To describe the differential diagnosis for phosphaturic neuromuscular conditions.

Conditions associated with renal phosphate wasting can result in myopathy and myalgia. Cystinosis is a lysosomal storage disease characterized by end-stage nephropathy and renal Fanconi syndrome.  A distal vacuolar myopathy can develop after the second decade of life. Phosphaturic mesenchymal tumors result in tumor-induced osteomalacia that clinically presents as progressive weakness and myalgia.

(1) A 37 year old man with pediatric renal failure status post renal transplant presented with two years of bilateral hand weakness. He had difficulty opening jars and buttoning clothes. On physical exam, he had marked hand atrophy and weakness. Needle electromyography showed myopathic change in the right finger extensors. 

(2) A 25 year old man with avascular necrosis of the left femoral head presented with four years of diffuse myalgia, arthralgia, and progressive gait instability. He required crutches for safe ambulation. On physical exam, he had a right inguinal mass and a power examination limited by pain. Serum CK was 55 U/L, alkaline phosphatase was elevated at 464 U/L, and phosphate was decreased at 1.2 mg/dL. Needle electromyography showed increased insertional activity and rare fasciculations in the right deltoid.

(1) Left extensor carpi radialis longus biopsy showed a vacuolar myopathy and rectangular crystals in perimysial macrophages consistent with cystinosis distal myopathy. 

(2) FGF-23 level was markedly elevated at 7150 RU/mL. Biopsy of the right groin mass showed bland spindle cells, osteoclast-like giant cells, and a “grungy” calcified matrix consistent with a phosphaturic mesenchymal tumor. Interestingly, MRI showed demyelinating lesions of the brain and cervical spinal cord with elevated CSF oligoclonal bands and IgG indices of uncertain etiology. After tumor resection, his symptoms improved.