Hyaline membrane disease (HMD)
Introduction
Also known as respiratory distress syndrome (RDS). The incidence
is inversely proportional to gestational age.
Etiology
Deficiency of pulmonary surfactant.
Vicious cycle: Decreased alveolar surfactant —
lungs collapse at end expiration with each breath —
increasing difficulty in breathing — exhaustion —
atelectases (airless areas) — hypoxemia —
endothelial and epitethelial damage: hyaline membrane formation
— profound hypoxemia, hypercarbia and acidosis —
progressive atelectases, abundant hyaline membranes.
Surfactant
- synthesized by type II. pneumocytes
- consists of lecithin, sfingomyelin and
surfactant associated proteins
- reduces surface tension at the air-liquid
barrier in the alveoli
- produced in considerable amounts after
35 weeks of gestation but modulation by variety of stimuli is
possible (hormones, intrauterine stress including natural
labor)
- increased indidence of HMD than expected for gestational age:
acute cesarean section before the onset of labor, asphyxia,
infants of diabetic mothers
- decreased indidence of HMD than expected for gestational age:
preeclampsia, reccurent vaginal bleeding
- there is a significantly higher risk of RDS in the
second as compared to the first of twin pairs
Clinical signs
- Signs of respiratory distress: tachypnea,
cyanosis, intercostal and subcostal recession,
nasal flaring, grunting, rapid increase in oxygen
requirement
- Prevention of HMD
- administration of surfactant (prophylactic or
treatment of symptoms)
- the incidence of HMD has been reduced by 50%
with the use of antenatal corticosteroids to promote
lung maturity. The corticosteroids are administered
to pregnant women with threatened premature delivery
at 24 – 34 weeks of gestation
Macroscopic appearance
- the classic appearance develops between 12 and 24 hours of life
- lungs are airless, congested, liver-like consistency
Histology
- in the early phase basophilic debris of necrotic epithelium
- full developed phase
- thick eosinophilic hyaline membranes (consist of
necrotic bronchiolar epithelium and fibrin) lining
the respiratory bronchioles and alveolar ducts,
- atelectases
- reparative changes occur in survivors by 48 hours by
phagocytosis of membranes, regeneration of the epithelium
and mild fibrosis
Hyaline membranes, newborn, HE 40x (72311)