What is mucoid impaction in the lungs on a computed tomography (CT) scan?

Mucoid Impaction in Lungs on CT Scan

Mucoid impaction on CT scan appears as branching tubular or nodular opacities representing bronchi filled with inspissated secretions, often displaying the characteristic "finger-in-glove" sign due to accumulated mucus plugs within dilated bronchi. 1

Definition and Imaging Characteristics

Mucoid impaction refers to the obstruction of proximal bronchi by mucous plugs and exudates, which can be readily identified on CT imaging with the following features:

• Tubular or branching opacities extending from the hilum peripherally (finger-in-glove sign)
• Relatively low-attenuation structures compared to surrounding lung tissue
• Often associated with bronchial dilatation
• May appear as branching opacities that follow the bronchial tree pattern 1

CT is superior to chest radiography for visualizing mucoid impaction, as it provides better differentiation between mucoid impaction and other pathologies such as arteriovenous malformations 1.

Clinical Significance and Associations

Mucoid impaction is an important indirect sign associated with bronchiectasis 2. It represents a significant finding that may indicate:

1. Underlying bronchiectasis: Bronchial wall thickening and mucus impaction are among the most common CT abnormalities found in patients with chronic cough 2

2. Allergic Bronchopulmonary Aspergillosis (ABPA): Mucoid impaction is strongly associated with ABPA, where it may represent a form of "mucoid microimpaction" 3, 2

3. Cystic Fibrosis (CF): Proton MRI can demonstrate the presence of bronchiectasis and mucus plugging in CF patients 2

4. Other conditions: May be seen in:

   • Primary ciliary dyskinesia (PCD) 2
   • Asthma 4
   • Bronchocentric granulomatosis 2
   • Bronchial obstruction from tumors or foreign bodies 5

Pathophysiology

Mucoid impaction develops through two primary mechanisms:

1. Abnormal mucus production and clearance:

   • Excessive mucus production
   • Impaired mucociliary transport
   • Abnormal mucus rheology

2. Obstructive mechanism:

   • Discrete lesions (tumors, foreign bodies) causing bronchial obstruction with secretion accumulation distal to the obstruction 5

Clinical Implications

The presence of mucoid impaction on CT has important clinical implications:

• May cause lobar collapse if a lobar bronchus is occluded 5
• Associated with higher exacerbation frequency in COPD patients 2
• In CF patients, mucoid impaction increases during pulmonary exacerbations and may reverse after antibiotic therapy 2
• Can mimic other pathologies like tuberculosis or neoplasm on imaging 4

Diagnostic Approach

When mucoid impaction is identified on CT:

1. Evaluate for underlying causes:

   • Assess for bronchiectasis patterns and distribution
   • Look for central bronchial obstruction
   • Consider ABPA if central distribution of disease is present 2

2. Consider complementary diagnostic tests:

   • Pulmonary function tests
   • Sputum culture and sensitivity
   • Aspergillus-specific tests in suspected ABPA (IgE levels, skin testing)
   • Bronchoscopy may be needed to remove mucus plugs and obtain samples 6

Management Implications

Identification of mucoid impaction should prompt:

1. Treatment of the underlying condition (ABPA, CF, asthma)
2. Consideration of bronchoscopic removal of mucus plugs in cases of lobar collapse
3. Appropriate antimicrobial therapy if infection is present
4. Airway clearance techniques and mucolytic therapy

Recognizing mucoid impaction on CT is crucial for proper diagnosis and management of various pulmonary conditions, particularly in patients with chronic respiratory symptoms.