A 36-year-old woman with history of hypertrophic cardiomyopathy, awaiting heart transplant, developed a 2-month clinical condition of progressive low-grade fever and dry cough, refractory to multiple cycles of antibiotics. Laboratory exams presented leukocytosis (17.000x109/mL) with neutrophilia and pronounced C-reactive protein elevation (260mg/dL). The remaining laboratory investigations were unremarkable.
Initial chest radiographs showed bilateral ill-defined opacities with basal predominance, more conspicuous on the right hemithorax (Fig. 1a,b).
An initial chest computed tomography (CT) scan was performed, which showed multifocal ground-glass opacities associated with septal thickening (crazy-paving pattern), mainly with peripheral distribution and lower lobe predominance, with scattered small foci of consolidation (Fig. 2a-c).
A follow-up CT, ten days later, showed more extensive opacities, which also increased in density, while maintaining basal and peripheral predominance. A small right pleural effusion was also seen (Fig. 3a,b), probably related to the underlying cardiac dysfunction.
The patient underwent a lung biopsy with histological evaluation, which showed enlarged alveolar septa caused by early fibrosis, inflammatory infiltrate and alveoli filled with blood and fibrin. These morphological aspects were compatible with acute fibrinous and organising pneumonia (AFOP). Therapy with methylprednisolone was started, with symptomatic, laboratory and imaging improvement in the following two months.
AFOP is a rare form of idiopathic interstitial pneumonia, which courses with pulmonary parenchymal inflammation and fibrosis. 
The dominant histopathological aspects are the presence of intra-alveolar fibrin, with formation of "fibrin balls", and concomitant foci of organising pneumonia. Thickening of alveolar septa can be seen as a result of inflammatory cell infiltration. 
Clinical manifestations include dyspnoea, fever and cough. 
There are two forms of the disease: 1) the acute and fulminant form, with rapid respiratory failure, associated with significant intra-alveolar fibrinous component; 2) the subacute form, with slower progression and good response to therapy, in which the organising component predominates. 
The definitive diagnostic of AFOP relies on pathology and requires the presence of the two dominant histological findings described above (refer to "Background" section). 
Imaging findings are nonspecific and demonstrate the severity of lung involvement. Chest radiography may show disperse segmental opacities and diffuse interstitial infiltrates. Chest CT scans demonstrate two types of findings: 1) the fibrotic component of the disease, with diffuse, basal predominant, ground-glass opacities and simultaneous reticular thickening (which often produces a crazy-paving pattern); 2) the organising pneumonia foci, with multifocal consolidations, with variable distribution. 
Although histologically both components co-exist, the first imaging findings are predominantly seen in the acute form of the disease, while the second ones are more common in the subacute form. 
The main imaging differential diagnoses are acute pulmonary oedema, acute respiratory distress syndrome and some atypical types of pneumonia. 
Outcome (therapeutic options, prognosis, impact of imaging on therapy planning)
Therapy involves high-dose corticosteroids for several weeks. As such, the exclusion of infectious entities is essential. 
Short term prognosis is variable, with a high morbidity and mortality rate in the acute form and an average recovery rate in the subacute form. 
In the long term, the prognosis is poor, with frequent recurrence and high mortality rate in the following three years, usually caused by pneumonia. 
Residual peribronchial fibrosis and architectural distortion are frequently found on CT scan reassessments. 
This case report describes a histologically proven case of AFOP in the subacute form, with a slow and progressive evolution and a good response to corticosteroids. The patient died one year later, during the post-operative period following heart transplantation, from pneumonia (Mycoplasma pneumoniae).
Take-Home Message / Teaching Points
AFOP is a rare idiopathic interstitial pneumonia with variable prognosis.
Imaging findings are nonspecific and include ground-glass opacities and foci of organising pneumonia.
The definitive diagnostic is histological, with identification of "fibrin balls" inside the alveoli and foci of organising pneumonia.
 Travis WD, Costabel U, Hansell DM et al (2013) An official American Thoracic Society/European Respiratory Society statement: Update of the international multidisciplinary classification of the idiopathic interstitial pneumonias. American Journal of Respiratory Critical Care Medicine 188(6):733-48 (PMID: 24032382)
 Damas C, Morais A, Moura CS, Marques JA (2006) Acute fibrinous and organizing pneumonia. Rev Port Pneumol XII(5):615-620 (PMID: 17117329)
 Kima JY, Doob KW, Jang HJ (2018) Acute fibrinous and organizing pneumonia: Imaging features, pathologic correlation, and brief literature review. Radiology Case Reports 13(4):867-870 (PMID: 30174771)
 Santos C, Oliveira RC, Serra P et al (2019) Pathophysiology of acute fibrinous and organizing pneumonia – Clinical and morphological spectra. Pathophysiology 26:213–217 (PMID: 31076239)
 Yang X, Kong J, Yang MX et al (2020) Analysis of clinical characteristics of 21 cases of acute fibrinous and organizing pneumonia. Chinese Journal of Tuberculosis and Respiratory Diseases 43(8):670-676 (PMID: 32727179)
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.