Incidental case of an idiopathic pulmonary arterial aneurysm masquerading as a stable pulmonary nodule for 17 years

Abstract

A true pulmonary arterial aneurysm is the dilation of all three vascular walls in a pulmonary artery. True pulmonary arterial aneurysms are very rare, and the relatively minor cases on this topic present with symptoms pertaining to the causes of trauma, Behçet’s disease, pulmonary vasculitis, granulomatous lung infections, and cartilage instability. A rupture of the pulmonary arterial aneurysm can be life-threatening and often requires surgical repair. Our case presents a 70-year-old male who was referred to pulmonary clinic for evaluation of an asymptomatic, incidental finding in his right lower lobe that was considered to represent a pulmonary nodule based on its presentation on computed tomography imaging. The referring provider had requested a possible biopsy. Before proceeding with a biopsy, the presence of the pulmonary aneurysm was confirmed only after the fluorodeoxyglucose-positron emission tomography scan and a computed tomography angiography were performed. Our case illustrates the importance of including pulmonary aneurysm in the differential during work-up of a pulmonary nodule, as diagnostic sampling could have been detrimental.

Introduction

A pulmonary arterial aneurysm (PAA) is the dilation of the three layers of the blood vessel wall 1.5 times greater than the maximum normal diameter. ¹ On computed tomography (CT) scans, 29 mm is the upper limit of normal measurement of the main pulmonary artery and 17 mm is the upper limit of the interlobar pulmonary artery. ² This condition is very rare with an estimated occurrence of approximately 1 in 14,000 patients based on post-mortem examinations of 110,000 cases. ³ PAA can often be lethal as progression of the aneurysm to rupture results in a 33% mortality. ⁴ Most causes of pulmonary artery aneurysms are due to trauma, often associated with instrumentation or treatment, and are also associated with diseases such as Behçet’s, pulmonary vasculitis, granulomatous lung infections, and/or cartilage instability. Less common causes would be due to congenital heart disease, pulmonary hypertension, connective tissue diseases, or neoplasms. ² Symptoms of PAAs include pulmonary arterial hypertension (PAH), dyspnea, chest pain, and hemoptysis. ¹ With the rare incidence of PAAs, there have not been any guidelines established for managing these cases. ⁵ However, cautious management should be employed due to the possibility of fatality from rupture. ⁵ The compression of a bronchi from the aneurysm as it enlarges could lead to post-obstructive pneumonia, which can be difficult to treat given the anatomic abnormality. ⁶ This case reports an unusual presentation of an idiopathic, incidental PAA in the right lower lobe that has been masquerading as a relatively stable pulmonary nodule for 17 years.

Case report

A 70-year-old male patient with a history of an abdominal hernia, diverticulitis, hepatitis C virus, hypertension, diabetes mellitus, and obesity presented for evaluation of an incidental right lower lobe solitary lung nodule. In July 2003, he was admitted for abdominal pain and later diagnosed with diverticulitis. Retrospective analysis demonstrated that during his 2003 hospitalization, CT abdomen was performed, incidentally noting a right lower lobe lung nodule. Given this abnormality, CT chest was obtained showing a 15-mm right lower lobe lung nodule with no other chest lesions. Of note, the patient was unaware of this lung lesion and, therefore, does not recall referral to a specialist in 2003.

In July 2020, the patient underwent evaluation for an abdominal hernia, which included CT imaging of the abdomen that displayed an 18-mm right lower lobe nodule (Figure 1(a)). The patient was then referred to the pulmonology clinic for further work-up. History was obtained and positive for prior tobacco use, and the patient had worked as a welder with previous asbestos exposure. He had no personal history of cancer or chest trauma. His notable family history included lung cancer with his mother and heart disease with his father. He had no significant history of hospitalizations for critical illnesses but had prior knee and wrist operations. During his pulmonary assessment, the patient endorsed dyspnea that was initially thought to be weight-related. However, the dyspnea persisted despite weight loss and tobacco cessation. Vital signs were as follows: temperature at 36.2°C, blood pressure of 128/68 mmHg, heart rate of 65 beats/min, respiratory rate of 16 breaths/min, and oxygen saturation of 98% on room air. He denied constitutional symptoms, hemoptysis, or appetite changes. In addition, he had no evidence of oral ulcers or visual complaints.

Figure 1.

(a) CT of abdomen and pelvis from July 2020 with soft tissue image (left) and lung window (right) displaying pulmonary artery aneurysm. (b) Non-contrast CT chest from August 2020 with lung window (left) and mediastinal window (right) displaying pulmonary artery aneurysm.

Given this lung nodule’s indolent appearance and possible endobronchial location, the primary differential included carcinoid, hamartoma, and pulmonary sclerosing pneumocytoma. Pulmonary aneurysm was thought to be less likely given its relatively rare incidence while a lung nodule secondary to malignancy was being entertained in view of the patient’s former tobacco use. Non-contrast chest CT (Figure 1(b)) and positron emission tomography (PET)-CT were performed for lung nodule evaluation. The PET-CT demonstrated a 2.1-cm right lower lobe nodular opacity with mild uptake (maximum standardized uptake value (SUV max) of 2.3) with no hypermetabolic or enlarged axillary, mediastinal, or hilar lymph nodes. While the uptake on PET was mild, neoplasm remained on the differential. Due to suspicion of a pulmonary artery aneurysm with its adjacent vascularity and enhancement similar to pulmonary arteries on prior CT scans, the patient was instructed to undergo CT pulmonary angiography before proceeding with a lung nodule biopsy. Interestingly, CT pulmonary angiography (Figure 2) illustrated an intensely enhancing 2.1 × 1.7 cm lesion with direct blood supply from a right lower lobe pulmonary artery. This prompted the radiologist to interpret this lesion as a pulmonary artery aneurysm or pseudoaneurysm. Mild emphysematous changes with bronchial wall thickening were also identified. Hence, biopsy was not performed.

Figure 2.

CT pulmonary angiography from September 2020 confirming the presence of a pulmonary artery aneurysm.

Given the slow growth of the aneurysm and the absence of prior trauma or instrumentation of the pulmonary vasculature, a diagnosis of idiopathic pulmonary artery aneurysm was made. Since the pulmonary aneurysm had been stable for at least 17 years after a thorough multispecialty evaluation, including thoracic surgery and interventional radiology, the decision was made to continue with serial imaging and clinic follow-up.

Discussion

The etiology of pulmonary artery aneurysms can be categorized as congenital, acquired, or idiopathic. ⁵ Congenital and acquired aneurysms are the most common, while idiopathic PAAs are more rare, thereby resulting in the absence of management guidelines. However, Greene and Baldwin ⁷ have offered four major criteria in defining idiopathic PAA: (a) simple dilation of the pulmonary trunk regardless of involvement in the arterial tree, (b) absence of abnormal intracardiac and extracardiac shunts, (c) absence of pulmonary or chronic cardiac disease, and (d) absence of arterial disease. Although scarce, multiple idiopathic PAA cases exist in literature. In a literature review conducted by Gupta et al., 41 idiopathic cases of PAAs were identified through a PubMed search. Of those cases, there were 21 males and 17 females with idiopathic PAA. The average age of patients was 54.4 years, and the most common symptoms included hemoptysis, dyspnea, and chest pain, while some were asymptomatic. Hemothorax and laryngeal nerve palsy were present in rare cases. Furthermore, 25 of the cases had proximal PAA with an average size of 59 mm while 14 were distal with average sizes of 38 mm. ¹

The pathogenesis of idiopathic PAAs is still a point of debate. In a single-center study presented by Deb et al., ⁸ cystic medial degeneration (CMD) was confirmed in 3 of 5 idiopathic cases, and they postulated that CMD is the most important factor in the pathogenesis of idiopathic PAAs. This idea is supported in existing literature as cystic medial necrosis has been a common finding in true pulmonary artery aneurysms. ⁹ These findings imply that the weakening of the arterial wall, or CMD, may be a strong factor in the development of idiopathic PAA.

The initial presentation of an apparent pulmonary nodule led to suspicion of either carcinoid (75% of bronchopulmonary carcinoid tumors are located in the central airways) ¹⁰ or hamartoma (pulmonary hamartomas account for 77% of all benign lung tumors and 4% of all solitary lung nodules)^11,12 due to its appearance and minimal growth rate. In view of the indolent growth pattern over 17 years and the benign characteristics of this lesion coupled with lack of diseases/processes associated with PAA (e.g. Behcet’s, pulmonary vasculitis, granulomatous lung infection, cartilage instability, and instrumentation/trauma), a vascular disease process was not in the initial differential diagnosis. Follow-up imaging with contrast could have led to consideration of sclerosing pneumocytoma versus aneurysm in the differential. The former deserves particular attention in view of the solitary nature of our patient’s lesion, its proximity to vasculature, and the well-defined borders. Interestingly, there was a mild fluorodeoxyglucose uptake, but given that there was no appearance of malignant lymph nodes, this uptake was thought to be due to inflammatory changes of vessel wall and/or focal atherosclerosis. This case illustrates the importance of considering contrasted chest imaging in scenarios where a lung lesion appears adjacent to a vessel for additional characterization, especially when diagnostic biopsy is requested. CT angiogram of the chest timed for the pulmonary arteries was most useful in our patient’s work-up.

Given the prolonged stability of the lesion, the patient opted not to undergo operative repair after further evaluation by multiple specialists. Furthermore, the risk of rupture is highest in patients with large aneurysms (>5 cm) and/or pulmonary hypertension in which case operative or procedural intervention may be warranted.^13,14 Given the potential challenges of accessing the PAA during intervention ¹⁵ and the patient’s preference, continuous monitoring via follow-up imaging and history was deemed more appropriate in our case.

Conclusion

We report a case of an idiopathic PAA disguising as a stable pulmonary nodule over a prolonged time course. The indolent growth pattern coupled with the patient’s preference supported continued surveillance as opposed to intervention, which carries its own set of potential complications. Despite the rarity of such cases, it is imperative to consider PAAs masquerading as a potential lung nodule prior to proceeding with biopsy, as this could have been ominous in our case. The paucity of literature makes management of idiopathic PAAs challenging. Furthermore, studies should be considered to elucidate the best follow-up course for patients with idiopathic PAAs. Presently, we propose that a multidisciplinary approach linked with patient preference should be taken to optimize management plans.