Karnofsky Performance Scale and modified Rankin Scale as indicators of functional capacity in patients with mitochondrial disease

Abstract

Mitochondrial disease is a common form of genetic neurometabolic disease that affects the mitochondrial respiratory chain function resulting in impaired aerobic energy production. We investigated whether two quantitative measures of functional capacity, modified Rankin scale (mRS) and Karnofsky Performance scale (KPS) scores could be determined retrospectively based on electronic patient records (EPRs), and whether they could provide insights into disability and disease progression. In this retrospective observational study, we investigated a previously identified cohort of patients with genetically defined mitochondrial disease at Turku University Hospital (TUH, Turku, Finland). Using the EPR data at TUH, we reviewed all available neurology and emergency medicine patient notes of these patients. Based on the EPR texts for all clinical visits, the mRS and KPS scores were retrospectively determined whenever sufficient information was available. N=52 (28 women) mitochondrial disease patients with available data were identified. Mean age was 50 years (range 10-85 years). The patients were grouped based on their genetic background to m.3243A>G mtDNA variant (N=21, 13 women), other pathogenic mtDNA (N=22, 11 women) variants and nuclear gene variants (N=9, 4 women). There were on average 9 data points per patient in the EPR system and average follow-up time was 8.8 years. All groups showed a decline in functional capacity over time. Patients with nuclear gene variant (particularly POLG) had the most severe functional capacity scores and earlier disease onset with higher mortality rates at a younger age. Patients with mtDNA variants showed a slower progression of decline in functional capacity. Those with chronic progressive external ophthalmoplegia and myoclonic epilepsy with ragged red fibres phenotypes maintained relatively stable functional capacity for longer period. All groups experienced multiple acute and emergency visits due to worsening of mitochondrial disease often leading to decline in KPS and mRS scores. The KPS and mRS scales may offer a simple and quick tool for long-term overview of mitochondrial patients’ functional capacity, especially in non-specialist and primary healthcare.