Advances in the past two decades of perinatal interventions as well as strides in technology have resulted in the dramatic improvement in the survival of recipients of neonatal intensive care. In India, the survival of Extremely Low Birth Weight (ELBW) babies is now improving all over the country ranging from 48.7% to 58%<xref ref-type="bibr" rid="bib001">1</xref>. A major concern with improved survival of high risk neonates is new generation of high risk infants with plethora of challenges in their management during convalescence & post discharge. Nutrition is a major area of concern to all those dealing with these smallest human beings. To provide NICU graduates optimal nutrition for growth & maturation is a medical challenge. Osteopenia of prematurity is an underrecognized nutritional challenge.</sec><sec id="sec002"><title>What is osteopenia of prematurity?

Journal of Neonatology

0971-2179

National Neonatology Forum

jn-21-2-011

REVIEW ARTICLE

Osteopenia of Prematurity: The brittle challenge

Nanavati

Ruchi

*Shrinivas

Chinta

Department of Neonatology, Seth G. S. Medical College & K. E. M. Hospital, Mumbai.

*E-mail: drruchinanavati@gmail.com

4–62007

212127132

Advances in the past two decades of perinatal interventions as well as strides in technology have resulted in the dramatic improvement in the survival of recipients of neonatal intensive care. In India, the survival of Extremely Low Birth Weight (ELBW) babies is now improving all over the country ranging from 48.7% to 58%<xref ref-type="bibr" rid="bib001">1</xref>. A major concern with improved survival of high risk neonates is new generation of high risk infants with plethora of challenges in their management during convalescence & post discharge. Nutrition is a major area of concern to all those dealing with these smallest human beings. To provide NICU graduates optimal nutrition for growth & maturation is a medical challenge. Osteopenia of prematurity is an underrecognized nutritional challenge.</sec><sec id="sec002"><title>What is osteopenia of prematurity?

Osteopenia of prematurity, a metabolic bone disease, refers to the hypo mineralized skeleton of the premature infant. The term osteopenia refers to decreased amount of bone matrix (scarcity of bone) and can be caused by either insufficient synthesis and/or increased resorption of organic bone matrix. This condition encompasses a variety of disturbances ranging form mild under mineralization to severe bone disease with fractures.^2,3

Rickets and osteopenia are used interchangeably although they are not synonymous, Rickets refers to defective mineralization of growth plate cartilage's is characterized by radiological changes at the growing ends of the long bones and is usually due to vitamin D deficiency. The use of the word ‘rickets’ should be reserved for cases in which there are specific histologic, radiographic, & physical findings that may be associated with different disease states including the more severe degree of osteopenia of prematurity. ^2,3

In contrast to rickets, incorporation of Calcium (Ca) and Phosphorus (P) into bone matrix is not affected in osteopenia. Accordingly, mineralization of growth plate cartilage proceeds normally. However, like rickets bone mineral density is decreased.

The burden

In growing Very Low Birth Weight (VLBW) infants (birth weight <1500 g and gestational age <32 weeks), osteopenia of prematurity occurs almost without exception.⁴ The incidence of rickets or fractures in infants with a birth weight <1500 g reportedly ranges from 20–32%^5–7, increasing to 50–60%^8–10 in infants with a birth weight <1000 gm. The incidence of rickets or fractures is increased in infants fed unsupplemented human milk or soy formulas.^6,11,12 The frequency of osteopenia is also correlated directly with postnatal morbidity especially Bronchopulmonary Dysplasia (BPD), Necrotizing Enterocolitis (NEC), and prolonged parental nutrition.

The challenge

Meeting the intrauterine accretion rates of minerals in VLBW infants has been one of the challenges faced by neonatologists & failure to achieve this results in development of Osteopenia of Prematurity, an under diagnosed entity as overt clinical features are seen only in a small proportion of infants especially those having the most severe form of illness. Conventional radiologic methods can not detect decrease in bone mineral density until 30 to 40% of loss of bone mineral has occurred.⁵ Dual Energy Xray Absorptiometry (DEXA) is not available to masses in developing countries. Finally treatment is also a difficult task. Standard formulas and preterm human milk do not contain enough minerals to meet with intrauterine accretion rates. The most of the oral mineral preparations available in Indian market contain only calcium & not phosphorus while Total Parenteral Nutrition (TPN) solutions containing Ca & P are not available. Hence many preterms are supplemented only with calcium though emphasis has been laid in the literature on supplementation of phosphorus in addition to calcium in prevention of Osteopenia of Prematurity. The risk of creating hyperosmolar milieu in the immature intestinal lumen with mineral supplementation exists. Even during high risk follow up clinics NICU graduates are subjected to detailed neuromotor, neurodevelopmental, neurosensory examinations, what often goes neglected is a condition which never kills nor results in any neurological impairment i.e. Osteopenia of Prematurity. The long term effects including the effects on bone mineralization & stature during adolescence as well as effect on the risk of osteoporosis in adults remains unknown.

Etiopathogenesis

Skeletal development begins early in embryonic life and continues throughout fetal life, infancy and childhood. Fetal skeletal mass accumulation & skeletal growth are totally dependent on the maternal mineral supply. The third trimester of fetal life and early infancy are the most critical stages of skeletal development and mineralization. Thus adequate mineral supply prenatally during the critical period of third trimester as well as postnatally during infancy is essential to maintain bone mineralization and mineral homeostasis. In pregnancy, placenta transports large amounts of calcium and phosphorus to allow rapid fetal skeletal mineralization. In term neonates, the total body calcium is approximately 28 grams, most (about 80%) of which is accrued during the last trimester of pregnancy at a rate of upto 150mg/kg fetal weight/day. ⁵ Study conducted in rats by Comer shows that the fetal calcium content increased from 0.45 mg on day 17 of gestation to 12.3 mg at term (22 days).¹³ Klemm observed that the transfer of maternally injected radioactive phosphorus ³²P (per gram placenta) in the rat, increased linearly from day 19 of pregnancy until term.¹⁴ The total amount of phosphorus in normal adults is about 1 kg. In term newborns, it is approximately 16g. Eighty percent of total P is accumulated during the last trimester of pregnancy at a rate of 75mg/kg fetal weight/day⁸ and is closely linked to the accretion of Ca, with a Ca: P ratio of 1.7:1. Preterm infants experience a cumulative deficit of all minerals, in particular Ca & P, because they miss the peak period of fetal mineral accretion. If there are deficiencies in mineral availability, mineral homeostasis is maintained at the expense of skeletal mineral content leading to osteopenia. Even term infants may have decreased stores owing to maternal complications such as severe preeclampsia with fetal growth retardation and placental insufficiency. Osteopenia has been found to occur more commonly after deliveries complicated by preeclampsia.¹⁵

Substrate deficiency

By the virtue of being preterm, the substrate deficiency is the main cause of metabolic bone disease of prematurity. This is compounded by the fact that many premature infants suffer from a deficient Ca & P intake in the postnatal period, either from an inability to tolerate adequate enteral feeding for prolonged periods or from deficient mineral intake when fed low Ca & low P milk such as human milk or standard term infant formula.^16,17 Preterm human milk containing 25–35 mg/dl of calcium and 10–15 mg/dl of phosphorus¹⁸ is not sufficient to support skeletal growth at the intrauterine rate, even assuming 100% retention. Though preterm infants fed human milk have a high percentage of Ca absorption & retention, the absolute retention of calcium is significantly below intrauterine accretion because of low calcium & phosphorus content of preterm milk. Thus preterm milk can not sustain normal skeletal mineralization of a VLBW infant. Additionally once the preterm infant has survived an acute initial illness and enters a phase of rapid growth it needs further supplementation of minerals to support the “catch up” growth.^16,17

Evidence is accumulating that phosphorus deficiency is an important factor in bone disease of prematurity. This is because of overriding need for phosphorus for soft tissue growth and metabolism in the body resulting in withdrawal of phosphorus from the skeleton. Elevated parat-hormone resulting in phosphaturia leads to hypophosphatemia. Calcium can not be used for bone growth in the absence of phosphorus and hence is lost in the urine. It remains high in most formulas with the exception of soy-based formulas since phosphate is bound by phytates present in soy pro-tein.

However, meeting the intrauterine accretion rates of minerals extends be-yond that of simple substrate supply. Specially designed formulas with a high mineral content (up to 171 mg/dl of calcium and 85 mg/dl of phosphorus) for the VLBW infants are available. Other products are fortifiers that contain high levels of minerals and can be added to human milk fed to premature infants. These products of high mineral content do not yield the direct rela-tionship between mineral intake and mineral retention. Bioavailability of the various mineral salts in these products is therefore a concern because in vivo mineral insolubility may limit mineral absorption and retention. Thus, simply enhancing the mineral content of formulas or human milk may not result in the desired retention rate of minerals in premature infants. ⁸

Bronchopulmonary dysplasia

Long term diuretic usage results in increased urinary excretion of Ca which compounds the usually low Ca intake associated with fluid restriction. Long term corticosteroid therapy may predispose the infant to abnormal bone mineralization.

Chronic illness may prevent normal nutrition & result in poor intake of all nutrients because of fluid restriction. NEC requiring bowel resection may be associated with complication of short gut syndrome & deficient nutrient absorption.

Total parenteral nutrition (TPN)

The American Academy of Pediatrics Committee on Nutrition recommends a minimum of 30–40 mg/kg of elemental calcium and phosphorus in the TPN solutions for LBW infants, an amount well below the intrauterine accretion rate (AAP Committee on Nutrition, 1993). Thus standard TPN solution for LBW infants has had low concentrations of calcium (20 mg/dl) and phosphorus (15.5 mg/dl).⁴ Aluminum contamination of amino acid preparations and of mineral supple-ments used in TPN has been associated with metabolic bone disease in pre-mature infants. However, the exact contribution of aluminum in TPN solutions to the bone pathology of LBW infants remains to be determined. Liver disease with cholestasis is another factor in bone disease in this population.

Medications

Several medications have been associated with increased calcium losses in premature infants. Furosemide inhibits electrolyte reabsorption from the ascending loop of Henle, resulting in hypercalciuria proportional to the increased excretion of sodium. Theophylline & caffeine can increase urinary Ca loss. Other drugs associated with bone disease in adults are corticosteroids. Though increasingly used in infants with severe BPD (or for prophylaxis of the disease), their effects on bone formation and bone reabsorption in this population particularly at risk for osteopenia have not been adequately studied.

Passive exercise

The importance of mechanical stimulation has been neglected in neonatology. Skeletal development is driven by functional requirement, which in turn depends upon muscle strength.² Importance of this muscle bone interaction is particularly obvious in neonates with muscular hypotonia of intrauterine onset, who often have fractures at birth. When mechanical challenge posed by muscles is lacking the skeletal system “sees” no reason to increase its strength and less of new bone is added.²

Vitamin D deficiency

There is no evidence that vitamin D deficiency plays a significant role in metabolic bone disease of prematurity. Most studies have reported that infants with metabolic bone disease were receiving adequate vitamin D supplementation at the time diagnosis was made and had normal serum 25-OH D₃ concentration. Renal 1 alpha-hydroxylase, which is essential for l,25 (OH)₂D synthesis, appears to be functional in the human fetus immediately after birth. This finding may explain why 1,25 (OH)₂D is almost always elevated in LBW infants with significant osteopenia. In preterm infants, intestinal Ca absorption appears not to be vitamin D dependent.

Clinical manifestations

The clinical onset of metabolic bone disease of prematurity usually occurs between the 6^th and 12^th postnatal week. The clinical picture ranges from decreased bone mineralization to overt rickets¹⁹ (growth retardation, frontal bossing, craniotrabes, costochondral beading, wrist widening) or fractures. Softening and/or fracture of ribs due to poor mineralization of the rib cage secondary to low bone phosphorous & muscular weakness due to hypophosphatemic myopathy in babies with respiratory distress are the important factors that contribute to difficulty in weaning them from the ventilator. ²⁰ Osteopenia also makes the preterm infants more susceptible to fractures during minimally invasive procedures and even routine handling.²¹ Rib fractures are especially common in infants with chronic lung disease requiring chest percussions. Koo et al in 1989 have described 82 fractures in 19 VLBW infants. ²²

Diagnosis

As mentioned earlier, overt clinical features are seen only in a small proportion of infants especially those having the most severe form of illness, hence it is important to resort to certain investigative modalities to detect the subclinical disease.

Serum calcium

Serial measurements of serum calcium in very low birth weight infants have shown no significant change in serum calcium with time or with development of osteopenia. Serum calcium in infants with osteopenia is comparable to those without osteopenia.^6,23

Serum phosphorus

Most of the studies have shown that serum phosphorus levels in infants with osteopenia is significantly less (< 4.0 mg/dl) as compared to controls.^6,23 Kulkarni et al^24,25 have shown that the lowest mean serum phosphorus level precedes radiological changes of rickets by two to four weeks.

Serum alkaline phosphatase

Plasma alkaline phosphatase is simple screening test for significant demineralization in preterm infants.^26,27 Changes in plasma alkaline phosphatase are usually the earliest to occur, preceding the radiological changes by four to five weeks. Reference range of alkaline phosphatase value in preterm neonates is not available. The values are normally higher (as compared to adult reference range: 90–330 IU/L)²⁶ due to increased osteoblastic activity of rapidly growing bones. Serial estimates are of value rather than a single absolute value. However studies have shown that for screening purposes plasma alkaline phosphatase activity (in absence of liver disease) of up to the five times the upper limit of the adult reference range may be normal for the preterm infants, a value of six times the upper limit indicates an X - ray to exclude rickets and a value of seven and a half times the upper limit is compatible with the active disease. However several studies have shown that alkaline phosphate activity does not correlate with radiologically apparent disease.^25,26,28

Urinary calcium

Studies have revealed a significant inverse correlation between bone mineral density and urinary calcium level (as determined by urinary calcium/creatinine ratio). In general, urine calcium/creatinine ratio (mg/mg) exceeding 0.2 suggests hypercalciuria, although normal ratios in infants may be as high as 0.8. ²⁹

Unpublished data from our institution has revealed significant correlation between lower serum phosphorus levels (< 4.0 mg%) & low BMD as well as between higher urinary calcium/creatinine ratio (>0.4) & low BMD in VLBW infants both at corrected date of birth & 3 months.

Bone densitometry

It is still in the incipient stage in neonatal practice. Following techniques of bone densitometry are in use in clinical practice

X-ray

Radiological changes in bone attributable to osteopenia of prematurity have been found in more than 30% of VLBW babies⁵ and 57% of ELBW babies.³⁰ However, changes are not visible before bone mineralization is reduced by at least by 20–30%. Thus radiological examination does not allow reliable diagnosis unless the disease is advanced.

Koo et al³¹ have devised a scoring system to assess the severity of osteopenia based on the radiological changes in wrist X-ray. Four grades have been given.

Grade 0 - Normal density of bone cortex along shaft with normal dense white line at metaphysis and normal band of lucency in submetaphyseal region.

Grade 1 - Loss of dense white line at metaphysis, increased submetaphyseal lucency and thinning of cortex.

Grade 2 - Changes of grade 1 plus irregularity and fraying of metaphyses, with splaying and cupping - i.e. changes of rickets

Grade 3 - Changes of rickets with evidence of fracture

X ray absorptiometry

DEXA has been used in neonatal field in assessing BMD of lumbar spine, forearm and whole body. It is considered to reflect most accurately the state of bone mineralization in preterm infants. ³² The fact that DEXA is sensitive in measuring the small changes in the bone mineral content and density and the fact that radiation exposure is small makes this technique a reasonable and acceptable choice for serial measurement of BMD in pediatric patients. Limitations of DEXA^33,34 include movement artifacts in neonates and lack of free availability of the instrument. No standard reference values are available for preterm infants of various gestational ages.

Single photon absorptiometry (SPA)^35,36 & Dual Photon Absorptiometry (DPA)^35,36 have been used in neonates to measure BMD.

Quantitative ultra sound (QUS) <xref ref-type="bibr" rid="bib035">35</xref>,<xref ref-type="bibr" rid="bib036">36</xref>

Ultrasound waves over a particular frequency range, (0.2–0.6 MHz), passing through bone are attenuated depending on the bone mineral density. This is called broad band ultrasonic attenuation (BUA). The most commonly used bone is calcaneum. Precision of this technique is modest and shows poor agreement with DXA diagnosis. The diagnostic use of QUS has not yet been substantiated in children.

Quantitative computed tomography (QCT) <xref ref-type="bibr" rid="bib035">35</xref>,<xref ref-type="bibr" rid="bib036">36</xref>

Its advantages are precise three dimensional anatomic localization thereby providing a true volumetric mineral density in gm/cm³, its ability to distinguish cortical from cancellous bone, its ability to exclude extraosseus mineral (such as aortic calcification) for measurement and its higher precision and accuracy than DEXA (2–4% and 4–15% respectively). However it is highly operator dependant requiring a careful slice positioning. Also, due to high spatial resolution it is more sensitive to movement artifact.

Magnetic resonance imaging (MRI)

Although still in early stages, recent reports indicate its potential use as a means of assessing bone mineral density. T1 and T2 relaxation times of vertebral marrow show a progressive and statistically significant decrease with age.

Prevention of osteopenia

There are numerous studies^37–43 showing that Ca and P supplementation of human milk to preterm infants is essential. The ideal postnatal rate of bone mineralization in preterm infants is not clearly established. The optimum goal may be to achieve Ca and P intakes in amounts sufficient to allow bone mineralization similar to intrauterine bone mineralization rates.

Supplementation

In general, Ca requirements in healthy preterm infants range from 120–140 mg/kg/day and P requirements would range from 60–75 mg/kg/d.

Day et al³⁸, using Ca supplementation of 143 mg/kg/d and P supplementation at ½ this dose in healthy preterm neonates found that net calcium accretion rate was similar to intrauterine accretion. Steichen³⁷et al found that BMC of healthy preterm infants taking 100-150mg/kg/d of calcium and ½ this dose of phosphorus was comparable to those taking much higher dose (220 - 250 mg/kg/d). These intakes may not be sufficient for infants with medical complications such as BPD or long term diuretic therapy. Thus, depending on the variability associated with absorption, losses and net retention, and intakes of Ca of upto 250 mg/kg/d and of P upto 140 mg/kg/d is probably sufficient to achieve intrauterine bone mineralization rates. These intakes can be achieved either by feeding them with specially designed formulas or using human milk fortifiers. Supplementation of P without Ca or Ca without P is physiologically inappropriate and can have significant side effects.

The currently available special formulas for preterm infants contain a mixture of MCT & long chain fats & are reported to have an average fat absorption of 92 to 95% & adequate Ca & P retention without excessive feed loss of minerals or fat. Formula fed neonate fare better in terms of bone mineralization then do fortified human with fed neonates.⁴⁰

Total parenteral nutrition

In infants who do not tolerate adequate enteral feeding for prolonged periods because of chronic illness, BPD or NEC, TPN solutions with Ca content of 60 mg/dl and P content of 47mg/dl result in average retention of 88–94% for Ca and 71–97% for P. Thus about 70% of peak in utero accretion of Ca and >90% of peak in utero excretion of P can be achieved. Running these solutions at 120 ml/kg per day results in a much improved calcium and phosphorus delivery, although it is still less than the intrauterine rate. However, for the extremely LBW infant requiring very small daily fluid volumes, such high amounts of calcium and phosphorus in TPN solutions still often result in precipitation of these minerals. Newer mineral preparations hold promise for increasing the calcium and phosphorus concentrations in TPN solutions for the very LBW infant, namely calcium-glycero-phosphate and calcium-glucose-phosphate.

The ideal age at which supplementation should be started is not clearly defined. It is reasonable to begin supplementation when enteral intake is well established arbitrarily at greater than 75 kcal/d.⁴⁴

The age at which high mineral intake should be discontinued is not well defined. Data on whole body BMC indicate that preterm infants near the time of hospital discharge have low BMC compared to newborn infants matched for post menstrual age. This supports a period of supplementation beyond 1800 to 2000g in body weight.⁴⁵ It seems appropriate to continue mineral supplementation at least until a body weight comparable to that of full term infant is reached. An unpublished study from our institution has revealed better BMD at 3 months (corrected gestational age) with calcium phosphorus supplementation of VLBW infants however the BMD was significantly less when compared with term neonates at 3 months of age.

However, there are few studies showing that mineral supplementation or fortification is not necessary. Steven J Gross et al⁴⁶ showed that BMC of preterm neonates fed with breastmilk alone was comparable to those given Ca and P supplementation as well as fortification. Carey et al⁴⁷ showed that preterm infants fed with human milk absorb more than 90% of ingested P and retain virtually all P that is absorbed. Any supplementation with P alone or with Ca and P will only result in urinary excretion of these supplemented minerals. Moreover, there are potential risks associated with any method of mineral augmentation of human milk such as increased osmolality of milk, dilution of immunological properties, undesirable intakes of other nutrients and impedance of fat and magnesium absorption secondary to high Ca intake. Jan Faerk et al⁴⁸ found that neither phosphate, fortifier (protein, Ca and P) nor preterm formula supplementation of human milk to preterm infants (<32 weeks) had any significant effect on bone mineral content at term compared with those exclusively breastfed. Hence they concluded that neither mineral supplementation nor use of preterm formula significantly improves bone mineralization outcome at term.

Vitamin D

The use of vitamin D in the prevention or treatment of osteopenia is not defined. Since infants receiving routine amounts of vitamin D (400-500IU/d) have been found to be vitamin D sufficient and no defect in vitamin D metabolism has been found in preterm infants, supplementation beyond 800 IU/d is not recommended There is no evidence that supplementation with vitamin D metabolites such as 25(OH)₂ D is useful.

Role of estrogen and progesterone

The effective treatment of postmenopausal osteoporosis with hormone replacement and the fact that the preterm babies suffer from premature withdrawal from exposure to estradiol and progesterone has inspired the use of these hormones in the prevention of osteopenia of prematurity. Trotter A et al⁴⁹ conducted a randomized controlled pilot study in which they aimed to maintain intra-uterine plasma levels of estradiol and progesterone (in addition to calcium and phosphorus supplementation) for 6 weeks postnatally and demonstrated that the highest median bone mineral accretion rate was in the hormonal replacement group, Extensive research is however, needed to address the potential benefits and to rule out adverse effects.

Treatment of osteopenia

Aims of treatment of established osteopenia are to

Optimize nutrient intake

Minimize pharmacological therapies that interfere with nutrient retention and bone mineralization.

Minimizing various physical therapy that might contribute to fractures of the weakened bones.

It is important to ensure normal (or returning to) status by frequent monitoring of serum Ca, P and alkaline phosphatase. Continue mineral supplementation until there is radiographic healing of rickets and fractures.⁵⁰ BMD using DEXA is a more sensitive means to measure changes in bone mineralization and can be used whenever it is available.

Long term outcome

There is scarce data on long term consequences of osteopenia of prematurity. Cogdon et al⁵¹ followed up preterm infants who were fed with standard infant formula and vitamin D supplementation and demonstrated spontaneous resolution of bone mineral depletion by 60 wk of post conceptional age. Bowden LS et al⁵² found that bone mineral content in ex preterm infants, at 8 years of age was significantly low as compared to controls. They were also shorter by 4.9 cm and lighter by 2.6 kg. Fewtrell MS et al⁵³ demonstrated that biochemical evidence of metabolic bone disease as indicated by high peak plasma alkaline phosphatase during neonatal period was associated with stunting at 18 months, 7.5 to 8 years and 9 to 12 years.

In a multicentric study involving 857 preterm infants, Lucas A et al⁵⁴ demonstrated that osteopenic infants showed a significant reduction in length at 9 months and 18 months post term.

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. High alkaline phosphatase activity and growth in preterm neonates. Arch Dis Child, 1989; 64: 902–909.