MFM patients could serve as new markers to quantify the extent of biochemical changes in individual muscle regions. Further longitudinal evaluation is required to validate whether they are sensitive to changes prior to a high degree of fat replacement.