Sodium (²³Na) MRI detects elevated muscular sodium concentration in Duchenne muscular dystrophy

Objective: In boys with Duchenne muscular dystrophy (DMD), ¹H MRI suggested muscular edema before fatty degeneration. Using specific ²³Na MRI sequences, we tested the hypothesis that the edema is caused by an osmotic effect due to increased myoplasmic Na⁺ content rather than inflammation that would lead to extracellular edema.

Methods: Eleven patients with DMD (mean age, 10 ± 5 years) and 16 healthy volunteers of similar age were examined on a 3-T system with ¹H MRI and ²³Na density-adapted 3-dimensional radial MRI sequences. The muscle edema was quantified on short-tau inversion recovery images using background noise as reference. Fatty degeneration was quantified on T1-weighted images using subcutaneous fat as reference. Na⁺ was quantified by a muscular tissue sodium concentration (TSC) sequence. A novel inversion recovery (IR) sequence allowed us to determine mainly the myoplasmic Na⁺ by suppression of the extracellular ²³Na signal from vasogenic edema. A reference tube containing 51.3 mmol/L Na⁺ with agarose gel was used for standardization.

Results: The normalized muscular signal intensity of ²³Na as assessed by the IR sequence was significantly higher for patients with DMD than for volunteers. TSC was markedly increased at 38.4 ± 6.8 mmol/L in patients with DMD compared with 25.4 ± 2.1 mmol/L in volunteers. The muscular edema-like changes were much more prominent in patients with DMD than in volunteers. In addition, the muscular fat content was significantly higher in patients with DMD than in volunteers.

Conclusions: The elevated myoplasmic Na⁺ concentration in DMD is osmotically relevant and causes a mainly intracellular muscle edema that contributes to the pathogenesis of DMD.