Amyloidosis is a rare disease that occurs when amyloid proteins accumulate in organs and tissues of the body. The human heart is frequently impacted by two distinct types of amyloid proteins: the primary or light-chain (AL) and the transthyretin (TTR) amyloidosis proteins ^[1]. Amyloid fibrils accumulating in the myocardium can lead to heart failure due to infiltrative cardiomyopathy ^[2][3]. Most patients with TTR amyloidosis have cardiac involvement. TTR is synthesized by the liver and is present in both serum and cerebrospinal fluid. One of its primary functions is to act as a carrier protein for two essential compounds: retinol (vitamin A) and thyroxine (T4) ^[4][5]. Under normal physiological conditions, TTR is present in the bloodstream in the form of a tetramer that comprises four monomers with beta-sheet rich structures. Changes in the protein’s structure can cause it to lose its tetrameric shape and misfold. As a result, the misfolded TTR can accumulate in different organs of the body and lead to amyloid diseases, such as transthyretin amyloid cardiomyopathy (ATTR-CM) ^[6]. Invariably, the accumulation of the amyloid protein in various organs of the body can cause other diseases that may hint at the presence of cardiac amyloidosis. Conditions such as carpal tunnel syndrome, lumbar spinal stenosis, bicep tendon rupture, and polyneuropathy affecting the autonomic or sensory systems are associated with amyloidosis ^[7].

ATTR-CM is classified into two types: hereditary (hATTR-CM) and wild-type (wtATTR-CM). hATTR-CM is a genetic disorder caused by a mutation in the transthyretin gene. On the other hand, wtATTR-CM is a condition that occurs due to aging. In both forms, the transthyretin protein becomes structurally unstable, leading to its accumulation in the heart ^[8]. The onset of hATTR-CM can occur in people in their 50s to 60s, whereas wtATTR-CM typically develops in older individuals, usually in their late 70s to 80s ^[8]. The detection of ATTR-CM has frequently been overlooked or delayed because of inadequate diagnostic methods ^[9].

Given the range of cardiac and extra-cardiac symptoms associated with ATTR-CM, it is crucial for clinicians to be cognizant of common disease patterns, additional clues, and populations that are commonly affected to ensure an effective diagnosis ^[10][11]. The most prevalent symptoms of ATTR-CM are those of heart failure, which can be either with preserved or reduced ejection fraction. Additionally, atrial fibrillation is also a common symptom ^[10][11]. Unexplained peripheral neuropathy or gastrointestinal symptoms are the most common extra-cardiac systemic manifestations of amyloidosis. Additionally, a history of bilateral carpal tunnel syndrome or bicep tendon rupture can increase suspicion of the disease ^[11].

2.2. Clinical Suspicion

Given that the clinical manifestations of ATTR-CM can be non-specific, maintaining a high index of suspicion is crucial for making an accurate diagnosis. The  suggested that it is crucial to comprehend the challenges that patients encounter in their journey to facilitate an early diagnosis, and they drew attention to the inadequate awareness regarding ATTR-CM in primary and secondary healthcare centers. The panel emphasized that coordination and communication between the cardiac centers and the referring personnel are important for an effective diagnosis. The diagnostic process for ATTR-CM begins with a clinical history and examination, followed by ECHO and CMR imaging ^[7]. ECHO can help identify diagnostic clues such as increased left ventricular wall thickness (≥12 mm), atrioventricular valve/right ventricle free wall/interatrial septum thickening, diastolic dysfunction, decreased mitral annular systolic velocity, and biatrial enlargement ^[12][13]. Strain imaging is important, and the apical sparing pattern could be a clue that helps general cardiologist establish the diagnosis. Additionally, the tissue Doppler of the mitral annulus is important. CMR imaging may be necessary for certain patients when ECHO images are suboptimal or when other potential diagnoses, often referred to as “mimickers,” are being considered. CMR can help diagnose ATTR-CM by detecting certain features such as an expansion of the extra-cellular volume, abnormal gadolinium contrast kinetics, and diffuse late gadolinium enhancement ^[14][15]. Although CMR is an effective tool to rule out amyloidosis in suspected cases of cardiac amyloidosis, it is important to note that it cannot be used as a standalone test. Furthermore, it is neither necessary in all cases nor sufficient for confirming the presence of ATTR-CM or AL amyloidosis as it is unable to distinguish between the two types of cardiac amyloidosis ^[16][17]. The panel emphasized the importance of a multi-modality diagnostic imaging approach, with an emphasis on ECHO, strain imaging, and CMR imaging in the diagnosis process. Literature provides an established diagnostic framework for ATTR-CM, as well as detailing the identification of several red flags that can raise suspicion of ATTR-CM. The initial step to suspect ATTR-CM involves using ECHO to evaluate myocardial wall thickness. Further evaluation is recommended for patients who present with heart failure symptoms or red flag symptoms and have a left ventricular wall thickness of ≥12 mm ^[9][10][18].

2.3. Diagnosis

The diagnostic confirmation of ATTR-CM requires multiple imaging modalities and may require invasive procedures such as endomyocardial biopsy ^[12][19]. However, recent developments in non-invasive imaging have made endomyocardial biopsy less essential ^[20]. A combination of serum/urine electrophoresis and immunofixation tests, along with nuclear scintigraphy findings, is used to diagnose ATTR-CM and rule out AL amyloidosis ^[10][21][22]. While nuclear scintigraphy has become a crucial diagnostic tool for ATTR-CM, it is important to note that this method alone cannot distinguish between ATTR-CM and AL amyloidosis ^[11]. ATTR-CM and AL amyloidosis can be differentiated by concomitant testing for light chains ^[7][22]. Serum/urine electrophoresis and immunofixation may be requested concurrently for nuclear amyloid scan order or before it. An elevated serum light chain should prompt a consultation with a hematology specialist to rule out AL amyloidosis or monoclonal gammopathy of undetermined significance.

Nuclear Scintigraphy for Diagnosing ATTR-CM

In contemporary medicine, nuclear scintigraphy is the mainstay for non-invasively diagnosing ATTR-CM. During nuclear scintigraphy, a bone-avid radio tracer known for its affinity to bind amyloid fibrils is administered intravenously. Subsequently, images are acquired using a combination of SPECT (single photon emission computed tomography) and low-dose CT for radio-tracer uptake localization. These images reveal the distribution of the radio tracer within the myocardium. The physician evaluates the images for abnormal radio-tracer uptake in the myocardium, particularly the left ventricular wall. Increased radio-tracer uptake in these areas is suggestive of ATTR amyloid deposition. Semi-quantitative and quantitative analysis techniques may also be employed to assess the extent and severity of myocardial radio-tracer uptake. This can involve calculating ratios such as heart-to-contralateral lung (H/CL) ratios or myocardium-to-background ratios to provide a more objective measurement of amyloid deposition. Bone-avid radio-tracers such as Tc-99m-DPD (3,3-diphosphono-1,2-propanodicarboxylicacid), Tc-99m-HMDP (hydroxy methylene diphosphonate), and Tc-99m-PYP (pyrophosphate) are commonly used for nuclear scintigraphy, due to their high sensitivity and specificity for ATTR-CM ^[23]. Tc-99m-PYP is the most preferred radio tracer used in Saudi Arabia, while HMDP has been used in times of Tc-99m-PYP shortages. Nuclear scintigraphy is usually ordered after ECHO. However, in a few rare situations, PYP can be performed without ECHO if other red flags or criteria are met (for example bilateral carpal tunnel syndrome with positive histology for transthyretin amyloidosis, or a positive genotype for common ATTR mutations). The panel also advised that the most preferred radio tracer for screening is PYP (nuclear amyloid scans) and emphasized that it can be used judiciously for cases with clinical suspicion following the use of ECHO and/or CMR. The panel stressed the importance of screening patients with ECHO before proceeding with nuclear imaging. The nuclear scans should be performed in accordance with the ASNC protocol for cardiac nuclear imaging, with slight modifications as per the patient’s needs and the available infrastructure ^[14]. Almost all the scans are undertaken with SPECT imaging for the localization of the tracer uptake within the myocardium ^[14]. The images acquired through bone scintigraphy for the diagnosis of ATTR-CM can be analyzed by semi-quantitative and quantitative methods. The Perugini grading system is used to semi-quantitatively assess cardiac uptake, which is divided into four categories: Grade 0 indicates no visible cardiac uptake, Grade 1 indicates mild cardiac uptake visible but inferior to skeletal (rib) uptake, Grade 2 indicates moderate cardiac uptake visible equal to or greater than skeletal (rib) uptake, and Grade 3 indicates strong cardiac uptake with little or no skeletal (rib) uptake ^[20][24][25]. An uptake of Grade 2 and above is considered significant; scans with Grades 2 and 3 have a positive predictive value of 100% for detecting ATTR-CM with a specificity of 87% and sensitivity of 97%. Quantitative assessment involves comparing counts from a region of interest situated over the heart to a region of comparable intensity located on the chest’s opposite side. For all scans that test positive, SPECT (or SPECT/Computed Tomography [CT]) is evaluated to ensure that the uptake signifies the tracer’s retention in the myocardium and not a signal from the blood pool. This is especially crucial in patients with low cardiac output, as the radio tracer may remain in the blood pool ^[26][27][28]. For an accurate diagnosis, it is crucial to exclude AL amyloidosis while identifying ATTR-CM, given that there is a risk of up to 13% for false positive outcomes in patients suffering from AL amyloidosis ^[22][29]. To achieve this, hematologic assessments, such as serum free light chain (FLC) assay (Kappa/Lambda Ratio), serum protein electrophoresis with immunofixation (SPIE), and urine protein electrophoresis with immunofixation (UPIE), are often used in combination with nuclear scintigraphy, for a comprehensive diagnostic evaluation of ATTR-CM. The FLC assay is helpful in ruling out AL amyloidosis, as an abnormal Kappa/Lambda ratio suggests the presence of clonal plasma cell disorders, which are indicative of AL amyloidosis or monoclonal gammopathy of undetermined significance. A normal Kappa/Lambda ratio supports the possibility of ATTR-CM as the underlying cause. SPIE can help detect and characterize abnormal protein bands that may indicate the presence of monoclonal proteins associated with AL amyloidosis or other systemic conditions. UPIE is like SPIE but focuses on protein analysis in urine. It aids in identifying abnormal protein bands in the urine, which can be indicative of AL amyloidosis or other underlying conditions.

3. Conclusions

ATTR-CM remains challenging and often goes unrecognized due to its non-specific clinical presentation and lack of awareness among physicians. However, as treatment strategies advance, awareness of the disease is expected to increase. The use of 99mTc-labeled bone radio-tracer scintigraphy has introduced a non-invasive and highly accurate diagnostic method for the early and definitive detection of ATTR-CM. Presently, studies are focused on the development of novel radio-tracers that are specifically designed to target amyloid deposits in the myocardium. This advancement aims to improve the precision of visualization and quantification of amyloid burden ^[9]. Additionally, the integration of hybrid imaging technologies, such as SPECT/CT or PET/CT, has demonstrated potential in enhancing anatomical localization and aiding in the differentiation of cardiac amyloidosis from other conditions ^[30]. Moreover, the implementation of quantitative analysis methods and artificial intelligence algorithms has shown promise in streamlining the interpretation of bone scintigraphy images and enhancing diagnostic accuracy ^[30][31]. These advancements indicate that bone scintigraphy will increasingly play a crucial role in the early and accurate determination of ATTR-CM, enabling timely intervention and improved patient outcomes.