Etymology:
Derived from the accumulation of mucus (mucoid) within the airways, leading to obstruction and radiological findings of impacted mucus.
AKA:
Bronchial mucoid impaction (BMI), Bronchocele (in focal cases with associated bronchial dilatation).
What is it?
Mucoid impaction refers to the accumulation of mucus within the bronchial or bronchiolar lumen, resulting in obstruction and potentially causing localized or diffuse lung abnormalities. It can arise from either obstructive or non-obstructive processes and is associated with impaired mucociliary clearance or airway obstruction. Radiologically, it is characterized by branching tubular opacities that may resemble vascular shadows or a finger-in-glove appearance.
Caused by:
Non-Obstructive Causes:
Chronic bronchitis: Excessive mucus production due to chronic airway inflammation (most common).
Asthma: Particularly in poorly controlled cases with mucus hypersecretion.
Allergic bronchopulmonary aspergillosis (ABPA): Often associated with central mucus plugs, which may have high attenuation on CT.
Cystic fibrosis: Thickened secretions due to impaired mucociliary clearance.
Obstructive Causes:
Congenital:
Bronchial atresia: Most commonly affects the apicoposterior segment of the left upper lobe.
Aspiration: Particularly affects the lower lobes due to gravity-dependent pooling of gastric contents.
Resulting in:
Airway obstruction with subsequent hyperinflation, atelectasis, or infection.
Impaired mucus clearance leading to recurrent infections and chronic inflammation.
Structural Changes:
Bronchial obstruction: Lumenal occlusion by thick mucoid material.
Bronchial wall thickening: A key finding often associated with chronic bronchitis or bronchiectasis.
How to assess bronchial wall thickening:
Absolute measurement: Add the thickness of the opposing walls of the bronchus. If the sum exceeds 40?50% of the lumen diameter, wall thickening is present.
Relative comparison to adjacent airways: Compare the wall thickness of the affected bronchus to other airways in the same region with a similar lumen diameter. Normally, bronchial wall thickness should be consistent across airways of comparable size.
Ratio of bronchus to adjacent pulmonary artery: Normally, the bronchial wall thickness is less than the diameter of the adjacent pulmonary artery branch. Thickened walls exceed this ratio.
Lung parenchyma: Atelectasis, post-obstructive consolidation, or hyperinflation.
Pathophysiology:
Impaired mucociliary clearance leads to mucus accumulation.
Obstruction causes localized air trapping, atelectasis, or post-obstructive pneumonia.
Aspiration, especially in the lower lobes, exacerbates inflammation and mucus production, leading to impaction.
Chronic inflammation and recurrent infections promote structural changes, such as bronchial wall thickening and bronchiectasis.
Pathology:
Gross: Thick, inspissated mucus filling airway lumens, often with a jelly-like appearance.
Microscopic:
Dense mucus with inflammatory cells (eosinophils in ABPA).
Evidence of infection or fungal elements in secondary infections.
Diagnosis:
Clinical: Symptoms include chronic cough, wheezing, and recurrent respiratory infections.
Radiology: Characteristic findings on imaging are key for diagnosis.
Radiology:
CXR:
Findings: Tubular or branching opacities (finger-in-glove appearance, especially in ABPA).
Associated Findings: Atelectasis, hyperinflation, or consolidation.
CT (High-Resolution Computed Tomography):
Parts: Central airways in ABPA and bronchitis; lower lobes in aspiration cases.
Size: Varies depending on the extent of airway involvement.
Shape: Finger-in-glove appearance due to mucus-filled dilated bronchi.
Position:
Lower lobes: Often impacted due to aspiration or gravity-dependent mucus pooling.
Central airways: Common in ABPA or chronic bronchitis.
Character:
Low-attenuation mucus in non-infectious cases.
High-attenuation mucus in ABPA or fungal infections (due to calcium or fungal elements).
Time: Chronic or recurrent in predisposed individuals.
Associated Findings:
Bronchial wall thickening, bronchiectasis, or post-obstructive consolidation.
Air trapping or mosaic attenuation.
Finger-in-Glove Appearance and Bronchiectasis:
Finger-in-glove typically indicates mucoid impaction but does not necessarily imply bronchiectasis.
Bronchiectasis is suggested if airway dilation and failure to taper are present alongside the mucoid impaction.
Other Relevant Imaging Modalities:
MRI: Rarely used but may highlight mucus-filled airways.
Ultrasound: Limited role except in pleural complications.
Pulmonary Function Tests (PFTs):
May show obstructive patterns in cases with airway involvement.
Mixed patterns possible in chronic bronchiectasis or fibrosis.
Management:
Underlying cause:
Bronchodilators and corticosteroids for chronic bronchitis and asthma.
Antibiotics for bacterial superinfection.
Antifungals in ABPA or fungal colonization.
Mucolytics and airway clearance techniques:
Chest physiotherapy and mechanical clearance in conditions like cystic fibrosis.
Surgical intervention: Rarely required but may be necessary for obstructing lesions.
Recommendations:
Early imaging to identify and address airway obstruction.
Close monitoring for recurrent infections or progressive structural damage.
Aggressive management of underlying conditions to prevent chronic complications.
Key Points and Pearls:
Mucoid impaction commonly presents as finger-in-glove opacities on imaging.
Lower lobe airway impaction is often due to aspiration, particularly in gravity-dependent regions.
Finger-in-glove appearance is most commonly associated with ABPA but does not imply bronchiectasis unless airway dilation is also present.
Early and aggressive therapy can prevent irreversible structural lung damage.
