Clinical presentation | MRC scale 4/5 for both proximal arm and leg muscles and for foot dorsiflexors | MRC scale 5/5 for both proximal and distal arm and leg muscles, mild difficulty in heelwalking |
| Exercise intolerance and painful muscle cramps; scapular winging | No exercise intolerance, no muscle cramps; mild scapular winging |
Somatic growth | Height: 3rd–10th percentile; weight: below 3rd percentile | Height: 50th percentile; weight: 5th percentile |
Serum lactate (NR 0.5–2.2 mmol/l) | Highly elevated: up to 12.8 mmol/l; abnormal increase after exercise | Mildly elevated: 2.9 mmol/l; normal increase after exercise |
Electromyography | Myopathic changes, polyphasic, low amplitude action potentials in deltoid and quadriceps muscles | Normal in deltoid and quadriceps, but very mild increase in spontaneous activity in the right tibialis anterior muscle |
Muscle histology | Chronic myopathy; 50% ragged red fibres; severe lipid accumulation; severe COX defect in all fibres | Mild myopathic changes; 47% ragged red fibres; milder lipid accumulation; 14% COX positive fibres |
Immunohistochemistry | Virtual absence of COX II/III but normal staining for COX IV in all fibres | COX positive fibres were positive for COX II/III, IV, and Vab, but some of the COX negative fibres were also positive for COX IV and Vab |
COX (NR 0.90–4.70) | 0.03 U/U CS; | 0.08 U/U |
CS: 45–105 U/gNCP) | 256 U/g NCP | 136 U/g NCP |
Rate of mutant mtDNA | 93% mutant mtDNA in muscle; 0% in hair follicles; 0% in blood | 50% mutant mtDNA in muscle; 0% mutant in myoblasts |