ADULT-ONSET IDIOPATHIC PHOSPHATE DIABETES .1. CHRONIC PSEUDOINFLAMMATORY BACK PAIN AND OSTEOPENIA

Study objective: to investigate clinical, laboratory test, and bone mi neral density abnormalities in 19 adults with phosphate diabetes of un known etiology diagnosed in a rheumatology department on the basis of a maximal rate for tubular reabsorption of phosphate (TmPO4/GFR) of 0. 77 or less. Results: there were 14 males and five females with a mean age of 36.7 years (range 20 to 68 years) at symptom onset and 43.9 yea rs (24-70) at diagnosis. Seventeen patients (90%) had back pain and 13 (68%) had nerve root pain. The pain was nocturnal only or both noctur nal and diurnal in 14 cases (74%). Other manifestations were fatigue ( n = 7, 37%), myalgia (n = 6, 32%), fracture (n = 6, 32%), renal colic (n = 4, 21%), and pseudodepression (n = 10, 53%). Laboratory test abno rmalities were as follows: serum phosphate, 0.72 mmol/L (0.58-0.89); r ate for tubular reabsorption of phosphate, 74% (54-84%); maximal rate for tubular reabsorption of phosphate, 0.58 (0.4-0.76); urinary calciu m/urinary creatinine > 0.48 in nine patients (47%); and fractional pot assium excretion > 20% in seven patients (37%). Normal values were fou nd for serum levels of Ca++, Na++, Mg++, creatinine, cortisol, T3, T4, TSH, 25(OH)D3, and 1,25(OH)2 D3. Tests for glycosuria and amino acidu ria were negative. Bone mineral density measurements showed z-scores o f -2.13 (+0.9 to -4.25) at L2-L4, and -1.34 (+1.5 to -3.2) at the femo ral neck. Bone histology showed osteoporosis with a mild increase in o steoid deposition. Conclusions: idiopathic adult-onset phosphate diabe tes manifests as chronic back pain and nerve root pain, sometimes with fatigue and depression. Bone mineral density values are decreased and histology shows osteopenia. Differential diagnoses include spondyloar thropathy, disk disease, fibromyalgia, and depression. Determination o f the maximal rate for tubular reabsorption of phosphate is the only m eans of establishing the diagnosis.