Myocarditis is an inflammatory disease of the myocardium that is diagnosed on the basis of established histological, immunological, and immunohistochemical criteria . Although the specific pathogenesis of myocarditis after a spider bite remains uncertain, possible mechanisms are the direct toxic effect of alpha-latrotoxin, excessive noradrenaline/acetylcholine release, and immunological mechanisms (i.e., allergy or hypersensitivity reactions) .
Our case report is an interesting example of a rare myocarditis form. Diagnosis was challenging and was mainly based on a few clinical signs, results from diagnostic tools (ECG, CMR), and laboratory values, which ultimately revealed the hidden cause of patient’s illness.
Physicians have several instruments at their disposial for diagnosing myocarditis. Endomyocardial biopsy (EMB) is considered the gold standard for diagnosis, but focal dissemination of the pathogens, rare right ventricle involvement, and immune-mediated forms with little/no cellular infiltrate strongly limit EMB in terms of performance . In one study, only 30% of post-mortem-proven myocarditis met the Dallas criteria . EMB is not sensitive, has a relatively high false negative rate, is expensive, and requires expertise to perform. Current guidelines promote its use only in patients with acute, unexplained heart failure with hemodynamic instability . Our patient was hemodynamically stable and did not meet any of the criteria for being transferred to an EBM-performing center.
CMR is accurate in describing ventricular morphology and functions well in characterizing cardiac tissue. CMR has a diagnostic accuracy for myocarditis of close to 80% and a negative predictive value of 70% ; T1- and T2-weighted images in addition to late gadolinium enhancement (LGE) improve the final diagnosis [11,12,13]. Nevertheless, CMR also has limitations: minimal variations can not be detected; borderline myocarditis or cytokine/humoral-mediated forms might not induce injuries to myocytes, thus no LGE can be seen; finally, the time from symptom onset to CMR performance may influence the results. Therefore, some patients with clinical myocarditis may show normal findings on CMR images . Our case illustrates the limitations of CMR to diagnose myocarditis: despite the signs and the symptoms of the patient clearly being related to a myocarditis condition, no overt features of myocarditis were demonstrated with CMR mapping.
Echocardiography ameliorates the diagnosis of myocarditis. Some findings include: LV dilatation and thickness (edema), systolic/diastolic dysfunction, regional wall motion abnormalities, pericardial effusion, and changes in myocardial texture . Echocardiography is the first approach in acute myocarditis due to its non-invasiveness and good performance in follow-up . Nevertheless, no specific echocardiographic features have been recognized in acute myocarditis, especially in patients with preserved LV systolic function. Initially our patient showed no alteration in LVEF; later, the reduction in GLS value associated with the natural course of the disease suggested the negative influence of inflammation on the myocardium .