SAP Scintigraphy for Amyloidosis Diagnosis

Introduction:

Amyloidosis is an infrequent group of diseases characterized by pathological buildup of amyloid fibril proteins within the body’s tissues. Diagnostic tools should be highly efficient and specific to detect the presence of such abnormal proteins and the kind of protein deposition. The medical world has revolutionized technologies for diagnosing rare diseases, and one such great diagnostic tool for detecting amyloidosis is serum amyloid P component (SAP) scintigraphy, which was introduced by Sir Mark Pepys in 1987.

What Is SAP Scintigraphy?

Amyloidosis can cause amyloid fibrils to be deposited in various tissues in the body. A nuclear imaging tool is employed to detect the presence of such abnormal fibril proteins. This is called SAP scintigraphy or serum amyloid P component, which uses the unique properties of the amyloid P component. Serum amyloid P component is a normal protein present in the bloodstream of all humans but in very minimal quantities. In patients with amyloidosis, the SAP proteins are seen in high quantities in circulation and various organs and tissues. These infiltrated SAP proteins will light up during the scintigraphy to help locate and diagnose amyloidosis.

What Unique Properties Does the Serum Amyloid P Component (SAP) Possess?

• SAP has the greatest affinity to the misfolded amyloid fibrils in amyloidosis. This is due to its ability to recognize and attach to the beta-sheet structure characteristic of amyloid deposits. This specificity ensures that when radiolabeled SAP is introduced into the body, it will preferentially bind to amyloid fibrils wherever they are present.

• SAP is considered to be a pentraxin (which means that it has high structural stability). This structural stability helps SAP components to be intact and functional in the bloodstream long enough to circulate throughout the body and bind to amyloid deposits.

• SAP is a protein normally present in the plasma. This means that it is not an antigen and cannot induce an immune response in the administered individual. This non-immunogenicity is highly beneficial for detecting amyloid proteins.

• The binding of SAP to amyloid fibrils does not interfere with the pathology of the deposits. Still, it allows for their better visualization only through imaging techniques used by gamma cameras.

How Is SAP Scintigraphy Performed?

• The first step in SAP scintigraphy is preparing the patient and assessing whether this procedure is essential. During the preparation stage, the patient is asked to consume a small amount of potassium iodide. This prevents the thyroid gland from taking up the radiolabeled SAP component (radioactive iodine) injected.

• This will be followed by radiolabelling serum amyloid P component (SAP) with a gamma-emitting isotope. The isotope used for the SAP scintigraphy procedure is Iodine-123.

Radiolabeled SAP Injection:

• The radiolabeled SAP will be injected slowly through the patient’s vein (intravenous).

• The injected SAP (radiolabelled with I-123) will be allowed to circulate throughout the body for 24 hours. During this time, the SAP will distribute and bind with amyloid fibrils present all over the body. Leaving enough time for circulation helps in better visualization and accurate diagnosis.

Imaging:

• After allowing the SAP to circulate and bind, the patient will be subjected to the imaging procedure.

• The radiolabeled SAP bound to amyloid proteins will start to emit gamma rays, which will be detected with a gamma camera. The whole body will be imaged to identify amyloid deposits, and the patient will be requested to stay still for better image accuracy.

Analysis of Obtained Image:

• The images obtained from gamma cameras will be thoroughly assessed to determine the location of the amyloids. The intensity of the images formed in different organs will be analyzed. Higher intensity indicates a greater accumulation of amyloid deposits in that particular region or organ. Thus, quantitative assessment can aid in determining the extent of amyloid burden in a specific region.

Interpretation:

• A nuclear medicine specialist will analyze the images and compare old SAP scintigraphy images with new ones to have an overview of the disease progression.

• A detailed report about the deposited amyloid fibrils' pattern, distribution, and intensity will be provided.

What Are the Advantages of SAP Scintigraphy?

• SAP scintigraphy is a non-invasive diagnostic tool. Thus, it is a great alternative to biopsy.

• SAP scintigraphy can be beneficial in identifying the presence of amyloid deposits even in regions where biopsy is impossible (for example, the spleen or adrenal glands).

• SAP scintigraphy can provide a comprehensive idea about the presence of amyloid proteins throughout the body.

• The procedure is highly specific and sensitive as the radiolabeled SAP components bind specifically to amyloid fibrils.

• SAP scintigraphy is invaluable for the longitudinal monitoring of patients with amyloidosis. Repeat scans can track disease progression or regression, providing critical insights into how well a patient responds to treatment.

• Since the patient takes potassium iodide right before the procedure, the radiation exposure is highly minimized.

• The SAP molecule used for the procedure is obtained from healthy individuals and is radiolabeled only after proper purification and treatment. So, the SAP molecules cannot elicit an immune response in the affected patients when injected through the vein.

• SAP scintigraphy is also an effective tool to stage the amyloidosis disease in patients.

What Are the Limitations of SAP Scintigraphy?

• SAP scintigraphy cannot provide enough information about the presence of amyloids in moving organs, particularly the heart and the intestine. Therefore, a biopsy would be required to detect amyloid deposits in these organs.

• SAP scintigraphy cannot detect the amyloid deposits in the brain as the penetration of the radiolabeled SAP molecules into the brain is extremely slow.

• SAP scintigraphy centers are not available easily and are very scarce.

• Smaller molecules of amyloids in different organs cannot be detected effectively by SAP scintigraphy.

• SAP scintigraphy cannot classify or detect the type of amyloidosis the patient suffers from. The molecular composition of amyloid fibrils cannot be detected with this procedure.

Conclusion:

Overall, SAP scintigraphy is a cornerstone in diagnosing systemic amyloidosis in humans. Future research aims to develop better and improved radionuclides and combine other imaging modalities with SAP scintigraphy. Also, the number of centers where SAP scintigraphy can be performed should be increased to make the procedure more accessible to common people.