Back to back patients, both women in their late 50s, arrived at the
clinic with their DXA reports in hand and both very concerned about the wording
on the report. Both reports had the DXA images and in both, the spine images
had clear evidence of scoliosis rendering interpretation difficult.
Careful examination of the first woman confirmed the scoliosis a
condition previously unknown to her. The scoliosis was mild and corrected on
forward bending which she could do without difficulty. Correction of scoliosis
with forward bending is suggestive of leg length discrepancy which I confirmed
very simply by examining leg length. When she lay on her back with her legs
extended, the left was shorter than the right by ~3/4. With her knees
bent, the difference was still apparent. When lying prone with her knees bent
at 90° the difference was no longer apparent, confirming that the
discrepancy was between the hip joint and the knee. Since she had no prior
knowledge of this and no relevant symptoms, no intervention is necessary.
What effect does the scoliosis have on the validity of the DXA and its
interpretation? The simple answer is that no one really knows, particularly
with a mild degree of scoliosis such as seen in this patient. There are
problems when attempting to use DXA for detection of vertebral fractures
more on that later.
The second patient had known about her scoliosis for many years and has
long had both back trouble and difficulty walking. She had sustained more than
her share of fragility fractures and did not really need a DXA to confirm her
diagnosis of osteoporosis.
Not quite true! The correct diagnosis of osteogenesis imperfecta leapt out at me with the blueness of her sclerae along with the history of
multiple fractures, but thats a topic for another time.
DXA can accurately determine the bone mineral content (BMC) of the
analyzed vertebrae even in a person with severe scoliosis and can equally
accurately determine the area of bone analyzed (BA). BMC/BA provides the bone
mineral density, but how to best interpret the data is the issue at hand.
The technology will provide the BMD along with a T-score and a Z-score but at
least to me, it cannot confidently relate the result to persons matched for
age, sex, and ethnicity who do not have scoliosis.
The reference database is established from a population of subjects in
whom the X-ray beam is passed through the vertebral body truly perpendicular
to the orientation of the vertebra (both the vertebral body and the posterior
elements). In a patient with scoliosis, it is not possible for all assessed
vertebrae such the resultant T- and Z-score cannot be as accurately related to
the reference database. This will of course vary with the degree of scoliosis
and the extent to which there is spurring of the vertebral end plates. In some
cases, such as the first patient discussed above, this is not a major problem, but
in the second case it is.
The second case presented an even bigger technical issue because the DXA
technique of vertebral fracture analysis (VFA) was applied. VFA is an excellent
noninvasive, minimal radiation exposure method of evaluating whether or not a
patient has sustained a vertebral fracture without experiencing any clinical
symptoms. This simply does not work in patients with severe scoliosis. I only
had to look at the images obtained to see that the vertebral body markings on
the output were a hit and miss effort with respect to correctly
identifying the edges of the vertebral body.
Whats the solution?
The technician performing a DXA scan can see the image of the scanned
portion of the skeleton (forearm, lumbar spine, proximal femur) as the scan is
proceeding. She/he would be unlikely to know before this that scoliosis is
present since examination of the patient is appropriately not part of a DXA
study. Any evidence that something about the image produced will interfere with
the analysis and interpretation should alert the technician and reader to use
caution with the subsequent interpretation and consider not providing an
interpretation for a suspect scan.
Scoliosis is just one issue. I, and many colleagues, have seen DXA
studies that included a replaced hip which is probably the most egregious
error. Navel rings, zippers in slacks, coins in the pocket and many other
things can interfere with a DXA.
From the perspective of the clinician ordering the scan and receiving
the report, until you are satisfied that the DXA facility you are using is
meticulous in the work, ask to have all of the images and ancillary data
provided to you and not just the summary report. Most often you will be fully
satisfied, but dont rely on that up front!