Late onset (distal) myofibrillar myopathies
MFMs are histopathologically characterised by aberrant desmin aggregation and ultrastructurally by myofibrillar degeneration, which led to the introduction of the term `myofibrillar myopathy’ (MFM). Mutations in the human desmin gene (DES) were first shown to be associated with MFM; another form is associated with mutations in the gene encoding αB-crystallin (CRYAB). More recently, mutations in the human Z-disc proteins myotilin (MYOT), ZASP (LDB3) and filamin C (FLNC) have also been shown to cause MFM [51]. Practically, a definitive determination of the MFM subtype can only be established by direct gene sequencing. However, muscle imaging in combination with clinical information is very helpful for separation of distinct MFM subtypes and in scheduling of genetic analysis.
Recently, we performed muscle imaging in 46 MFM patients (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 αB-crystallinopathy and 3 ZASPopathy patients) and observed two major characteristic patterns of muscle involvement [26]. In desminopathy patients, at the thigh the semitendinosus was the most affected muscle, being more affected than the biceps femoris and semimembranosus muscles. Furthermore, the sartorius and gracilis muscles were often involved earlier and more severely than other thigh muscles. At the lower legs, peroneal muscles were more involved than the tibialis anterior muscle or at least equally as involved. Muscle imaging findings in a patient with a R120G αBC mutation were very similar to those in desminopathy patients. The most frequently involved muscles were the gluteus maximus, semitendinosus, sartorius, gracilis and the peroneal muscles.
Highly contrarily, all other MFM patients (caused by mutations of the Z-disc protein-encoding genes myotilin, filamin c and ZASP) showed an opposite posterior thigh affliction with more involvement of the biceps femoris, semimembranosus and the adductor magnus than the semitendinosus muscle. In filaminopathy, the gracilis and sartorius muscles were often relatively equally spared, while in myotilinopathy the sartorius muscle was slightly more involved than the gracilis muscle. In the lower legs, the soleus and the medial gastrocnemius were the most frequently affected muscles, and in the anterior compartment the tibialis anterior muscle was the most frequently affected. Differences among these three MFM subforms were much more subtle, but detectable on statistical analysis. This revealed highly sensitive and specific criteria (Fig. 5) that may be very useful for the detection of individual MFM forms.
Fig. 5 Muscle imaging differences in myofibrillar myopathies. Highly specific and sensitive statistical criteria were identified in a systematic retrospective muscle imaging assessment in a large series of 43 MFM patients [24]. Equal or greater affliction of the semitendinosus than the biceps femoris and equal or greater affliction of the peroneal group than the tibial anterior muscles is highly specific for primary desminopathy and crystallinopathy (18 out of 19 patients). Those of the remaining patients who had equal or greater involvement of the sartorius than the semi-tendinosus and equal or greater involvement of the adductor magnus than the gracilis usually had a myotilin mutation (eight out of nine patients). All eight patients who did not fulfil the criteria for desmin and myotilin mutations and who had more involvement of the medial than the lateral gastrocnemius showed a filamin C mutation. Two of the three remaining patients had a ZASP and the third a myotilin mutation