A retrospective study at the Mayo Clinic in Rochester (Minnesota, USA) was based on 94 cases of mitochondrial myopathy diagnosed in adulthood and followed up between 2005 and 2021:
- the mean time between the first signs and diagnosis was 11 years and the median age at diagnosis was 48 years (range 32 to 63 years);
- the most frequent phenotypes were multisystem involvement in 32% of cases, MELAS in 15%, limb-girdle myopathy in 14%, chronic external ophthalmoplegia in 13% and external ophthalmoplegia-plus also in 13% of cases;
- the mutation affected mitochondrial DNA in 48 patients and nuclear DNA in 29; the genes most frequently involved were MT-TL1 in 27 cases and POLG in 17 cases;
- skeletal muscle involvement of the limbs was found in 43% of cases, 69% had involvement of the central nervous system and 21% had cardiac involvement;
- median follow-up was 4.25 years (from just over two years to eight years);
- loss of muscle strength was moderate and slowly progressive, with the need for walking assistance on average 17 years after the onset of the disease;
- thirty patients died on average 33.4 years after the onset of the disease, with cardiac involvement being associated with higher mortality.
