PAVMs are characterized by an abnormal vascular connection between an afferent pulmonary artery and one or more efferent pulmonary veins without an interposed capillary bed. The pulmonary capillary bed works as a filter of the bloodstream that removes small thrombi and septic particles. Bypassing the pulmonary capillary bed provides a direct right-to-left shunt that depends on the diameter of the feeding artery [
1], allowing the systemic venous blood to bypass gas exchange and pulmonary capillary bed processing. As a consequence, thromboembolic and septic emboli arising in the pulmonary circulation may evoke cerebrovascular disorders, one of which is the formation of a brain abscess.
Approximately 80-95% of PAVMs are associated with HHT, which is also known as the Osler-Weber-Rendu syndrome. HHT is a rare systemic angiodysplasia inherited as an autosomal dominant disorder, characterized by telangiectases and arteriovenous malformations. The widely accepted
Curaçao criteria is the standard for diagnosis of HHT, and is based on the most characteristic features of the disease: i) spontaneous and recurrent nosebleeds, ii) mucocutaneous telangiectasia, iii) familial history of HHT in the first-degree relative, and iv) visceral AVM. A diagnosis of ‘definite HHT’ is reached if a patient exhibits at least three of the four criteria, ‘suspected HHT’ if two criteria are present, and ‘unlikely HHT’ for fewer than two criteria [
2]. For patients with definite clinical HHT, genetic testing is not required to confirm the diagnosis. The goal of genetic testing for HHT is to identify the causative mutation in a family with clinically confirmed HHT, to establish a diagnosis in relatives of a person with a known causative mutation, and to assist in establishing a diagnosis of HHT in individuals who do not meet the clinical diagnostic criteria. Since more than 80% of all HHT cases are due to mutations in either the endoglin gene (
ENG) on chromosome 9 (coding for the endoglin protein) or activin receptor-like kinase gene (
ACVRL1) on chromosome 12 (coding for the activin receptor-like kinase 1 protein), the primary recommended genetic test includes analysis of these genes [
3,
4]. More recently, mutations of the gene called mother against decapentaplegic homolog 4 (
MADH4, coding for the SMAD4 protein) have been described in 1-3% of HHT patients who present with a rare syndrome of combined familial juvenile polyposis and HHT. Hence, the HHT Guidelines Working Group recommends that if the complete gene analysis for the
ENG and
ACVRL1 genes is negative, SMAD4 testing should be considered to identify the causative mutation [
5]. However, no matter how remarkable the genetic testing techniques have developed, the final diagnosis of HHT is still based on clinical manifestations because the existing genetic testing is only about 80% sensitive. In the current case, we clinically diagnosed the patient with ‘suspected HHT’ according to the
Curaçao criteria; however, the characteristic genetic tests were negative, with no definite family history or any other condition associated with acquired PAVMs. Therefore, the patient was considered to have a brain abscess resulting from idiopathic PAVMs.
The clinical manifestations and complications of idiopathic PAVMs are similar to those associated with HHT and anatomically similar to HHT-related PAVMs, with notable differences of a more significant proportion of solitary PAVMs and a lack of lower lobe predominance [
6]. Chest CTA is the gold standard for diagnosis, and transcatheter embolization is the treatment of choice for PAVM. However, because this modality can fail if the PAVM is large or has multiple complex feeding vessels, surgical resection is sometimes necessary for those kinds of patients [
7].