BRITTLE BONES AND LEAKING PHOSPHATE

Abstract

INTRODUCTION
Fanconi syndrome is an established cause for low bone mineral density (BMD). Confirmed cases of acquired Fanconi syndrome due to tenofovir have been reported worldwide. The mean duration of therapy with tenofovir before the development of Fanconi syndrome is reported to be 11 months. The implicated agent was discontinued in all cases after which renal function tests and electrolytes normalised. We present a case of acquired Fanconi syndrome following tenofovir use.

CASE
A 57-year-old male with Hepatitis B infection who had been on tenofovir for 5 years presented with a low impact calcaneal fracture while standing up from a squatting position 3 years ago. Over the past two years, he experienced gradually worsening proximal muscle weakness and weight loss of 6 kg. Physical examination revealed a man of small build with proximal muscle weakness and tenderness. Blood parameters showed hypokalaemia and hypophosphatemia with inappropriately elevated urinary phosphate and potassium clearance. Thyroid function and serum testosterone were normal. His 25-hydroxyvitamin D levels were sufficient. Electromyography reported diffuse neurogenic pattern with secondary myogenic changes suggestive of a metabolic cause. Abdominal ultrasound revealed bilateral renal calculi. Dual x-ray absorptiometry scan showed osteoporosis at the lumbar spine and distal third of the radius. A diagnosis of acquired Fanconi syndrome associated with tenofovir therapy was made. He was started on oral phosphate and potassium supplements while tenofovir was replaced with entecavir. Upon review 6 months later, he was much better with no muscle pain or weakness. Repeated serum potassium and phosphate levels were within normal limits.

CONCLUSION
Tenofovir use is associated with acquired Fanconi syndrome which can lead to osteoporosis. A high index of suspicion is necessary among patients on this medication who present with low impact fractures as timely intervention can prevent significant morbidity.