The Pathology of Acute Lung Injury(ALI) / Acute Respiratory Distress Syndrome (ARDS) by Dr. Anunoy Samanta

1. By,
Dr. Anunoy Samanta
MBBS, MHA, DNB (Patho) Resident, JLNH & RC, Bhilai

2. 1) Definitions of ALI/ARDS
2) Epidemiology
3) Etiology
4) Risk Factors for ARDS
5) Pathophysiology
6) Three Phases
7) Clinical Features
8) Clinical Diagnosis
9) Radiological Diagnosis
10) Biomarkers
11) Pathological Diagnosis
12) Are Pathologists Needed?
13) Gross Appearance
14) Histologic Patterns
15) Findings in COVID-19
16) Approach to a Lung Bx
specimen
17) Summary

3. Acute Lung Injury (ALI) refers to a stereotypical
pattern of lung injury, secondary to a wide range of
pulmonary & extra-pulmonary insults, encompassing a
continuum of clinical & radiographic changes that affect
the lungs, with Acute Respiratory Distress Syndrome
(ARDS) representing the severe end of this continuum.
It’s too generalized. Clinicians need a working definition!

4.  In 1915, although the terms ALI/ARDS weren’t used
per se, a physician serving with the Canadian Forces,
described the condition of a soldier’s lungs exposed to
poison gas as “Shock Lung”.
 In 1967, Ashbaugh and colleagues coined the term
ARDS to describe a clinical syndrome characterized by
“an acute onset of tachypnea, hypoxemia and loss of
compliance after a variety of stimuli”.
 In 1994, after decades of confusion with myriad
definitions, finally the American-European Consensus
Conferences (AECC) on ARDS recommended the
adoption of an acceptable definition for ALI/ARDS.

5.  Acute onset of diffuse, B/L pulmonary infiltrates
documented by CXR, consistent with pulmonary edema.
 PaO2:FiO2 <= 300 (for ALI) & <=200 (for ARDS)
 PAWP <= 18 mm of Hg, or no clinical evidence of
cardiac failure/ left atrial hypertension.
This 3 criteria definition by AECC was clinically
relevant, easy to use & capable of quantifying the
severity of lung injury. Thus, it was ideal for use both
in the clinical setting & for research purposes.

6.  The radiological criteria
are nonspecific & subject to
inter-observer variability.
 ‘Acute’ was ill defined.
 There was no standard
ventilation protocol for
patients before meeting the
criteria for hypoxemia. E.g.
PaO2:FiO2 can simply be
raised by increasing PEEP.
 PAWP may oscillate above
& below cut-off. It can also
be elevated for reasons
other than heart failure.

7. According to the Berlin definition framed by a panel of
experts in 2012, ARDS is an acute diffuse, inflammatory
lung injury, leading to increased pulmonary vascular
permeability, increased lung weight, and loss of aerated
lung tissue, with hypoxemia, & B/L radiographic opacities
associated with increased venous admixture, increased
physiological dead space & decreased lung compliance. It
categorized ARDS (on PEEP >= 5 cm of H2O) as:-
 Mild: 200 < PaO2:FiO2 <= 300
 Moderate: 100 < PaO2:FiO2 <= 200
 Severe: PaO2:FiO2 <= 100

8.  Acute- means onset <= 1 week
 B/L opacities consistent with pulmonary edema must
be present & may be detected on CT (thorax) or CXR
 PaO2:FiO2 <= 300 with a minimum of 5 cm H2O PEEP
 “must not be fully explained by cardiac failure or
fluid overload,” in the physician’s best estimation using
available information. An objective assessment (e.g.
Echocardiogram) should be performed in most cases in
the absence of a clear cause such as trauma or sepsis.

9.  Berlin definition doesn’t include underlying aetiology
& lacked a direct measure of lung injury.
 It doesn’t allow early identification of patients who
may be amendable to therapies before establishment of
ARDS.
 It still allows CXR to be used for diagnosis which
compares poorly with CT (thorax).
 It has low sensitivity (True Positivity Rate) when
compared to autopsy findings (~ 89%).
 Although slightly better than AECC, its ability to
predict mortality is still poor.

11. Berlin definition, although widely in practice, is not
suitable in resource-constrained settings with difficult
access to arterial blood gas data, CXR & mechanical
ventilation. In 2015, Riviello & colleagues developed a
modification of the Berlin criteria to define ARDS &
called it Kigali modification. It differed in:-
 No minimum PEEP requirement
 SpO2:FiO2 <= 315
 B/L chest opacities evaluated by using of USG: B
Lines or lung consolidation without pleural effusion in at
least one field on each side of the chest is considered
to be consistent with B/L lung opacities. CXR done when
available.

13.  Occur in all age groups. Mean age is 60 years.
 Male : Female = 3:2
 Incidence 15-30 per 1,00,000 person/year
 Incidence of ARDS in ICU:
- 10.4% of all ICU patients
- 67.2% of all patients with acute hypoxemic respiratory
failure
- ARDS is frequently undiagnosed or late diagnosed
 Mortality- 30-40% for past 2 decades
 ARDS : ALI = 70%
 Not all ARDS show DAD (diffuse alveolar damage)
pattern (58% in open lung Bx Vs 45% in autopsy findings)

14.  ALI is a multi-factorial disease process that occurs
due to an environmental trigger on the background of a
genetic predisposition.
 The most common cause of ALI in our country is
pneumonia, whereas in UK, sepsis accounts for the
majority of case load.
 Although genetic markers have been identified that
are associated with a phenotype that predisposes to
ALI, the exact genetic basis of ALI still remains elusive
due to:- (i) diversity in environmental stimuli involved, (ii)
the complex gene-environment interaction, (iii) the
heterogeneous nature of the disease itself & (iv) the
presence of myriad co-morbidities.

16. We classify these causes as direct or indirect
depending on the mechanism of injury to the lung:-
(i) Direct / Pulmonary Causes:
 Infectious Pneumonia- Bacteria (Mycoplasma,
Mycobacteria, Legionnela, Rickettsia), Virus ( Influenza,
Herpes, CMV, Hanta, COVID-19), Fungi (Pneumocystis
jirovecii)
 Aspiration of gastric contents
 Inhalation injury (Smoke, SO2, NO2)
 Near drowning
 Fat emboli
 Pulmonary contusion

17. (ii) Indirect / Extra-Pulmonary Causes:
 Sepsis (Pseudomonas Sp, E. coli, Staph aureus)
 Severe trauma/burn especially with shock & multiple
transfusions.
 Drug (MTX, AZA, Bleomycin, Amiodarone, Gold,
Penicillamine, Nitrofurantoin)
 Autoimmune diseases (Amyopathic dermatomyositis,
SLE, RA, Scleroderma, Polyarteritis nodosa)
 Acute Pancreatitis
 Cardio-pulmonary bypass

18.  Increased severity and extent of illness as measured
by Injury Severity Score (ISS) or APACHE Score.

19. Patients with ALI progress through a similar patho-
physiological process irrespective of whether the
damage is a direct effect on the alveolar epithelial cells
by an external stimulus or an indirect process resulting
from a more distant systemic inflammatory process
mediated via cytokines:-

23.  ALI is a consequence
of alveolar injury which
produces diffuse
alveolar damage (DAD).
The injury causes the
release of pro-
inflammatory cytokines.
 Cytokines recruit
neutrophils to the lungs
where they get
activated & release
toxic mediators that
damage the capillary
endothelium & alveolar
epithelium

24.  In healthy
lungs, there is
a small amount
of fluid that
leaks into the
interstitium.
Our lymphatic
system
removes that
fluid & returns
it into the
circulation,
maintaining
the alveoli dry.

26.  Damage to
the capillary
endothelium
and alveolar
epithelium
allows
protein to
escape from
the vascular
space!

27.  Now, the
oncotic
gradient that
allows
resorption of
fluid is lost &
fluid pours
into the
interstitium,
overwhelming
the lymphatic
system.

28.  Breakdown of
the alveolar
epithelial barrier
allows the air
spaces to fill with
bloody,
proteinaceous
edema fluids &
debris from
degenerating cells.
 Functional
surfactant is lost
resulting in
alveolar collapse!

29. Healthy lungs can regulate
the movement of fluid to
maintain a small amount of
interstitial fluid and dry
alveoli. But, lung injury
interrupts this balance,
causing excess fluid in
both the interstitium &
alveoli. This excess fluid
results in:-
 Impaired gas exchange
 Decreased compliance
 Increased pulmonary
arterial pressure

32. (1) Acute/Early/Exudative Phase (4-7 days):-
 Alveolar Changes:
- Hyaline membrane on alveolar duct or sacs
- Interstitial & intra-alveolar edema
- Collapsed alveoli
 Epithelial Changes: Denudation & necrosis of Type I
pneumocytes
 Vascular Changes:
- Necrosis of endothelial cells
- Neutrophil aggregation
- Micro thromboembolism
- Hemorrhage

33.  Acute exudative
phase of DAD
showing prominent
hyaline membranes
 Interstitial
edema &
inflammation are
present
 Alveolus contains
edema fluid as well
as a number of
inflammatory cells
including RBCs.

34. (2) Proliferative/Organizing Phase (>7 to 21 days):-
 Alveolar Changes:
- Organizing or remnants of hyaline membrane
- Interstitial & intra-alveolar proliferation of
fibroblasts /myofibroblasts
- Lymphocytic infiltration
 Epithelial Change: Proliferation of Type II
pneumocyte
 Vascular Changes:
- Endothelial injury
- Thromboembolism in arteries & arterioles

35.  The alveolar
walls are markedly
expanded by
extensive
fibroblastic
proliferation,
accompanied by
mild inflammation
& edema
 Remnants of
hyaline membranes
are seen
 Thrombus seen
in an arteriole

36.  Micrographs of proliferative phase highlighting Type
II pneumocyte hyperplasia

37.  Micrograph of late proliferative phase with extensive
interstitial & intra-alveolar fibroblastic proliferation

38. (3) Resolving/Fibrotic Phase (>21 days):-
 Alveolar Changes:
- Diffuse collagenous fibrosis
- Microscopic honeycomb like change
- Traction bronchiolectasis
 Epithelial Change: Squamous cell hyperplasia
 Vascular Changes:
- Remodelling of arteries
- Fibrosis in intima
- Thickening of media

39.  Fibrotic phase of DAD showing collapsed/enlarged
alveoli along with Squamous metaplasia

40. (1) Acute Phase:-
 Acute & progressive Respiratory Failure: Usually starts
12-48 hrs after the initial insult, dyspnoea on exertion
followed by dyspnoea at rest
 Hypoxemia resistant to O2 therapy: PaO2:FiO2 <= 300
 Diffuse, fine crepitations on auscultation
 Often accompanied by signs of sepsis and MODS!
(2) Organizing Phase:-
 Resolution of hypoxemia
 Improved lung compliance
(3) Fibrotic Phase:-
 Continued hypoxemia
 Decreased pulmonary compliance & Right heart failure

41. ALI/ARDS is typically a diagnosis based on clinical &
radiological features, with DAD being the histological
counterpart only . Presently we’re using well defined
clinical criteria as proposed in Berlin definition, with a
keen focus on ABG analysis.

42.  Geographic
distribution of patchy
ground-glass densities
& areas of lobular
sparing are radiologic
hallmark of Chest X-
Ray in ALI/ARDS.
 Acute phase shows
B/L whiteout
appearance with
patchy consolidations
 Fibrotic phase may
show interstitial
appearance.

43. Most common CT (thorax)
findings are:-
 B/L, basilar (68%),
patchy (42%),
predominantly dependent
(86%) abnormalities
 Areas of consolidation &
air bronchograms (89%)
 Details of the disease
status can be best studied
around the areas of
consolidation & fibrosis

45. Bronchoscopic
Lavage (BAL) sample
if obtained usually
shows:-
 Cytology- Reactive
epithelial cells,
alveolar
macrophages and
neutrophils
 Giemsa, ZN
Staining might
identify the specific
organism

46.  BAL of a patient with ARDS who tested + for
COVID19 shows: Reactive broncho-epithelial cells with
non-specific viral like nuclear & cytoplasmic changes in a
background of severe acute inflammation.

47. DAD is the main pattern
of lung injury. Ironically
most of our findings are
AUTOPSY BASED only.
Biopsies (Wedge Bx) are
seldom sought for in the
diagnosis, when:-
 Presentation isn’t
straightforward
 Specific infection is
being suspected
 Therapeutic response
is poor
 Autopsy cases

48.  Patel et al (2004): In selected patients with clinical
ARDS, open lung Bx (OLB) can be performed safely,
often reveals an unsuspected Dx, & frequently leads to
alterations in therapy.
 Alexandra K. Et al (2015): Among mechanically
ventilated patients with respiratory failure of unclear
etiology, lung Bx yielded a wide range of Dx & was
associated with change in therapy in most patients.
 Carole et al (2018): OLB can play an useful role in the
Mx of ICU patients with ARF of undetermined origin,
including ARDS mimickers.
 Gerard et al (2018): OLB was able to identify a
steroid-sensitive pathology in a significant proportion of
non resolving ARDS patients. These patients had a
better outcome with lower hospital mortality.

49. All these studies clear our doubts.
Yes, PATHOLOGISTS ARE NEEDED
in managing ALI/ARDS cases too!
Pathologists can help in the:-
 Evaluation of a specific infectious
cause
 Correlation with clinical findings,
and thus understanding the
pathogenesis better, which in turn
shall aid in the evolution of new
targeted therapies
 Increasing sensitivity of the chest
radiograph so that steroids could be
added at the beginning of the
fibrotic process in ARDS.

50. Autopsy specimens
of lungs usually are:-
 Dark blue lungs
with dots of
haemorrhage on
pleural surface
 Heavy, firm due to
edema & fibrosis
Here we find diffuse
consolidation of all
lobes representing
DAD

51. Diffuse Alveolar Damage (DAD) is the classic histo-
pathological manifestation of ALI/ARDS & it has been
discussed earlier in fair details. Here we’ll checkout the
most important differentials of DAD:-
 Usual Interstitial Pneumonia (UIP)
 Non-specific Interstitial Pneumonia (NSIP)
 Cryptogenic Organizing Pneumonia (COP)
 Acute Eosinophilic Pneumonia (AEP)
 Acute Fibrinous & Organizing Pneumonia (AFOP)
 Diffuse Alveolar Haemorrhage (DAH) with capillaritis
 Organizing Pneumonia (OP)

52. Microscopic findings
in UIP pattern:-
 Dense Fibrosis &
Architectural
Distortion
 Patchy &
peripheral/
perilobular
involvement
 Honeycomb
appearance
 Fibroblastic foci

53. (a) OLB showing fibrotic lung dz with heterogeneous spatial &
temporal fibrosis consistent with UIP pattern. (b) Honeycomb
change with enlarged subpleural alveolar spaces, with
bronchiolar metaplasia, mucus plugs & mild chronic
inflammatory infiltrates are visible in the section studied

54. Microscopic findings in
NSIP pattern:-
 Diffuse & uniform
inflammation (on low
power) of alveolar wall,
bronchovascular bundles
& pleura.
 Lymphocytic or
plasmacytic infiltration
 Loose fibrosis
 Lung architecture
frequently preserved

55. (A) Uniform & diffuse thickening of the alveolar walls in NSIP
(B) Alveolar septa thickened by the inflammatory cell infiltrate
& mild interstitial fibrosis, without remodelling the basic lung
structure

56. Microscopic findings in
COP pattern:-
 OP- Fibroblastic
plugs in alveolar sacs,
ducts & bronchiolar
lumen
 Mild to moderate
cellular infiltrate
 Alveolar
architecture usually
preserved
 Foamy macrophages
in surrounding airspace
may be present

57. (A) Involvement is
often patchy & the
border between
COP & normal lung
is relatively clear
(B) The basic lung
architecture is
maintained
(C,D) The polypoid
organization is
composed of
myofibroblasts.

58. Microscopic findings in
AEP pattern:-
 Alveolar &
interstitial infiltration
by eosinophils, plasma
cells & histiocytes
 Variable angitis,
fibrosis, mucus
plugging & bronchiolitis
with necrosis
 May have Charcot-
Leyden crystals

59. (A) Many eosinophils are visible in air spaces in a patient of
AEP
(B) Organization in alveolar spaces seen. Hyaline membrane are
rarely present

60. Microscopic findings in
AFOP pattern:-
 Intra-alveolar fibrin
called “fibrin ball”
 OP- Fibroblastic
plugs in alveolar sacs &
ducts with loose
collagen matrix
 Diffuse & patchy
distribution
 Lymphoplasmacytic
infiltrate

61. (1) “Fibrin balls”
without formation
of hyaline
membranes
(2) Intra-alveolar
fibrin & OP with
patchy
involvement
(3) AFOP with
diffuse
involvement
(4) Fibroblastic
tissue surrounding
alveolar fibrin

62. Microscopic findings in DAH
with capillaritis:-
 Diffuse, intra-alveolar
blood admixed with
hemosiderin laden
macrophages with coarsely
granular & golden brown
pigment
 Significant neutrophilic
infiltrates within alveolar
septae with vascular necrosis
 Hyaline membrane present
 Fibroblastic tissue forming
‘dumbbell’ shapes

63.  Capillaritis in DAH is distinctive but can be quite focal in a
Bx specimen. The section shows variable fresh blood in the
parenchyma, typically associated with fibrin, reactive Type
II cells & haemosiderin laden alveolar macrophages

64. Microscopic findings in
Organizing Pneumonia
(OP) pattern:-
 Exudate of fibrin &
neutrophils transforming
into fibromyxoid masses
with histiocytes
 May have necrotizing
changes in the bronchi
 We’ve already observed
this beautiful pattern
while discussing AFOP,
DAH with capillaritis &
COP

66. * Based on minimally invasive autopsy:-
 Pulmonary changes in the form of DAD
 Multinucleated enlarged pneumocytes with large
nuclei, amphophilic cytoplasm & prominent nucleoli in
alveolar spaces
 Intranuclear inclusions
 May be superimposed by bacterial pneumonia
* BAL Cytology:-
 Abundant activated plasma cells
 Alveolar macrophages may feature nuclear clearing or
intranuclear cytopathic inclusions

67. COVID-19 + Case: (A)
Proteinaceous exudates
in alveolar spaces with
granules; (B) Scattered
large protein globules;
(C) Intra-alveolar fibrin
with early organization,
mononuclear
inflammatory cells &
multinucleated giant
cells; (D) Hyperplastic
pneumocytes, some with
possible viral inclusions.

68. COVID-19 +
BAL: (A) Many
clusters of
activated
plasma cells;
(B) Alveolar
macrophage
intranuclear
cytopathic
inclusion;
(C,D) Groups
of polyclonal
CD-138 +
plasma cells.

70.  Coordinate with clinician colleague & extract maximum
Clinical information: relating disease onset, progression,
suspected underlying etiologies, radiographic findings,
whether the patient is on mechanical ventilation etc
 Careful evaluation for potential infective etiologies
should be done in every case: Special stains for
microorganisms namely an Acid-fast stain, a Grocott-
Gomori-methenamine-silver (GMS) & a bacterial stain
such as Brown-Hopps should be obtained, & should be
meticulously scrutinized for viral cytopathic changes.
 Although a wedge Bx is recommended, as a pathologist
we usually end up receiving small Bx specimens especially
in the initial stages of the clinical workup. A precise Dx
may not be possible in many cases.

71.  In this case of AFOP, numerous fungal organisms, compatible
with Aspergillus were present. These organisms were not
visible on the H&E section (B), but could be easily appreciated
on staining with Gomori-methenamine-silver.

72. Gomori-methenamine-silver stain showing:-
1) Pneumocystis carinii and
2) Cryptococcus neoformans respectively

73. 1) Brown-Hopps stain showing E. coli
2) Z-N stain showing M. tuberculosis

74. * Hyaline membrane may not be detectable. We’ve to still
check for the presence of:-
 Intra-alveolar edema/Myxoid interstitial fibrosis
 Marked pneumocyte hyperplasia with bizarre cytologic
features
 Alveolar fibrin/debris
* If these (or any of these) features are appreciated in a
patient with known respiratory failure, it’s best to:-
 Provide a descriptive Dx with a comment that the
findings are suggestive of, or consistent with ALI
 Avoid over-interpretation of findings & be conservative
when correlative information is unavailable

75.  This small Bx section
showing intra-alveolar
fibrin with mild
expansion of the
alveolar septa &
pneumocyte hyperplasia.
These findings aren’t
specific per se. But,
when applied on a
patient with diffuse
pulmonary infiltrates &
respiratory compromise,
it can be reported to be
consistent with ALI

76.  When a diagnosis of ALI is suspected we must evalute
for the presence of:-
* Microorganisms and viral inclusions
* Eosinophils (suggesting the possibility of AEP)
* Coarse hemosiderin & capillaritis (suggesting an immune
mediated vasculitis DAH)
 Be mindful of the presence of fresh haemorrhage
secondary to Bx procedure. Blood & associated fibrin in air
spaces shouldn’t be over-interpreted as hyaline membrane.
Here the fibrin would be loose & wispy.
 Procedure-related haemorrhage unless supported by
macrophages with coarse hemosiderin granules or reactive
pneumocytes shouldn’t be over- interpreted as DAH.

77.  Hemosiderin secondary to alveolar haemorrhage
typically consists of large coarse granules (A), in contrast
to the finely granular pigment (B) associated with
cigarette smoking.

78.  (A) Neutrophilic capillaritis is characterized by prominent
PMNs within the alveolar septa. Neutrophilic debris or fibrin
thrombi suggests underlying vascular damage. (B) Healing
capillaritis is characterized by organizing fibroblastic tissue.
Often the organizing tissue bridges the previously damaged
alveolar septa in a ‘dumbbell’ fashion.

79.  ALI/ARDS clinically represent a significant cause of
pulmonary morbidity & mortality.
 Majority of patients with these conditions will have a
histologic pattern of DAD, but not all. So, conditions
like AFOP, AEP, UIP, NSIP, COP, DAH with capillaritis
may also be encountered which comprise genuine
consideration for the differential diagnosis.
 Determination of potential infective etiologies is
important from the pathologist’s perspective, and due
caution should be exercised in interpreting small Bx
specimens.