Whole-body muscle MRI in patients with spinal muscular atrophy

BACKGROUND: Spinal muscular atrophy (SMA) is a motor neuron disease with loss of musculature, which is replaced by fat. Previous magnetic resonance imaging (MRI) studies have focused on imaging muscles either in lower or upper extremities, but whole-body MRI can provide additional information on the involvement pattern. This study examined whole-body muscle fat replacement and the relationship between muscle structure, function, and bulbar symptoms.

METHOD: We conducted a descriptive, cross-sectional study. We assessed the fat replacement in skeletal muscles using whole-body MRI, the muscle function using the Motor Function Measurement 32, and bulbar muscle strength using the Bulbar Rating Scale. The presence of bulbar symptoms and function was assessed using the Voice Handicap Index, Eating Assessment Tool questionnaires, and a swallowing test.

RESULTS: We recruited 20 adult patients with type II and III SMA. The most affected muscles were the psoas major, soleus and rectus femoris, while the least affected muscles were the biceps brachii, deltoideus, and pterygoideus medialis. The tongue was involved in nearly half of the patients. Most patients reported issues with swallowing (75%) and voice (95%) but had relatively preserved strength of bulbar muscles.

CONCLUSION: Certain muscles are more prone to fat replacement than others in SMA, with a predominant proximal-distal and extensor-flexor involvement. Nearly half of the patients had increased fat content in the tongue, which is associated with dysphagia. In addition, most patients retained muscle strength in the bulbar muscles, despite advanced muscle weakness in the rest of the body.