Isolated respiratory chain complex-IV deficiency (ICIVD) usually manifests clinically as an early-onset, severe, multisystem mitochondrial disorder (MID) and only rarely with mild manifestations. Here we present an adult patient with late onset ICIVD with slowly progressive, mild clinical manifestations.

In a 57-years old Caucasian male with exercise-induced myalgia, muscle cramps, ptosis, and recurrent creatine-kinase (CK) elevation, muscle biopsy and biochemical investigations of the left lateral vastus muscle revealed ICIVD. He additionally had developed diabetes, arterial hypertension, hyperlipidemia, retinal detachment, transient hypothyroidism, and a hearing fall. The family history was positive for diabetes, Parkinsonism, and dementia in the mother and myopathy in the brother, suggesting maternal transmission of the MID.

Conclusions: ICIVD may manifest in adulthood with only mild manifestations and may take a slowly progressive course. Patients with mild hyper-CKemia and mild multisystem manifestations, including the muscle, profit from muscle biopsy

and biochemical investigations.