Kearns–Sayre syndrome (abbreviated
KSS), also known as
oculocraniosomatic disorder or
oculocraniosomatic neuromuscular disorder with ragged red fibers, is a
mitochondrial myopathy with a typical onset before 20 years of age. KSS is a more severe syndromic variant of
chronic progressive external ophthalmoplegia (abbreviated CPEO), a syndrome that is characterized by isolated involvement of the muscles controlling movement of the eyelid (levator palpebrae, orbicularis oculi) and eye (extra-ocular muscles). This results in
ptosis and
ophthalmoplegia respectively. KSS involves a combination of the already described CPEO as well as bilateral pigmentary retinopathy and
cardiac conduction abnormalities. Other symptoms may include
cerebellar ataxia, proximal muscle weakness,
deafness,
diabetes mellitus,
growth hormone deficiency,
hypoparathyroidism, and other
endocrinopathies. In both of these diseases, muscle involvement may begin unilaterally but always develops into a bilateral deficit, and the course is progressive. This discussion is limited specifically to the more severe and systemically involved variant.