High resolution micro-ultrasound in neuromuscular disorders

Objectives: Muscle ultrasound plays a minor part in diagnosis of muscle weakness, because only echointensity could be assessed by prevoius ultrasound technique, that depends very much on examination conditions. By ultrasound frequencies >10MHz and realtime-compound imaging the resolution is within the muscle cell, so that comparison of ultrasound morphology to histology will be possible.

Material and Methods: 21 children with presumption diagnosis of neuromuscular disorders (5 x SMA, 4 x DMD, 4 x Myositis, 2 x structure myopathy, 3 x isolated CK-emia, 3 x unknown) and controls of same age were examined by ultrasound. The digitalized images were compared to histologic results.

The ultrasound examinations were done by means of a high resolution ultrasound system and SonoCT (HDI 5000 and 15–7MHz-Compact-Linear-Broadband-Scanhead from ATL/Philips).

Results: In SMA, DMD and structure myopathy especially muscle fibers of the musculus vastus intermedius are atrophic. Additional signs:

In SMA spontaneous contractions of muscle fiber groups (fasciculations) and single muscle fibers are specific signs in the first year of life, at further age hypertrophy of muscle fiber groups is specific.

“Fibrotic reflexes“ are characteristic of DMD and very echogenic “fat bundles“ are typical of structure myopathy.

Conclusions: For the first time the cause of echointensity is distinguishable: parenchymal (muscle cell atrophy or hypertrophy) or mesenchymal (fibrosis or fat). By means of high resolution ultrasound it was possible to assign the typical “moth-eat“-like or “puffy-pastry“-like echotexture to histologically well-known hypertrophy of muscle fiber groups in SMA. Until now it was doubted, whether spontaneous movements of single muscle fibers (termed “fibrillations“) will be detectable in spite of improved ultrasound resolution. With this sonographic proof succeded and gave reason to suggest more correctly the term “fibrations“ (lat. fibra=fiber), because we are not able to see movements of myofibrils but rather of single muscle fibers.