Adult-onset idiopathic phosphate diabetes. I. Chronic pseudoinflammatory back pain and osteopenia.

OBJECTIVE

to investigate clinical, laboratory test, and bone mineral density abnormalities in 19 adults with phosphate diabetes of unknown 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 years (24-70) at diagnosis. Seventeen patients (90%) had back pain and 13 (68%) had nerve root pain. The pain was nocturnal only or both nocturnal 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 abnormalities were as follows: serum phosphate, 0.72 mmol/L (0.58-0.89); rate for tubular reabsorption of phosphate, 74% (54-84%); maximal rate for tubular reabsorption of phosphate, 0.58 (0.4-0.76); urinary calcium/urinary creatinine > 0.48 in nine patients (47%); and fractional potassium excretion > 20% in seven patients (37%). Normal values were found 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 aciduria were negative. Bone mineral density measurements showed z-scores of -2.13 (+0.9 to -4.25) at L2-L4, and -1.34 (+1.5 to -3.2) at the femoral neck. Bone histology showed osteoporosis with a mild increase in osteoid deposition.

CONCLUSIONS

idiopathic adult-onset phosphate diabetes 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 spondyloarthropathy, disk disease, fibromyalgia, and depression. Determination of the maximal rate for tubular reabsorption of phosphate is the only means of establishing the diagnosis.