Osteomalacia

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Reduced mineralisation of bone matrix (osteoid) is the cardinal feature of all osteomalacic syndromes. Rickets is the form that exclusively occurs in children where there is impaired mineralisation of cartilage matrix (chondroid) with arrest of primary spongiosa formation. Osteomalacia can occur at any age once lamellar bone has formed.

Aetiology

Malabsorption, gastric bypass surgery, caeliac sprue, chronic hepatitis, chronic kidney disease.

Clinical Features

Features include bone pain, tenderness, muscle weakness, difficulty walking, and a waddling gait.

In a report of 17 patients with osteomalacia due to vitamin D depletion on bone biopsy:

• Bone pain and muscle weakness in 16 (94 percent)
• Bone tenderness in 15 (88 percent)
• Fracture in 13 (76 percent)
• Difficulty walking and waddling gait in four (24 percent)
• Muscle spasms, cramps, a positive Chvostek's sign, tingling/numbness, and inability to ambulate in one to two (6 to 12 percent)

Bone pain is most pronounced in the lumbar spine, pelvis, and lower extremities where fractures have occurred. There may be tenderness with palpation. The pain is dull, aching, and aggravated by activity and weight bearing.

Fractures can occur without significant trauma. Fractures generally occur in the ribs, vertebrae, and long bone.

The myopathy is generally in a proximal pattern. There may be muscle wasting, reduced tone, and a waddling gait. It is thought that increased PTH and decreased phosphate and calcitriol contribute to the myopathy.

Investigations

Laboratory Findings

Standard investigations include: measurement of serum concentrations of calcium, phosphate, alkaline phosphatase, 25-hydroxyvitamin D, PTH, electrolytes, creatinine, and urea

The findings depend on the underlying cause of osteomalacia. Typical findings of a nutritional osteomalacia are elevated alkaline phosphatase, elevated PTH, decreased calcium, decreased phosphate, and decreased 25-hydroxyvitamin D.

Laboratory findings in different causes of osteomalacia and osteoporosis

Disorder	Serum Phosphate	Serum calcium	Serum ALP	PTH	Calcidiol	Calcitriol
Vitamin D deficiency	↓ or N	↓ or N	↑	↑	↓↓	N or ↑ or ↓
Urinary phosphate wasting conditions	↓↓	N	N or ↑	N	N	N
Proximal renal tubular acidosis	↓	N	N	N	N	N
Hypophosphatasia	N	N	↓	N	N	N
Osteogenesis imperfecta and axial osteomalacia	N	N	N or ↑	N	N	N
Osteoporosis	N	N	N	N	N or ↓	N
Calcidiol: 25-hydroxyvitamin D₃, calcitriol: 1,25-dihydroxyvitamin D. Testing differences, PTH: parathyroid hormone, ALP: serum alkaline phosphatase, N: normal; ↓: reduced; ↓↓: very reduced; ↑: elevated.

Radiologic Findings

• Looser pseudofractures, fissures, narrow radiolucent lines.
• Loss of radiologic distinctness of vertebral body trabeculae and concavity of vertebral bodies, called codfish vertebrae
• BMD isn't required. Reduced BMD doesn't distinguish osteoporosis from osteomalacia.

Bone Biopsy

Transiliac bone biopsy and histomorphometry with tetracycline labeling should only occur if the diagnosis is in doubt or when the cause of the osteomalacia has not been determined.

Diagnosis

Based on combination of clinical features (bone pain, tenderness, fractures, muscle weakness), laboratory results, radiologic findings. Rarely transiliac bone biopsy is required with double tetracycline labeling.

Differential Diagnosis

Differential diagnosis of bone fractures, bone pain, and reduced BMD

• Osteoporosis
• Osteomalacia
• Malignancy including multiple myeloma
• Paget disease
• Hyperparathyroidism

Osteoporosis and osteomalacia may be confused. In osteoporosis there is a decreased apparent density of normally mineralised bone matrix. While in osteomalacia there is an increased, normal, or most commonly decreased mass of insufficiently mineralised bone.

	Osteoporosis	Osteomalacia
Definition	Bone mass decreased Mineralisation normal	Bone mass variable Mineralisation decreased
Age of onset	Generally elderly, postmenopausal	Any age
Aetiology	Endocrine abnormality Age Idiopathic	Vitamin D deficiency Abnormality of vitamin D pathway Hypophosphataemic syndromes Renal tubular acidosis Hypophosphatasia
Symptoms	Pain referable to fracture site	Generalised bone pain
Signs	Tenderness at fracture site	Tenderness at fracture site and generalised tenderness
Bone mineral density	Reduced	May be reduced, especially in vitamin D deficiency
Serum Calcium	Normal	Low or normal (high in hypophosphatasia)
Serum P1	Normal	Low or normal (high in renal osteodystrophy)
Alkaline phosphatase	Normal	Elevated, except on hypophosphatasia
Urinary calcium	High or normal	Normal or low (high in hypophosphatasia)
Bone biopsy	Tetracycline labels normal	Tetracycline labels abnormal

Treatment

The treatment depends on the underlying cause.

Osteomalacia secondary to severe vitamin D deficiency

• Treatment with vitamin D and calcium.
• Vitamin D treatment normally results in dramatic improvements in pain, bone tenderness, and muscle strength within a few weeks. A common regimen is 50,000 IU once per week for 6-8 weeks, then maintenance dosing.
• Following vitamin D treatment, monitor serum alkaline phosphate, PTH, calcium, and urinary calcium extretion at one month, three months, then every six to twelve months.
• Serum calcium concentration measurement allows detection of early hypercalcaemia from excessive dosing.
• At three to four months measure 25-hydroxyvitamin D levels, and adjust the dose to prevent hyeprcalciuria or hypercalcaemia.
• Radiologic abnormalities can take up to a year or more to resolve

See Also

• Narang & Reid. Osteomalacia in subtropical Auckland. BMJ case reports 2019. 12:. PMID: 31289164. DOI. Full Text.
• Bone Structure
• Bone Biomechanics

Literature Review

Literature Review

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