An Unusual Case of Polyostotic Paget Disease of Bone with Normal Alkaline Phosphatase Level

Presentation Number: SUN 324
Date of Presentation: April 2nd, 2017

Hadoun Jabri* and Michael G Jakoby IV
SIU School of Medicine, Springfield, IL

Abstract

Background. Paget disease of bone (PDB) is fairly common, occurring in 1-3 % of adults 44-55 years of age and up to 10% of individuals older than age 80 years. It is characterized by accelerated bone remodeling at discrete skeletal sites that results in bone overgrowth and reduced mechanical integrity of affected bone. Abnormal osteoclasts appear to be the cause of increased bone turnover, and levels of serum alkaline phosphatase (sAP) and bone-specific alkaline phosphatase (bAP) are typically elevated in proportion to disease extent and activity. We present an unusual case of polyostotic PDP with normal levels of both sAP and bAP.

Case. A 72-year-old male presented to clinic with severe low back pain of two months duration. Pain was not preceded by trauma. History was notable for PDB diagnosed 20 years earlier and treated with a six month course of high dose alendronate. No focal areas of low back pain were elicited on examination. Review of CT and MRI studies available from 2009 to present revealed stable Pagetic changes in the superior lumbar spine and a large area of left iliac crest, edema and changes of impending compression fracture of the L2 vertebra, and a chronic compression fracture of the L3 vertebra. Bone scintigraphy was ordered and showed significantly increased uptake of technetium-99m-methyldiphosphonate (99mTC-MDP) at sites of PDB in the lumbar spine and pelvis. sAP was unremarkable (82 IU/L, 30-130), and the patient’s bAP level was similarly within the laboratory reference range (13.3 µg/dL, 6.5-20.1). Due to active low back pain and imaging changes of the L2 vertebra, it was decided to treat the patient with zoledronic acid. Screening alternative markers of bone turnover -- serum C-telopeptide of type I collagen (CTX), serum procollagen I N-terminal propeptide (PINP), urinary N-telopeptide of type I collagen (NTX), and urinary hydroxyproline – to follow the biochemical response to bisphosphonate therapy was notable only for a modest elevation of urinary hydroxyproline (16 mmol/g, 0-15).

Conclusions. sAP is usually a reliable marker of PDB activity and response to treatment, though approximately 15% of patients will present with normal sAP levels, including patients like this one with symptomatic disease. However, PDB with normal sAP is typically monostotic or confined to the spine, and the patient had evidence of active PDB in both spine and pelvis on bone scan. bAP is only elevated in 60% of cases where sAP is normal, and the sensitivity of other bone turnover markers is also limited at approximately 40-70% depending on the specific marker. Bisphosphonates are indicated when patients have symptomatic PDB and normal sAP level, with status of symptoms, activity on bone scans, and change in sAP level from baseline serving as measures of response to therapy.

 

Nothing to Disclose: HJ, MGJ IV