Osteoporosis in erythropoietic protoporphyria patients

Abstract

Background

Erythropoietic protoporphyria is a rare metabolic disease with painful photosensitivity due to protoporphyrin IX accumulation, resulting in lifelong light avoidance behaviour.

Objectives

The aim of this study was to evaluate bone mineral density and known osteoporosis risk factors in erythropoietic protoporphyria patients.

Methods

In this cross-sectional study all erythropoietic protoporphyria patients attending the Erasmus MC outpatient clinic that had undergone bone mineral density measurements were included. Plasma 25 OH-vitamin D, alkaline phosphatase, parathyroid hormone and total protoporphyrin IX levels were measured and information on life style, sun light exposure and a bone relevant physical exercise index (BPAQ score) was obtained using questionnaires. Bone mineral density scores and the prevalence of osteopenia and osteoporosis in the erythropoietic protoporphyria population were compared to a reference population.

Results

Twenty-three female and twenty-one male erythropoietic protoporphyria patients with a mean age of 37.6 years were included. The mean SD of the T-scores were -1.12 for the lumbar spine and -0.82 for the femoral neck (p<0.001 for both). Osteopenia was present in 35.9% and osteoporosis in 23.1%. Based on the reference population the expected prevalence was 15% and 0.6%, respectively. The prevalence of vitamin D deficiency was 50% (defined as a 25-OH vitamin D level under 50 nmol/l). The mean self-reported BPAQ score was 19.4 units (reference 19-24 units). Multiple linear regression analysis showed a significant influence of vitamin D deficiency and bone relevant physical exercise score on bone mineral density in erythropoietic protoporphyria patients.

Conclusions

The prevalence of osteoporosis and osteopenia is greatly increased in erythropoietic protoporphyria patients. Factors that significantly correlate with low bone mineral density in this population are alkaline phosphatase (related to vitamin D deficiency) and the amount of weight bearing exercise.

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