Lymphocytic Myocarditis: Seaport Criteria for Non-Biopsy Specimens

Definition and Core Classification of Lymphocytic Myocarditis

Lymphocytic myocarditis is formally defined as myocardial inflammation predominantly composed of lymphocytes, accompanied by myocyte injury that is not attributable to other causes, such as ischemia, trauma, foreign body, or amyloid. When both lymphocytic infiltrate and myocyte injury are present, the diagnosis should be invoked as active lymphocytic myocarditis. This condition represents one of the most frequently encountered patterns of non-ischemic myocardial inflammation and is most often attributed to unproven viral infections, though other infectious and non-infectious causes exist.

The diagnostic framework provides for cases where inflammation is present but evidence of injury is lacking. If a lymphocytic infiltrate is identified within the myocardium in the absence of discernible myocyte injury, the term lymphocytic infiltrate of uncertain significance (LIUS) is recommended. The presence of LIUS should prompt a thorough review and possibly an increase in the number of examined tissue sections to search for overt myocarditis elsewhere in the heart. Notably, lymphocytes located strictly within areas of fibrosis, fat, or isolated within blood vessels and lymphatics do not meet the criteria for any form of myocarditis.

Histopathologic Indicators of Myocyte Injury

To establish a diagnosis of active lymphocytic myocarditis, myocyte injury must be distinct from changes observed in non-inflamed areas. A spectrum of myocytologic changes is considered indicative of such injury. These include myocytolysis, characterized by the overt breakdown of myocyte structure and disruption of the sarcolemma, and sarcolemmal scalloping, where the lymphocytic infiltrate imparts sharp and irregular borders to the sarcolemma. Other sufficient findings include single-cell hypereosinophilia, vacuolar degeneration containing prominent vacuoles within the cytoplasm, and lymphocyte internalization, where lymphocytes are present within the sarcoplasm of the myocytes. Nuclear changes such as pyknosis (shrinkage with chromatin condensation), karyorrhexis (nuclear fragmentation), and karyolysis (nuclear dissolution) are also definitive markers of injury, as is myocyte dropout.

Specimen Types and Technical Requirements

The established criteria apply to ventricular myocardial specimens derived from surgical resections and autopsy samples, specifically excluding atrial specimens like atrial appendages or reduction atrioplasties due to the uncertain clinical significance of inflammation in atrial tissue. There are four primary specimen types: autopsy hearts, surgically explanted hearts, septal myectomy samples, and apical core resections. Because myocarditis is often patchy, specific minimum sampling requirements are necessary to reasonably exclude the diagnosis.

For autopsy and explant specimens, a minimum of six full-thickness sections (extending from endocardium to epicardium) of ventricular myocardium must be histologically evaluated. These should be distributed across five or six blocks and should include any gross abnormalities observed. A tiered approach is suggested, where these six sections are processed initially, with a minimum of one mid-ventricular short-axis slice retained for additional sampling if the first round is inconclusive. Septal myectomy specimens require a minimum of two cassettes of myocardium, depending on the sample size. Apical core resections must be entirely submitted as cross-sections to visualize the full transmural thickness from epicardium to endocardium.

Extent and Distribution of Inflammation

Upon identifying active lymphocytic myocarditis, the extent of the process must be characterized as focal, multifocal, or diffuse based on the area of involvement within a single tissue section. Focal myocarditis is defined as a single focus of lymphocytic myocarditis that does not involve $\ge 50\,\%$ of the area of myocardium on the examined section. Multifocal myocarditis is used when two or more non-contiguous foci are identified, either on a single section or across multiple sections, but the total area involved remains < 50\,\% of the examined tissue section. The term diffuse is reserved for instances where $\ge 50\,\%$ of the area of a single tissue section is involved by confluent active lymphocytic myocarditis.

Temporal Qualifiers and Chronicity

The temporal stage of the disease is classified based on the presence of healing or remodeling features. These qualifiers provide clinical relevance and guide interpretation of the disease progression. Acute lymphocytic myocarditis is diagnosed when there is no granulation tissue present. Subacute lymphocytic myocarditis is characterized by the presence of granulation tissue, indicating an intermediate stage of healing. Chronic lymphocytic myocarditis is defined by the observation of mature, replacement-type interstitial fibrosis in association with persistent inflammatory cells.

As the disease "burns out," it may leave only residual scarring. Distinction between myocarditis-related fibrosis and ischemic injury is often possible through distribution patterns. Myocarditis-related fibrosis typically favors subepicardial regions. In contrast, ischemic injury usually originates in the subendocardium, follows the vascular territories of epicardial coronary arteries, and may specifically spare the immediate subendocardial myocytes due to oxygen proximity in the cardiac chambers—a feature not seen in myocarditis. Chronic cases may involve biventricular dilation, hypertrophy, and progressive loss of myocytes, which may lead to heart failure and warrants terms such as chronic myocarditis-related heart disease.

Forensic Application and Attribution of Death

In the autopsy setting, the pathologist must determine if the identified myocarditis is causal, contributory, or incidental to the death. While more extensive disease, such as fulminant or multifocal myocarditis, is generally accepted as a reliable cause of death in the absence of other findings, focal myocarditis presents an interpretive challenge. It may be entirely incidental, occurring in individuals with clear alternative causes of death, yet it also possesses arrhythmogenic potential, particularly if it involves the conduction system. Currently, no histological features definitively differentiate incidental focal myocarditis from significant focal myocarditis.

Confounding factors can mimic or obscure the significance of inflammatory cells. Cardiopulmonary resuscitation, cardiac arrhythmias, and catecholamine surges can induce myocyte injury that subsequently attracts inflammatory cells if there is sufficient interval survival. Furthermore, substances such as cocaine and antipsychotic medications (e.g., clozapine) can cause cardiac inflammation with ambiguous significance. A comprehensive evaluation—including circumstances of death, medical history (e.g., recent viral illness, fever, chest pain, palpitations, or syncope), clinical data (ECG, cardiac MRI, troponin levels), and complete toxicology—is required to attribute causality accurately.

Limitations of Histopathologic Evaluation and Future Directions

Current histopathologic methods are limited by sampling error, as the patchy distribution of myocarditis means inflammation can be missed even with multiple sections. Additionally, microscopic findings are often non-specific and do not always correlate with clinical prognosis. The current Seaport criteria rely on hematoxylin and eosin (H&E) staining, which can be subjective. While immunohistochemistry (IHC) increases diagnostic yield, it is not currently mandated for non-biopsy specimens due to cost and accessibility limitations in many forensic settings. Troponins, postmortem imaging, and molecular genetics for viral nucleic acid detection are noted but remain nascent for routine diagnostic incorporation.

Future research is needed to validate ancillary techniques, better define the clinical significance of LIUS, and utilize systems biology and artificial intelligence to minimize subjectivity. Emerging technologies like spatial transcriptomics may provide better insight into the relationship between inflammation and myocyte injury, especially for injury lacking a clear morphologic counterpart. Multidisciplinary approaches involving novel radiologic tracers may eventually allow for the characterization of inflammation by cell type in vivo, potentially reducing the reliance on tissue-based diagnosis.