Is a well-defined round nodule in the lung tuberculosis (TB) or a pulmonary arteriovenous malformation (AVM)?

Distinguishing Tuberculosis from Pulmonary Arteriovenous Malformation

A well-defined round nodule in the lung is more likely to be tuberculosis than a pulmonary arteriovenous malformation (AVM), but specific imaging features on CT can definitively distinguish between them.

Key Distinguishing Features

Pulmonary AVM Characteristics

• Pulmonary AVMs appear as well-defined nodules with visible feeding and draining vessels connecting a pulmonary artery to a pulmonary vein, creating a characteristic vascular morphology that distinguishes them from other nodular lesions 1
• The presence of a visible feeding artery and draining vein on CT is pathognomonic for pulmonary AVM and represents the abnormal vascular connection bypassing the normal capillary bed 1
• Pulmonary AVMs have a benign pattern of morphology that can be recognized on imaging, specifically the vascular connections that are visible on thin-section CT 2

Tuberculosis Nodule Characteristics

• TB nodules are extremely common in Asia, with one study from Thailand observing a very high prevalence of TB in nodules ≤8 mm, particularly in the 4.5-11 mm range 2
• TB can present as well-defined round nodules but lacks the characteristic feeding and draining vessels seen with AVMs 2
• In endemic areas, TB should be strongly considered for any indeterminate pulmonary nodule, and nonsurgical biopsy may be useful to avoid unnecessary thoracotomy 2

Diagnostic Approach

Initial Imaging

• Perform thin-section CT (≤1.5 mm sections) without IV contrast to characterize the nodule and identify vascular connections 2, 3
• Look specifically for feeding arteries and draining veins - their presence confirms pulmonary AVM 1, 4
• Review prior imaging to assess stability; a nodule stable for ≥2 years suggests benignity and may represent a tuberculoma 3

Risk Stratification

• For nodules >8 mm, calculate malignancy probability using validated models like the Brock model, though this applies primarily to malignancy risk rather than differentiating TB from AVM 3
• Consider clinical context: history of TB exposure, symptoms of hypoxemia or paradoxical embolization (suggesting AVM), or hereditary hemorrhagic telangiectasia (strongly associated with AVM) 1

When Diagnosis Remains Uncertain

• If vascular connections are not clearly visible and TB is suspected based on endemic area or clinical features, consider nonsurgical biopsy for specific diagnosis requiring medical treatment 2
• Transthoracic contrast echocardiography is the most sensitive test for identifying intrapulmonary shunting if AVM is suspected but CT findings are equivocal 1
• Be aware that false-positive PET scans can occur with TB due to inflammatory uptake, reducing PET's discriminative value in endemic areas 2

Critical Pitfalls to Avoid

• Do not assume all well-defined round nodules are malignant - both TB and AVM can present this way but require completely different management 2, 1
• Do not miss the vascular connections on CT - use thin sections and multiplanar reconstructions to identify feeding/draining vessels that confirm AVM 2, 4
• In TB-endemic regions, do not proceed directly to surgery without considering biopsy for tissue diagnosis, as an infectious etiology may be the correct diagnosis 2
• If initial biopsy shows only inflammation or granuloma, implement careful surveillance during therapy, as the initial diagnosis may be incorrect and a second diagnosis should be considered if the patient fails to respond to treatment 2