Osteoporosis in premenopausal women

Most osteoporosis guidelines recommend that bone density testing is not appropriate in healthy premenopausal women, but we all see on referral younger women in whom a bone density has been performed and reported as abnormal.

The T-score DXA classification of normal, low bone mass (osteopenia) and osteoporosis should not be applied to healthy premenopausal women. Instead, the age-related Z-score should be used, and concern should be raised in those with a Z-score below –2.0. This does not indicate disease but does indicate that the patient’s bone mineral density is >2 standard deviations below the mean value for women of her age and ethnicity. This patient may still be perfectly normal with respect to bone health since, by statistical definition, 2.5% of the “normal” population will have values below the fifth percentile.

That is not a justification for ignoring the result but must prompt you to look for potential secondary causes for low bone mass. Please note that I did not use the phrase “bone loss!" Many patients interpret a negative number (ie, the T- or Z-score) as an indication that they lost bone. On a single time point measurement, it is not possible to make such a dynamic statement, and many of these younger women simply have a low peak bone mass. There are many potential reasons for having a low peak bone mass, many of them genetic such as strong positive family history of osteoporosis or a subclinical inborn error in bone metabolism such as osteogenesis imperfecta.

The list of secondary causes of low bone mass in premenopausal women is quite long but not too different from the list that applies to postmenopausal women. One important mechanism more common in younger women is anorexia/bulimia and the female athlete triad.

Perhaps more important than anything is the personal history of fracture in this younger age group since this is often the trigger for ordering the DXA study. Stress fractures, even if recurrent, do not often represent underlying metabolic bone disease. Minimal trauma fractures should alert you to consider inherent bone disease including osteoporosis. The difference between major and minimal trauma remains arbitrary, but the time-honored definition (probably incorrect) is that a minimal trauma fracture is one that results from "trauma equal to or less than a fall from a standing height."

A recent issue of the Journal of Women’s Health carries a “must-read” report from the bone clinic at Columbia University in New York. Of the 61 women included in their report, 37 had an obvious cause for secondary osteoporosis, and of that group, only 40% had a history of fracture. In contrast, 58% of the 24 women without an identified secondary cause for osteoporosis had a history of fracture. There were a number of additional findings of similarities and differences between these younger women who did or did not have secondary osteoporosis or who did or did not have a history of fracture.

There are too many important details to include here (get a copy of the article and keep it on hand), but there are two that stand out. The first is that smaller women tended to have lower BMD, underscoring the limitations of this two-dimensional approach to “density." The second is that a substantial proportion of the women identified as having secondary osteoporosis or “idiopathic” osteoporosis had been prescribed bisphosphonate therapy prior to referral to this top-notch bone clinic, including six of the 10 with low bone mass, no identified secondary cause for bone loss and no history of fracture. That’s probably not a good thing — and note the word “probably” — since there is very little data about the use of bisphosphonates in this population nor is there much data about any potential adverse effects on the fetus should one of these women become pregnant. And remember, the “in the bone" half-life of bisphosphonates is quite long.

BMD measured by DXA is a very valuable clinical tool, but as with all clinical studies, it is important to order the correct test/procedure and in the correct patient. This article very clearly articulates the authors’ one year of experience in one of the most highly regarded osteoporosis referral centers. The discussion about their observations adds substantially to the impact of the study.

Cohen A. J Women’s Health. 2009;18:79-84.