the cerebral abscess is a common central nervous system infection that can result from trauma , hematogenous spread , or spread from an adjacent infection such as otitis media or sinusitis . despite exhaustive searches , 15 to 30% of abscesses are termed cryptogenic when no source of infection is identified20 ) . a distant infection focus that can cause brain abscesses is a cardiac right to left shunt , which is related to patent foramen ovale15 ) , cyanotic cardiac disease15 ) or pulmonary arteriovenous malformation or fistula ( avf ) . pulmonary avf is a rare congenital vascular malformation involving direct communication between the pulmonary artery and vein without an intervening capillary bed . approximately 8095% of pulmonary avfs are associated with hereditary hemorrhagic telangiectasia ( hht ) , known as osler - weber - rendu syndrome5,8,9 ) . the clinical triads of pulmonary avf is cyanosis , exertional dyspnea , and digital clubbing ; however , 56% of one large series were asymptomatic8 ) . the most prominent central nervous system ( cns ) complications associated with pulmonary avfs are neurologic events , including transient ischemic attacks31 ) , recurrent stroke1,6,18,27 ) , brain abscesses , and seizures26 ) . . we will focus on the cryptogenic brain abscess as developed by patients with idiopathic pulmonary avf , which may be detected only if we consider it may cause the brain abscess . a 65-year - old woman was admitted with a 1-month history of headache and cognitive impairment that had become aggravated 10 days prior . she showed impairment in orientation and judgment , but there were no lateralization signs of motor paralysis or cranial nerve deficits . there was no history of diabetes mellitus , hypertension , lung disease or heart disease . her blood pressure was 110/70 mmhg , pulse rate was 74/min , body temperature was 36.5c , and her respiration rate was 20 breaths / min . erythrocyte sedimentation rate was 54 mm / h and c - reactive protein ( crp ) was 0.22 mg / l . arterial blood gas analysis revealed a ph of 7.425 , pco2 of 42.5 mmhg , po2 of 90.4 mmhg , and an hco3 of 22.2 mmol / l on room air . total cholesterol was 249 mg / dl and ldl - cholesterol was 172 mg / dl . brain computed tomography ( ct ) showed a mass with perilesional edema on the right frontal lobe . brain magnetic resonance ( mr ) imaging revealed a 43-cm ring enhanced mass in the right frontal lobe , which was associated with severe edema and midline shifting to the left side ( fig . a chest x ray showed nodular infiltrations on the right mid - lung zone and the left upper and lower lung zones . 2 ) and antibiotics were maintained for 8 weeks . her past history gave no indication of exertional dyspnea or episodes of hemoptysis , melena , hematemesis , epistaxis or hematuria that might suggest underlying hht . a 45-year - old woman presented with a 7-day history of a progressive left hemiparesis . on admission , blood tests showed a hb of 13.4 g / dl , an hct of 39.6% , a white blood cell ( wbc ) count of 6290 cells/l , and a crp elevated to 2.62 mg / l . diffusion mr imaging showed a diffusion - restricted ovoid mass on the right motor and sensory cortex measuring 1.50.9 cm that was surrounded by diffuse vasogenic perilesional edema ( fig . 3 ) . mr imaging and mr spectroscopy revealed an enhancing lesion involving the right motor and sensory cortex and increased lactate / lipid complex , which was compatible with a brain abscess . a chest x ray did not suggest underlying lung disease . however , a chest ct revealed a pulmonary avf in the right upper lung ( fig . the brain abscess progressed despite treatment with vancomycin and ceftriaxone , so it was removed via craniotomy and the pulmonary avf was embolized . a 65-year - old woman was admitted with a 1-month history of headache and cognitive impairment that had become aggravated 10 days prior . she showed impairment in orientation and judgment , but there were no lateralization signs of motor paralysis or cranial nerve deficits . there was no history of diabetes mellitus , hypertension , lung disease or heart disease . her blood pressure was 110/70 mmhg , pulse rate was 74/min , body temperature was 36.5c , and her respiration rate was 20 breaths / min . erythrocyte sedimentation rate was 54 mm / h and c - reactive protein ( crp ) was 0.22 mg / l . arterial blood gas analysis revealed a ph of 7.425 , pco2 of 42.5 mmhg , po2 of 90.4 mmhg , and an hco3 of 22.2 mmol / l on room air . total cholesterol was 249 mg / dl and ldl - cholesterol was 172 mg / dl . brain computed tomography ( ct ) showed a mass with perilesional edema on the right frontal lobe . brain magnetic resonance ( mr ) imaging revealed a 43-cm ring enhanced mass in the right frontal lobe , which was associated with severe edema and midline shifting to the left side ( fig . a chest x ray showed nodular infiltrations on the right mid - lung zone and the left upper and lower lung zones . 2 ) and antibiotics were maintained for 8 weeks . her past history gave no indication of exertional dyspnea or episodes of hemoptysis , melena , hematemesis , epistaxis or hematuria that might suggest underlying hht . a 45-year - old woman presented with a 7-day history of a progressive left hemiparesis . , blood tests showed a hb of 13.4 g / dl , an hct of 39.6% , a white blood cell ( wbc ) count of 6290 cells/l , and a crp elevated to 2.62 mg / l . diffusion mr imaging showed a diffusion - restricted ovoid mass on the right motor and sensory cortex measuring 1.50.9 cm that was surrounded by diffuse vasogenic perilesional edema ( fig . 3 ) . mr imaging and mr spectroscopy revealed an enhancing lesion involving the right motor and sensory cortex and increased lactate / lipid complex , which was compatible with a brain abscess . however , a chest ct revealed a pulmonary avf in the right upper lung ( fig . the brain abscess progressed despite treatment with vancomycin and ceftriaxone , so it was removed via craniotomy and the pulmonary avf was embolized . cryptogenic brain abscesses can occur due to rare diseases that are not addressed in routine clinical practice . congenital cyanotic heart disease3 ) , patent foramen ovale12,15 ) , thoracic infection19 ) or asymptomatic dental infections7 ) are commonly associated with cerebral abscesses . idiopathic asymptomatic pulmonary avfs are also a cause of brain abscess , especially recurrent brain abscess ( table 1 ) . the incidence of idiopathic pulmonary avf - related cns complications is between 19 and 59 % 11,25,29 ) . associated neurological events included migraine , transient ischemic attack , stroke , abscess , and seizure17,26,30 ) . the incidence of brain abscess in patients with a pulmonary avf is around 1 to 5%8 ) . the most likely mechanism for these neurological events is a paradoxical embolism across the pulmonary avf or across a coexisting cerebral arteriovenous malformation in patients with hht17 ) . the pulmonary capillary bed acts as a filter that removes small thrombi and bacteria as they enter the bloodstream , even during daily activities such as oral hygiene . in a pulmonary avf , the pulmonary capillary bed is bypassed , providing a direct right to left shunt that depends on the diameter of the feeding artery . as a consequence , patients may develop paradoxical emboli that present as a transient ischemic attack , stroke , or brain abscess5 ) . the fundamental defect that we found was a right - to - left shunt from the pulmonary artery to the pulmonary vein , and the degree of shunting determines the clinical effects . if shunting is minimal , cyanotic symptoms are usually absent . if the right - to - left shunt is greater than 20% of the systemic cardiac output , or if there is reduction of hemoglobin by more than 50 g / l , the patient will have obvious cyanosis , clubbing , and polycythemia17 ) . the characteristic findings of cyanosis , clubbing and an extra - cardiac murmur do not always accompany pulmonary avf , and diagnosis may be difficult . a review of the mayo clinic experience suggested a morbidity of 2633% and mortality of 816% in untreated patients with pulmonary avf26 ) . the international guidelines for the management of pulmonary avf in hht recommends that treatments should be applied to all adults with avfs and children with symptomatic pulmonary avfs . the decision to treat in asymptomatic children ( no dyspnea , no exercise intolerance , no growth delay , no cyanosis or clubbing , no previous complication ) should be made on a case - by - case basis . the selection of pulmonary avfs for embolization is based on feeding artery diameter , generally 3 mm or greater10 ) . a literature review ( table 1 ) indicates that idiopathic pulmonary avfs can cause brain abscess in patients as young as 18 years old . based on these reported series , 10 of 13 patients did not have pulmonary avf - related symptoms until the brain abscess developed . idiopathic pulmonary avfs are more frequently solitary , around 80% , compared with that of hht ( < 40%)30 ) . it means idiopathic pulmonary avfs are less likely to be associated with large right - to - left shunt , this might in part explain why patients with idiopathic pavms might have a lower frequency of cyanosis and polycythemia . also , previous hht series have shown an association between number of pulmonary avfs and cerebral abscess risk22 ) . this might explain why idiopathic pulmonary avfs are associated with a lower frequency of cerebral abscess than in hht . the organisms in the brain abscess were not consistent but most frequently isolated ones were streptococci genus . 6 cases out of 13 did not reveal an organism at all . if young adults without a premorbid history present with a brain abscess , pulmonary problems must be evaluated . this report highlights the need to consider pulmonary avf as an etiology of cerebral abscess when routine investigations fail to detect a source .