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The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 10 4586-4589
Copyright © 2003 by The Endocrine Society

Are Adult Patients with Laron Syndrome Osteopenic? A Comparison between Dual-Energy X-Ray Absorptiometry and Volumetric Bone Densities

Carlos A. Benbassat, Varda Eshed, Moshe Kamjin and Zvi Laron

Endocrine Institute (C.A.B., V.E.), Rabin Medical Center, Beilinson Campus, Petah Tikva 49100, Israel; Nadrilony Institute (M.K.), Tel Aviv 52451, Israel; the Pediatric Endocrine and Diabetic Research Unit (Z.L.), Schneider Children’s Medical Center of Israel, Petah Tikva 49100, Israel; and Sackler School of Medicine (Z.L., C.A.B.), Tel Aviv University, Tel Aviv, 69978 Israel

Address all correspondence and requests for reprints to: Carlos Benbassat, M.D., Endocrine Institute, Rabin Medical Center, Beilinson Campus, Petah Tikva, Israel 49100. E-mail: carlosb{at}netvision.net.il.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Severe short stature resulting from a deficiency in IGF-I is a prominent feature of Laron syndrome (LS). Although low bone mineral density (BMD) has been noted in LS patients examined by dual energy x-ray absorptiometry (DEXA), this technique does not take volume into account and may therefore underestimate the true bone density in patients with small bones. The aim of the present study was to evaluate the BMD yielded by DEXA in our LS patients using estimated volumetric values. Volumetric density was calculated with the following formulas: bone mineral apparent density (BMAD) = bone mineral content (BMC)/(area)3/2 for the lumbar spine and BMAD = BMC/area2 for the femoral neck. The study sample included 12 patients (mean age, 43.9 yr; mean height, 123.7 cm). Findings were compared with 10 osteopenic subjects without developmental abnormalities (mean age, 56 yr; mean height, 164.8 cm) and 10 healthy control subjects matched for sex and age to the LS patients (mean height, 165.5 cm). BMAD in the LS group was 0.201 ± 0.02 g/cm3 at the lumbar spine and 0.201 ± 0.04 g/cm3 at the femoral neck; corresponding values for the osteopenic group were 0.130 ± 0.01 and 0.140 ± 0.01 g/cm3, and for the controls, 0.178 ± 0.03 and 0.192 ± 0.02 g/cm3. Although areal BMD was significantly lower in the LS and osteopenic subjects compared with controls (P < 0.02) at both the lumbar spine and femoral neck, BMAD was low (P < 0.01) in the osteopenic group only. In conclusion, DEXA does not seem to be a reliable measure of osteoporosis in patients with LS.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
LARON SYNDROME (LS), or primary GH insensitivity syndrome (GHIS), is a genetic disorder in which GH receptor deletions or mutations, or postreceptor defects, result in lack of IGF-I generation (primary IGF-I deficiency) and consequent developmental abnormalities, of which dwarfism is the most prominent (1, 2, 3). Current treatment of LS consists of replacement with recombinant IGF-I, but its availability is limited.

LS patients serve as a unique model for the study of the effects of IGF-I on bone, independent of GH action. Both GH and IGF-I have a well-recognized role in bone growth and skeletal maturation, but their effects on bone mass accretion are less clear (4). Children and adults with GH deficiency have a low bone mineral density (BMD), which improves after GH replacement (5, 6, 7, 8, 9, 10, 11). Little is known, however, about BMD in patients with primary IGF-I deficiency. Although lower than normal values have been reported in the few studies published to date (12, 13, 14), all comprised a small number of subjects, and areal density rather than volumetric density was measured. Areal density, measured by dual-energy x-ray absorptiometry (DEXA), is a function of the bone mineral content (BMC), and the projected area is expressed in square centimeters. Because it does not take vertebral depth into account, the findings are dependent on bone surface and may represent an underestimation in subjects with small bones (15, 16, 17, 18). Volumetric density has been considered by some authors to be a more accurate estimate of bone density in these subjects (15, 16, 17, 18). It is better assessed by quantitative computed tomography, but this technique involves high radiation exposure and is not widely available. Several authors have proposed estimating the volumetric density as derived from posteroanterior DEXA measurements (19, 20, 21, 22). In a recent study using the estimated volumetric density, Bachrach et al. (23) found a normal BMD in adult patients with LS. They suggested that the osteopenia found in LS subjects in earlier studies using DEXA technology was a consequence of the smaller bone dimensions in the affected subjects compared with healthy adults. The aim of the present study was to evaluate bone density in 15 adult patients from our LS series using estimated volumetric values compared with their DEXA measurements.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
The auxological, biochemical, and hormonal characteristics of the Israeli cohort of LS patients being followed in our clinic for decades have been described in detail previously (24, 25, 26). The medical files of 15 of these patients were reviewed in the present study. Three of them had been treated with IGF-I for a mean period of 3 yr and will be considered separately. The BMD data in the remaining 12 patients (seven females and five males; mean age, 43.9 ± 8.5 yr; mean height, 123.7 ± 11 cm) were compared with those of 10 osteopenic subjects (seven females and three males) without developmental abnormalities (mean age, 56 ± 12.1 yr; mean height, 164.8 ± 11.6 cm) and 10 healthy control subjects matched for sex and age to the patients with LS (mean height, 165.5 ± 8.3 cm). The hospital ethical committee approved this study, and all patients signed an informed consent form.

BMC and areal BMD were measured using DEXA (Lunar DPX, Lunar Radiation Corp., Madison, WI) at the lumbar spine and femoral neck, in the posteroanterior projection. Age- and gender-specific BMD, Z scores, and T scores were provided by Lunar. This study was performed at a time in which the new data from the National Health and Nutrition Examination Survey (NHANES) III study (27) had not yet been incorporated into the Lunar reference database. Estimated volumetric density [bone mineral apparent density (BMAD)] was calculated as reported by Katzman et al. (22), using the following formula:

Lumbar spine BMAD = BMC/(area)3/2

Femoral neck BMAD = BMC/area2

Values are expressed as mean ± SD. Statistical analysis was performed using the t test.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The characteristics of all subjects studied are shown in Table 1Go. Mean age in the LS group for females was 43 yr (37–49 yr) and for males, 45.2 yr (37–68 yr). There was a mean difference of 41.8 cm in height and 27 kg in weight between the control and LS groups. Patients with LS had a higher body mass index than the controls. Only one fracture was registered among the patients with LS (patient 12), and it was the result of a car accident.


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  		TABLE 1. Characteristics of patients with LS compared to osteopenic (OP) and healthy (Control) subjects

 
Table 2Go summarizes the areal BMD by routine DEXA measurements. In agreement with earlier reports (12, 13, 14), the patients with LS appear to be severely osteopenic. When patient 8, with an unusually high lumbar spine density (1.420 g/cm2; Z score, 2.42) was excluded, the mean BMD at the lumbar spine was in the osteoporotic range. BMD at the femoral neck was not measured in patient 12 because of previous hip replacement. Group comparison yielded significantly lower density values in the patients with LS and those with osteopenia than in the control subjects for both the lumbar spine and femoral neck (Table 3Go). Differences in mean T scores between male and female patients with LS were significant at the lumbar spine (-1.84 vs. -2.23, respectively; P < 0.5) but not at the femoral neck (-2.35 vs. -2.32, respectively; P = not significant). Mean BMC and projected area at the lumbar spine were 23.4 g and 23.3 cm2, respectively, in LS subjects, and 52.3 g and 42.5 cm2 in the controls. It should be noted that because current NHANES III reference data for femoral neck were not used in this study, there might be some underestimation of femoral density in our data.


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  		TABLE 2. BMD and BMC by DEXA in 12 subjects with LS at the lumbar spine and femoral neck

 

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  		TABLE 3. Area BMD (g/cm2) and BMC assessed by DEXA in patients with LS compared to osteopenic (OP) and healthy control subjects

 
Estimated volumetric bone mineral density (BMAD) for the three groups is shown in Table 4Go. No statistically significant differences were observed between the LS group and controls for the femoral neck values. The LS group had a higher BMAD than the control group at the lumbar spine, but the difference did not reach statistical significance. By contrast, the volumetric bone density in the osteopenic subjects was significantly lower at both sites compared with the control group (P < 0.01).


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  		TABLE 4. BMAD (in g/cm3) in patients with LS compared to osteopenic (OP) and healthy control subjects

 
Interestingly, the three LS patients previously treated with IGF-I for a mean period of 3 yr (between ages 13 and 18 yr) were 15 cm taller on average than the untreated patients and had a normal areal BMD 6 yr later (Table 5Go). Their BMAD at the lumbar spine and femoral neck was also higher than that of the untreated LS patients and the control group (lumbar spine, 0.225 vs. 0.201 and 0.189, respectively; P < 0.01; femoral neck, 0.224 vs. 0.201 and 0.196, respectively; P < 0.01).


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  		TABLE 5. Characteristics and densitometric measurements in three patients with LS previously treated with IGF-I

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Our study shows that untreated patients with LS have low BMD when measured by the standard DEXA technique. However, after adjustment for bone size using volumetric estimates derived from DEXA measurements, their BMD was not significantly different from normal subjects. These results agree with previous data reported by Bachrach et al. (23), who studied 11 patients with LS (eight females and three males; mean age, 30 yr) belonging to the Ecuadorian cohort. In that study, the areal BMD measured by DEXA was also low but became normal when volumetric density was estimated and compared with nonaffected relatives of the LS patients. Their results were confirmed by histomorphometric analysis (23). Maheshwari et al. (28) reported similar results in subjects with a congenital defect due to mutations in the GHRH receptor, the Dwarf of Sindh, a disease also resulting in IGF-I deficiency.

The question resulting from the present and previous studies is, which estimation of BMD is diagnostically valid and which should be considered to decide whether therapy is needed. BMD is calculated by dividing the BMC by the projected area of the specific region. The BMC is calculated from the amount of radiation attenuation at the region of interest. DEXA is the most common technique used to measure BMD, but it allows only a two-dimensional representation, resulting in area densities (grams per square centimeter). Because bone thickness is not taken into account, DEXA is not suitable for comparing BMD between subjects with significant differences in bone size (19). By contrast, quantitative computed tomography (QCT) gives a three- dimensional image of the scanned area, which is reported as volumetric bone density (grams per cubic centimeter), and has a higher sensitivity in differentiating normal from osteoporotic patients (29, 30). Nevertheless, a good correlation has been found between the two techniques. For a typical BMD of 1.0 g/cm2, the QCT measurement is 100 mg/cm3, and for each 0.05 g/cm2 change in BMD, there is a corresponding decrease of 10 mg/cm3 in QCT measurements (20). Using these data, a formula to extrapolate areal values obtained from DEXA into volumetric density was incorporated into the software of some early Lunar equipment. However, this formula did not solve the problem of comparing subjects with different bone size. Carter et al. (19) introduced a formula based on the good correlation between vertebral width and thickness that allowed for the geometric transformation of the two-dimensional data obtained by DEXA into a three-dimensional cuboidal representation. The transformed BMD, termed bone mineral apparent density (BMAD), is today widely used in clinical research. It should be noted, however, that it is only an estimate of volumetric density, not a direct assessment, and its correlation with fracture risk has yet to be investigated.

Another mathematical model to calculate apparent volumetric BMD derived from DEXA densitometry was proposed by Kroger et al. (21), in which the vertebra is assumed to have a cylindrical and not a cuboidal configuration. Baroncelli et al. (31) used this formula to study volumetric density in 22 children, aged 6–8 yr, with isolated GH deficiency (GHD). They found that areal BMD was significantly lower in children with isolated GHD than in healthy controls and remained lower, although to a lesser extent, when estimated volumetric density was considered. Boot et al. (32) also noted lower areal BMD measurements in children with GHD compared with their volumetric estimates, and a bigger increment was seen for areal BMD after 2 yr of GH treatment, indicating changes in bone size rather than in true bone density. DeBoer et al. (33), studying a group of 70 adult males with childhood-onset GHD (mean height, 165.8 ± 6.6 cm), observed Z scores of -1.50 for areal BMD at the lumbar spine, but only -0.90 when volumetric estimates were applied, using the formula of Kroger. Thus, the differences observed in our study between areal and volumetric bone density in LS patients are also present, albeit to a lesser extent, in patients with GHD.

On the basis of our data and those reported in the study of Bachrach et al. (23), the possibility arises that BMD in patients with LS is normal. This may be in accordance with the low fracture rate observed in our cohort of LS patients (Laron, Z., personal observation) and suggests that GH may not be involved in the process of bone accretion. Interestingly, the higher BMD observed in our three treated patients, compared with the untreated ones, indicates a GH-independent effect of IGF-I on BMD. The small number of subjects, however, precludes any definitive conclusions in this respect.

In conclusion, we are confronted with a serious dilemma, i.e. whether patients with Laron syndrome are osteopenic or not, and whether they need treatment by IGF-I or bisphosphonates for this condition. Although the low BMD found in these patients using conventional DEXA densitometry may be an artifact of the reduced bone size, the relationship between volumetric density and fracture risk has not been well established. Further studies including direct measurements of volumetric BMD by spine and peripheral QCT, rather than a derived calculation of areal BMD by DEXA, should be conducted to elucidate whether patients with LS need to be treated for osteoporosis.


   Footnotes
 
Abbreviations: BMAD, Bone mineral apparent density; BMC, bone mineral content; BMD, bone mineral density; DEXA, dual energy x-ray absorptiometry; GHD, GH deficiency; GHIS, GH insensitivity syndrome; LS, Laron syndrome; QCT, quantitative computed tomography.

Received April 10, 2003.

Accepted June 24, 2003.


   References
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 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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