**10. Intravenous drug use**

Intravenous drug use (IVDU), along with cardiac-devices, CHD and vascular access catheters, are the major risk factors for RSIE. Right-sided IE constitutes 5–10% of IE cases and approximately 90% of RSIE involves the TV [71]. Overall, IVDU use accounts for 5–10% of all cases of IE [3, 72]. The median age at time of infection is 30–40 years, not infrequently seen in patients with human immunodeficiency virus (HIV). The majority of cases (right-sided > left-sided) are thought to involve structurally normal cardiac valves [8, 72]. Staphylococcus is the usual microorganism however infections are not infrequently polymicrobial [57]. Various fungi and pseudomonas are noteworthy for severe cases of IVDU-associated IE [8]. Interestingly, streptococci and enterococci more commonly affect left-sided valves, often with underlying structural abnormalities [73, 74].

According to Mathew et al. [75], the overall incidence of left-sided cardiac involvement was similar to right-sided IE, with a minority involving both right and left-sided valves [75]. Others have reported a predominance of right-sided lesions in patients with IVDU [72, 76]. The overall incidence of IE in IVDU is estimated at 0.7–20 cases per 1000 patient-years [74, 77].

The increased risk of IVDU patients acquiring IE is likely attributable to a multitude of factors. Proposed explanations include: (i) particulate matter injury to endothelium from substance injection, (ii) drug-induced thrombus formation and vasospasm, (iii) immune complex deposition on valves, (iv) altered host immune function, (v) frequent exposure to high volume bacterial inoculation, (v) increased prevalence of staphylococcal skin carriage, and (vi) sympathomimetic -induced PHTN resulting in an increase in valvular regurgitation velocity and endothelial trauma. A preference for right-sided involvement of structurally normal valves may also be related to altered host and microorganism factors [71, 74, 78]. It is theorised particulate matter up to 8–10 μm in size can transit across the normal pulmonary vasculature and potentially traumatise left-sided valvular endothelium [75]. However, the relatively high prevalence of left-sided valve involvement in the IVDU cohort without apparent underlying valve disease, may not be completely explained by the above theories and warrants further research.

Transthoracic echocardiography is often very useful in IVDU patients for excluding predisposing underlying structural heart disease and providing confirmation of IE, especially for TV endocarditis. Patients with IVDU are often younger and with satisfactory acoustic windows. In addition, the TV is located anteriorly within chest, being in close proximity to the imaging transducer. The use of TOE is preferred for complicated cases of right and left-sided IE, such as periannular extension, prosthetic valves, nondiagnostic TTE, CHD and for excluding infection at other sites within the right heart, such as the Eustachian valve, atrial wall or vena cavae.
