Investigation of human mitochondrial myopathies by phosphorus magnetic resonance spectroscopy.

The investigation of human muscle disease by phosphorus (3'P) magnetic resonance spectroscopy began in Oxford in 1981 (Ross et al., 1981). Since then we have investigated over 300 patients. many of them several times, with a variety of primary muscle disorders or systemic disorders that are reflected in muscle metabolism (for summaries, see Radda et al., 1984; Radda & Taylor, 1985). We developed a relatively simple protocol in which the energetics of the flexor digitorum superficialis muscle is examined by placing the forearm of the subject inside a 1.9 T superconducting magnet of a Fourier transform n.m.r. spectrometer. Signals from phosphocreatine (PCr), ATP, Pi and hexose monophosphates are recorded with the aid of a carefully positioned surface-coil and intracellular pH is derived from the chemical shift of the Pi resonance. A patient examination takes between 30 and 45 min during which approx. 30n.m.r. spectra are accumulated at rest, during a 7 min two-stage exercise period and during a 10 to 15 inin recovery phase (Taylor et al., 1983). We measure a set of characteristic parameters in normal individuals and can use these to characterize and quantify different types of abnormalities, several of which are associated with mitochondrial myopathies. We use the following 'normal' indices: (i) At rest, intracellular pH and the relative concentrations of PCr, Pi and ATP are essentially invariant from normal individual to individual. From these values we calculate the concentration of free ADP assuming that creatine kinase is at equilibrium and obtain a low and relatively constant value for ADP concentration (6 t 3 pM). The (phosphorylation potential)-' (i.e. [ATP] / [ADP] x [Pi]) at rest is 2.8 f 1.3 x 1 0 6 M l in normal subjects (Arnold et al., 1985). (ii) During aerobic, dynamic exercise there is a characteristic relationship between the decrease in PCr and intracellular pH (Taylor et al., 1983). (iii) During recovery, the rate of PCr resynthesis has a t1,* of 52 t 1 6 s and represents the rate of oxidative phosphorylation. The PCr resynthesis rate also refects the rate of pH recovery (Arnold et al., 1984), the latter being relatively slow, and is likely to be a measure of H+ export from the muscle cell. (iv) The rapid decrease of ADP concentration to its resting level (within 2 min) is also characteristic. We have studied 12 patients with evidence of mitochondrial myopathies in whom the clinical manifestations ranged from mild external opthalmoplegia without symptoms or signs of limb weakness to severe generalized weakness and exercise intolerance (Arnold et al., 1985). At rest, an n.m.r. abnormality could be demonstrated in 11 of the 12 patients, 10 having evidence of a reduced muscle energy state with at least one of the following