Osteomalacia differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

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Overview

Osteomalacia must be differentiated from other diseases that cause lowering bone mineral density (BMD), such as osteoporosis, scurvy, osteogenesis imperfecta, multiple myeloma, and homocysteinuria.

Differential Diagnosis

 Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]  Associate Editor(s)-in-Chief:   Ibrahim Elkhayat, M.B.B.Ch.[3]

Osteomalacia should be distinguished from disorders that present with low bone mineral density (BMD), diffuse bone pain, proximal weakness, fractures, or multifocal skeletal abnormalities. The most important diagnostic distinction is from osteoporosis, because both conditions may have reduced BMD but require different evaluation and management. Other clinically relevant mimics include rheumatologic and neuromuscular disorders, Paget disease of bone, renal osteodystrophy, metastatic bone disease, multiple myeloma, and hypophosphatasia.

Differential diagnosis	Features that may overlap with osteomalacia	Key differentiating features	Useful diagnostic studies
Osteoporosis	Low BMD Fragility fractures May coexist with osteomalacia	Usually asymptomatic until a fracture occurs; diffuse bone pain and proximal weakness favor osteomalacia DXA cannot reliably distinguish osteoporosis from osteomalacia; because BMD is a valid index only in fully mineralized bone, osteomalacia reduces measured bone mass and is a recognized cause of a false-positive osteoporosis diagnosis. Abnormal calcium, phosphate, 25-OHD, ALP, or PTH should prompt evaluation for osteomalacia or another metabolic bone disorder Looser zones or pseudofractures strongly favor osteomalacia	Serum calcium, phosphate, ALP, 25-OHD, and PTH Skeletal radiography when clinically indicated Bone histomorphometry when the diagnosis remains uncertain
Polymyalgia rheumatica	Proximal girdle pain and stiffness, particularly in older adults Functional limitation may resemble osteomalacic myopathy	Prominent inflammatory symptoms and elevated ESR/CRP favor polymyalgia rheumatica Abnormal mineral biochemistry and skeletal pseudofractures favor osteomalacia	ESR and CRP Calcium, phosphate, ALP, 25-OHD, and PTH when osteomalacia is a consideration
Spondyloarthritis	Pelvic or axial pain Sacral or pubic lesions may mimic inflammatory skeletal disease	Osteomalacia may produce symmetric pseudofractures of the pelvis or sacrum that can be mistaken for sacroiliitis Mineral abnormalities and typical pseudofractures favor osteomalacia	Pelvic or sacral imaging Calcium, phosphate, ALP, 25-OHD, and PTH
Fibromyalgia	Widespread musculoskeletal pain and fatigue	Mineral biochemistry is generally normal Pseudofractures, focal bone tenderness, or objective proximal weakness favor osteomalacia	Focused metabolic bone evaluation when clinical features are atypical for fibromyalgia
Inflammatory myopathy	Proximal muscle weakness Difficulty rising from a chair or climbing stairs	Inflammatory myopathy is primarily a muscle disorder and commonly has elevated CK Bone pain and abnormalities of calcium/phosphate metabolism favor osteomalacia	CK and muscle enzymes Calcium, phosphate, ALP, 25-OHD, and PTH when osteomalacia is suspected
Paget disease of bone	Bone pain Elevated ALP Abnormal skeletal imaging	Typically produces focal rather than diffuse skeletal abnormalities Characteristic findings include bone expansion, cortical thickening, and coarsened trabeculae Calcium and phosphate are usually not reduced as part of the primary process	Serum ALP with calcium and phosphate Targeted skeletal radiography
Renal osteodystrophy / CKD-MBD	Low BMD Bone pain and fractures Abnormal bone turnover and mineralization	Occurs in the setting of chronic kidney disease Osteomalacia is one possible histologic pattern within renal osteodystrophy High-, low-, and mixed-turnover lesions may occur; clinical biochemistry and, when necessary, bone histomorphometry distinguish the patterns	Renal function and CKD-MBD biochemical evaluation Bone histomorphometry when the underlying bone lesion cannot be defined noninvasively
Metastatic bone disease	Bone pain Multifocal skeletal abnormalities Multifocal uptake on bone scintigraphy	Osteomalacia-related pseudofractures are often bilateral and symmetric and occur at characteristic sites Focal destructive lesions, an oncologic history, or a compatible primary malignancy favor metastatic disease	Targeted radiography/CT or MRI as indicated Cancer-directed evaluation when appropriate
Multiple myeloma	Bone pain and fractures Osteolytic skeletal lesions May cause low BMD	Typical myeloma produces lytic bone lesions and a monoclonal immunoglobulin or light-chain paraprotein with associated myeloma-defining features IgM paraprotein is not typical of multiple myeloma and should prompt consideration of Waldenström macroglobulinemia Multiple myeloma can itself cause hypophosphatemic osteomalacia through light-chain proximal tubulopathy/Fanconi syndrome Light-chain proximal tubulopathy is predominantly kappa-restricted; PTH and FGF23 may be normal, and osteomalacia may precede overt myeloma as the presenting manifestation	Serum and urine protein electrophoresis Serum free light chains Renal function, calcium, and hematologic evaluation Phosphate and other studies for proximal tubular dysfunction when hypophosphatemic osteomalacia is present
Hypophosphatasia	Osteomalacia-like skeletal pain Fractures and impaired bone mineralization	Characteristically low serum ALP distinguishes hypophosphatasia from the normal-to-elevated ALP commonly seen in other forms of osteomalacia Consider particularly when low ALP accompanies fractures, poor mineralization, or unexplained skeletal symptoms ALP ≤25 IU/L strongly predicts a pathogenic ALPL variant, but a higher ALP does not exclude clinical hypophosphatasia Recognition is important because antiresorptive therapy is relatively contraindicated in hypophosphatasia: bisphosphonates and denosumab have been associated with atypical femoral fractures in hypophosphatasia	Serum ALP PLP ALPL genetic testing when clinically appropriate
Osteogenesis imperfecta	Recurrent fractures Low BMD may occur	Blue sclerae, dentinogenesis imperfecta, hearing impairment, short stature, or a suggestive family history favor osteogenesis imperfecta Mineral biochemistry is generally not characteristic of osteomalacia	Clinical phenotype and family history Genetic testing when indicated
Scurvy	Bone pain Fragility and impaired skeletal health	Gingival bleeding, perifollicular hemorrhage, easy bruising, and other mucocutaneous findings favor vitamin C deficiency Mineral biochemistry is generally not characteristic of osteomalacia	Dietary history Plasma or leukocyte vitamin C when clinically indicated
Homocystinuria	Skeletal abnormalities and musculoskeletal symptoms	Ectopia lentis, marfanoid habitus, thromboembolism, developmental manifestations, or a relevant family history favor homocystinuria Elevated homocysteine is characteristic rather than a primary mineralization defect	Plasma total homocysteine Disease-specific metabolic/genetic testing

Osteomalacia versus osteoporosis

The distinction from osteoporosis is particularly important because low BMD alone does not establish osteoporosis. Osteomalacia can reduce measured BMD because inadequate mineralization reduces measured bone mass, and DXA therefore cannot reliably distinguish osteomalacia from osteoporosis. Diffuse bone pain, proximal muscle weakness, abnormal mineral biochemistry, or pseudofractures should prompt evaluation for osteomalacia rather than attributing the findings to osteoporosis alone.

Pseudofractures versus metastatic or inflammatory disease

Looser zones and other pseudofractures can produce focal or multifocal abnormalities on radiography, CT, MRI, or bone scintigraphy and may be mistaken for metastatic disease or inflammatory skeletal disorders. Bilateral or symmetric lesions at characteristic sites support osteomalacia, particularly when accompanied by compatible mineral abnormalities.

Hypophosphatemic osteomalacia as a presentation of multiple myeloma

Multiple myeloma should remain in the differential diagnosis of otherwise unexplained hypophosphatemic osteomalacia. Monoclonal light chains can cause proximal tubular dysfunction and acquired Fanconi syndrome, resulting in renal phosphate wasting and osteomalacia. Light-chain proximal tubulopathy is predominantly kappa (κ)-restricted, and PTH and FGF23 may remain normal. Osteomalacia may precede overt myeloma and can be the presenting manifestation.

Important diagnostic pitfalls

• Do not diagnose osteoporosis from low BMD alone. Osteomalacia can also reduce BMD, and DXA does not reliably distinguish the two disorders.
• Do not interpret low BMD as proof of osteoporosis when mineral biochemistry is abnormal. Calcium, phosphate, ALP, 25-OHD, and PTH abnormalities should prompt consideration of osteomalacia or another metabolic bone disorder.
• A low ALP changes the differential. In a patient thought to have osteomalacia, unexpectedly low ALP should raise suspicion for hypophosphatasia.
• Do not assume multifocal bone-scan uptake represents metastases. Symmetric uptake at typical pseudofracture sites may reflect osteomalacia.
• Do not exclude myeloma-associated osteomalacia solely because the skeletal presentation is metabolic. Light-chain proximal tubulopathy can produce hypophosphatemic osteomalacia as the presenting manifestation of multiple myeloma.