Reverse fiber type disproportion: A distinct metabolic myopathy

Cardiopulmonary Exercise Testing and Metabolic Myopathies

Skeletal muscle requires a large increase in its ATP production to meet the energy needs of exercise. Normally, most of this increase in ATP is supplied by the aerobic process of oxidative phosphorylation. The main defects in muscle metabolism that interfere with production of ATP are (1) disorders of glycogenolysis and glycolysis, which prevent both carbohydrate entering the tricarboxylic acid cycle and the production of lactic acid; (2) mitochondrial myopathies where the defect is usually within the electron transport chain, reducing the rate of oxidative phosphorylation; and (3) disorders of lipid metabolism. Gas exchange measurements derived from exhaled gas analysis during cardiopulmonary exercise testing can identify defects in muscle metabolism because [Formula: see text]o₂ and [Formula: see text]co₂ are abnormal at the level of the muscle. Cardiopulmonary exercise testing may thus suggest a likely diagnosis and guide additional investigation. Defects in glycogenolysis and glycolysis are identified by a low peak [Formula: see text]o₂ and absence of excess [Formula: see text]co₂ from buffering of lactic acid by bicarbonate. Defects in the electron transport chain also result in low peak [Formula: see text]o₂, but because there is an overreliance on anaerobic processes, lactic acid accumulation and excess carbon dioxide from buffering occur early during exercise. Defects in lipid metabolism result in only minor abnormalities during cardiopulmonary exercise testing. In defects of glycogenolysis and glycolysis and in mitochondrial myopathies, other features may include an exaggerated cardiovascular response to exercise, a low oxygen-pulse, and excessive ammonia release.

Congenital fiber-type disproportion in an ambulatory rehabilitation setting : A case report

Congenital fiber-type disproportion is a rare condition, histologically characterized by a relative type 1 fiber hypotrophy. The main clinical feature is mild-to-severe muscle weakness. In this report, we present the case of a 21-year-old patient with congenital fiber-type disproportion in an outpatient rehabilitative setting to evaluate the feasibility and results of an assessment battery, including bioimpedance analysis (BIA), dynamometry, 3D gait analysis, 6‑min walk test (6MWT), and the timed up and go test (TUG). The patient had a notable decrease in all functional scores. BIA: lean body mass, 38.4 kg (50.2 ± 5.3), body fat, 1.6% (12.4 ± 4.4); hand dynamometry: 18.5 kg left/20.0 kg right (44.8 ± 6.6); walking speed, 58 cm/s (122.7 ± 11.1), step length, 43.0 cm (61.6 ± 3.5); 6MWT: 478.5 m (638 ± 44); TUG: 9.4 s (8.1 ± 1.0). No adverse events were reported. The tests used were easily applicable in clinical routine and well tolerated by our patient.

Exercise responses in patients with chronically high creatine kinase levels

INTRODUCTION

Elevated serum creatine kinase (CK) is often taken to reflect muscle disease, but many individuals have elevated CK without a specific diagnosis. How elevated CK reflects muscle metabolism during exercise is not known.

METHODS

Participants (46 men, 48 women) underwent incremental exercise testing to assess aerobic performance, cardiovascular response, and ventilatory response. Serum lactate, ammonia, and CK were measured at rest, 4 minutes into exercise, and 2 minutes into recovery.

RESULTS

High-CK and control subjects demonstrated similar aerobic capacities and cardiovascular responses to incremental exercise. Those with CK ≥ 300 U/L exhibited significantly higher lactate and ammonia levels after maximal exercise, together with increased ventilatory responses, whereas those with CK ≥200 U/L but ≤ 300 U/L did not.

CONCLUSIONS

We recommend measurement of lactate and ammonia profiles during a maximal incremental exercise protocol to help identify patients who warrant muscle biopsy to rule out myopathy. Muscle Nerve 56: 264-270, 2017.