Pulmonary hypoplasia

Pulmonary hypoplasia refers to deficient or incomplete development of parts of the lung. It can develop as a result of a number of other in utero anomalies.

Epidemiology

Pulmonary hypoplasia has been reported to affect 1.4% of all births ¹³ however the true prevalence is not well known. In cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia is 9-28% ².

Pathology

Pulmonary hypoplasia is characterized by the presence of both bronchi (albeit rudimentary) and alveoli in an underdeveloped lobe. Both the size and the weight of the lung are reduced.

There are normally four stages of fetal lung development where the pathological process can occur at any level

• embryonic: conception to ~5^th week

• pseudoglandular: ~5^th to 17^th week

• canalicular: ~16^th to 24^th week: this stage may be more affected by oligohydramnios

• terminal sac or alveolar period: 24^th week to term and beyond

Etiology

There are several key factors required for the adequate development of the lung. These are:

• sufficient amniotic fluid volumes

• adequate volume of the thoracic cavity

• normal breathing movement

• normal fluid within the lung

A deficiency in any of these could lead to pulmonary hypoplasia.

Most cases of pulmonary hypoplasia are secondary to other congenital anomalies or pregnancy complications. Some cases however can occur as a primary event ⁵.

With secondary causes, it can result from factors directly or indirectly compromising the thoracic space available for lung growth.

Intrathoracic causes include:

• congenital diaphragmatic hernia: most common intrathoracic cause

• extralobar sequestration

• agenesis of the diaphragm

• mediastinal mass(es)/tumor(s)

  • mediastinal teratoma

• decreased pulmonary vascular (arterial) perfusion

  • from a congenital cardiovascular anomaly: e.g. tetralogy of Fallot

  • unilateral absence of the pulmonary artery

Extrathoracic causes include:

• oligohydramnios and its causes

  • Potter sequence: fetal renal anomalies

  • preterm premature rupture of membranes (PPROM)

• skeletal dysplasias, especially those causing a narrow fetal thorax

  • Jeune syndrome (asphyxiating thoracic dystrophy)

  • thanatophoric dysplasia

  • achondrogenesis

  • osteogenesis imperfecta

  • short rib polydactyly syndrome

  • campomelic dysplasia

• large intra-abdominal mass compressing the thorax

• neuromuscular conditions interfering with fetal breathing

Other associations include:

• Fryns syndrome

• Meckel Gruber syndrome

• Neu-Laxova syndrome

• Pena-Shokeir syndrome

Radiographic features

Ultrasound

Antenatal ultrasound may show ancillary features such as the presence of oligohydramnios and/or also show any of the causative anomalies.

Several sonographic parameters may give indirect clues as to the presence and extent of pulmonary hypoplasia. Such include:

• fetal lung : head ratio: reduced (ratios <1 usually indicate a poor prognosis)

• fetal chest circumference (or thoracic circumference) ⁶: reduced

In the case of intra-thoracic causes, both these parameters can be normal.

• thoracic : abdominal circumference ratio <0.6 ¹⁵

• femur length : abdominal circumference ratio <0.16 ¹⁵

Doppler

Peripheral pulmonary arterial resistance is often increased with pulmonary hypoplasia.

MRI

Fetal MRI may be useful to more accurately calculate lung volumes.

• T1: normal fetal lung is usually low signal on T1

• T2: normal fetal lung is usually high signal on T2; fetuses with pulmonary hypoplasia are thought to have low signal ¹

Treatment and prognosis

The presence or development of pulmonary hypoplasia is often a critical determining feature in fetal survival in context of other potential causative anomalies. Treatment will significantly depend on the presence of underlying/associated anomalies.

Differential diagnosis

In adult cases of unilateral pulmonary hypoplasia, consider conditions associated with a small hemithorax such as:

• Swyer-James syndrome

• Poland syndrome

• scimitar syndrome

• lung atelectasis

See also

• pulmonary aplasia

• neonatal respiratory distress (causes)