What is erdheim-chester disease?

Erdheim-Chester disease is an excessive accumulation of histiocytes (a type of cell) inside tissues in the body, leading to a range of symptoms. Often, the bones in the legs are most affected by this disease, with this cell accumulation causing changes in bone density, and bone pain is one of the first noticeable symptoms. Other symptoms can include the build-up of fatty plaques in the eyelids and skin, as well as general muscle and joint aches, weight loss and fatigue.¹ 

This condition can be difficult to diagnose due to its rarity and similarity to other diseases. Other associated conditions that could indicate an Erdheim-Chester diagnosis include:²

• Exophthalmos
• Retroperitoneal fibrosis
• Diabetes insipidus
• Dyspnea

The most prominent challenge when diagnosing Erdheim-Chester disease is due to its multisystemic involvement, meaning that several organ systems are affected, so the correct diagnosis is difficult to accurately pinpoint. This means that there is a lack of clear and specific symptoms, which, combined with the disease's rarity, can often lead to delayed diagnosis.²

Being able to image characteristic symptoms of this disease allows an accurate diagnosis to be made, allows the severity of the disease to be assessed and allows treatments to be monitored to ensure that they are working effectively. Several imaging techniques are used for Erdheim Chester disease, as some techniques are more suited to certain organ systems, and this is a disease that can affect several of these.³ This article will review the use of PET, MRI and CT scans and their findings to diagnose and assess Erdheim-Chester disease.

Role of imaging in erdheim-chester disease

It is always optimal to reach a diagnosis as early as possible, so that it can be treated as early as possible. This is often only feasible through high-quality imaging and, for this disease, requires several different imaging methods.³ The variety of symptoms associated with this disease can make it difficult to distinguish this condition from other similar diseases. Though certain indicators, such as a ‘Hairy Kidney Sign’ and a ‘Coated Aorta’, which both show up on CT scans, can help to differentiate Erdheim-Chester from other diseases.²

Another way in which imaging can be beneficial is in monitoring the efficacy of treatment. After first-line treatment such as interferon therapies or other targeted therapies such as BRAF inhibitors, PET and CT scans can be used to assess how effective the treatment has been by visualising and monitoring the metabolic responses of the patient.⁴

Positron Emission Tomography (PET)

PET scans are a functional imaging tool that can show a visual interpretation of metabolic pathways and are often used to assess the severity of cancers and monitor treatment. PET scans are particularly useful for assessing how well a treatment is working within the body.

One study found that PET scans could show skeletal issues, making this useful for diagnosis. PET uptake showed bone lesions, mostly in long bones in the legs. It also showed that a large proportion of patients also displayed abnormalities in the pelvis, which were shown on PET scans but difficult to detect with other imaging techniques.⁴

Additionally, PET scans can show lesions on the kidney which correspond to the ‘Hairy Kidney’ that can be seen on CT scans, further helping to clarify the diagnosis. Lesions on the Brainstem, Cerebellum, Spinal Cord and Brain Parenchyma can also appear in PET scans; these results usually correlate with enhanced masses detected by MRI scans.⁴

Overall, PET can be used to diagnose lesions throughout the whole body across several tissues and organ systems. This can be used with other imaging techniques to reach a diagnosis and monitor treatment, with the only negative being that it shows little anatomical detail, and so further techniques may be required to localise issues further.

Computed Tomography (CT)

CT scans image the soft tissues of the body so they can clearly show lesions and provide a clear structural and anatomical assessment. As previously mentioned, the distinguishing features of ‘Hairy kidneys’, ‘Coated Aortas’ are seen as abnormalities of soft tissue that show up uniquely on scans.⁵ Furthermore, thoracic findings demonstrate that thickening fluid or a build-up of material on chest CT scans also indicate an Erdheim-Chester diagnosis.⁶ 

The high resolution of CT scans is beneficial as it can clearly show bone and organ structure in detail; however, this technique has its downsides, including radiation exposure and its inability to image the central nervous system to a quality comparable to other organ systems. CT is a useful tool, but just like PET, it should be used alongside other imaging techniques to verify symptoms across different organs and tissue types.

Magnetic Resonance Imaging (MRI)

Soft tissue and the central nervous system are best imaged using MRI and is the greatest method to accurately diagnose issues with the nervous, cardiac and pituitary systems. 

MRI can be used to assess features such as: thickened tissue on the brain’s outer layer (though this can sometimes lead to a false diagnosis), abnormalities of the pituitary gland and hypothalamus, which can cause diabetes insipidus (a condition commonly associated with Erdheim-Chester disease), tissue build-up surrounding the eyes, brain and heart (particularly the right atrium). Varied bone density can also be shown clearly by MRI, which is a prominent symptom of Erdheim-Chester disease, as tissue build-up in the bones makes them denser and changes their appearance on scans.^2,7,8 

The benefits of using MRI include the high quality of image that can be produced without the need for radiation exposure, and it also has a high contrast between soft tissues, making them easier to distinguish. On the other hand, the process of MRI imaging is quite time-consuming and is not as good as other methods at distinguishing bone detail.

Comparative strengths of PET, MRI and CT

The most important strengths of PET scans are that they can visualise the metabolic activity of the whole body, providing a comprehensive image of how the drug is affecting metabolic pathways and what effect that is having on tissue build-up. This is useful in monitoring the response of the patient to treatment and can be used to evaluate how effective treatments are. This enables changes to be made; if it is evident that a treatment plan is working, the treatment can be continued with confidence, whereas if it is not working as well as it should, a different treatment may be suggested.⁴

CT scans can clearly show the bones in detail to identify abnormalities, particularly in the bones, lungs and kidneys and can indicate an Erdheim-Chester diagnosis by showing features such as ‘hairy kidneys’ and ‘coated aortas’. This method is also quick to do and readily available for many patients, and so is very useful to give an initial indication of health issues.⁵

MRI has proven to provide the best quality and highest resolution images of soft tissue and the central nervous system. This method has proven to be the best at diagnosing issues around the eyes and heart and shows abnormal tissue growth in these areas. Furthermore, this demonstrates a greater soft tissue contrast, making images clearer to see, and there is no radiation administered in this method, eliminating some of the negatives of other methods.^7,8

An integrated approach using a combination of all of these methods is the best way to diagnose Erdheim-Chester disease and monitor treatment. Using each method to its strengths and piecing the big picture together using this combined method gives the most comprehensive and accurate information, which is in the best interest of the patient.

Summary

Imaging is crucial in the diagnosis of Erdheim-Chester disease and the monitoring of treatment. PET imaging can provide a functional visualisation of the body and is instrumental in assessing the efficacy of treatment. CT provides a structural image of lesions and abnormalities that can then be investigated in further detail by MRI imaging. In order to make sure the information that we have is accurate, a combination of imaging techniques is crucial to show all parts of the disease in order to best combat it. Current methods tend to use a combination of PET and MRI imaging as an effective hybrid imaging technique. This shows structural detail and indicates how the metabolism is functioning within the body; this has proven an effective way to assess and help treat Erdheim-Chester disease.⁵