Diagnosis cannot be made based on clinical symptoms alone, and must therefore be supported by wide ranging evidence gained from different types of additional examinations.
PLAIN FILM X-RAY
Heading up these examinations is the plain film X-ray, which allows diagnosis to be questioned or confirmed. Often this technique is not sufficient and a body scan may be required or even a bone biopsy. In cases of polyostotic forms of the disease, where no other diagnosis is likely, this technique alone is generally enough.
A plain film x-ray is taken as a result of a culmination of symptoms being present (pain, fracture, deformation), or following a different event, when the lesion causing FD is discovered by chance. The characteristics of the lesion allows the diagnosis to be made, however, as is common in bone disorders, bone lesions can vary widely in appearance and a series of other imaging techniques or a bone biopsy is often necessary.
Plain film x-rays can also be used to monitor a known bone lesion, as part of an annual review for example.
Plain film x-rays use an x-ray beam, which is blocked in different ways depending on the composition of the tissue concerned. The skeleton blocks far more x-rays than soft tissue for example. If this technique is used sparingly then there should not be a problem. However, care should be taken in limiting the number of x-rays taken as this disease will need to be monitored for life, which may lead to a large x-ray dose over a long period of time. For example, the radiation received following a single facial x-ray is equivalent to that of the annual dosage that would be received naturally. Thus, x-rays taken for diagnosis, or as part of annual monitoring through selecting the most useful incidences, must not represent a significant risk to the patient. Limits to the number of x-rays taken of children are in place, and the ovaries of girls are protected by a specially made lead lined apron.
BODY SCANNING OR COMPUTERIZED (AXIAL) TOMOGRAPHY
The CT (OR CAT) scan allows sections of the body to be visualised, and thus provides additional information over a plain film x-ray. Whilst it allows the diagnosis to be confirmed, the main interest in this technique is that complications that are difficult to see on a plain film x-ray, such as fine cracks, are identifiable. It is also used to study the relationship between the lesion and the neighbouring tissue, in particular for cranio-facial forms, in order to assess the risk of neurological complications.
This technique also uses x-rays, but at a significantly higher dosage compared to plain film x-rays, which means that the number of examinations using this method must be limited, especially in children.
MAGNETIC RESONANCE IMAGING (MRI)
MRI is an imaging technique that uses variations in a magnetic field within which the patient is placed. It is probably less useful for diagnostic purposes than the techniques that use x-rays, with its main role being to determine the relationship between the disease and the neighbouring organs, essentially in the cranio-facial and spinal regions, in order to assess the risk of neurological complications. This examination technique does not emit any radiation. It can therefore be used preferentially on children, and on those who need to have limited exposure to x-rays, even if the data obtained is different to that from the CT scan.
Bone scintigraphy is a technique that works by the venous injection of a weakly radioactive solution into the body, which fixes to the bones. A camera scans the whole body and highlights all the skeletal lesions present. This examination is carried out once diagnosis has been made, to identify all FD bone lesions. Furthermore, this technique can be used to assess disease activity, especially in monostotic forms in which biochemical bone markers are not prominent, or even to identify cracks in the bone not visible on a plain film x-ray. This examination is very well tolerated, but is not carried out on pregnant women.
Bone densitometry is used to measure the density of the bones. This examination can be carried out on the hip of patients who are affected in this area, in order to measure the effects of treatment by medication. The technique uses x-rays, which are lower in energy those of plain film x-rays. Thus radiation is far lower, in the order of natural daily radiation levels.
Laboratory tests are very useful. Firstly, measurement of blood phosphorous levels must be carried out, at least initially, as it is low in patients with certain diseases. Often this information is not enough and an investigation of renal function is required, including a blood sample and urine collection over a 24 hour period. This is used to detect urinary phosphorus wasting. Analysing the blood for the protein responsible (FGF-23) for this phosphorus wasting is also possible. Biochemical bone markers are measured in the blood (osteocalcin, phosphatase alcalcin and serum CTX) and are used to assess disease activity. The higher the level, the more active the disease. Their concentrations generally reduce in response to appropriate treatment with medication. Research into the mutation can be made from bone tissue, obtained via a biopsy.
A bone biopsy consists of taking a small fragment of bone and analysing it under a microscope. The sample can be taken with monitoring by x-ray (or CT scan), or in a surgical procedure. A bone biopsy may be necessary if imaging techniques have not been successful in reaching a definite diagnosis. Indeed, abnormalities observed in x-rays are often such that other diseases are suspected, rather than FD. This is sometimes the case if a single bone lesion is present.
Created: 09 june 2010