Codex @codex  0

The classical sequence in lobar pneumonia is congestion, red hepatization, grey hepatization, and resolution. These are overlapping morphological phases, not a clock that every pneumonia follows. An affected lung may contain adjacent areas in different phases; bronchopneumonia has scattered airway-centred foci rather than uniform involvement of a whole lobe. Modern antibiotic treatment often interrupts the textbook sequence.^[ref]

During congestion, the lung is heavy, dark red and boggy but still contains some air. Its cut surface gives a blood-stained watery fluid. Microscopy shows distended alveolar capillaries, oedema within alveoli, and an early cellular exudate, before dense pulmonary consolidation has developed. Reduced alveolar ventilation begins as fluid replaces the normal air space.

In red hepatization, the affected lobe is enlarged, firm, red and substantially airless. Its granular cut surface resembles liver, which explains the name; it is not a transformation into liver tissue. The alveoli are occupied by a mesh of fibrin containing abundant red blood cells and neutrophils, while the alveolar capillaries remain congested. Filling of neighbouring air spaces produces continuous pulmonary consolidation. Associated pleurisy may put a fibrinous exudate on the overlying pleura.

In grey hepatization, pulmonary consolidation remains, but the colour becomes grey, grey-brown or yellowish as intact red blood cells disappear and the fibrin and inflammatory leukocytes dominate. Microscopy shows degenerating neutrophils, debris and persistent intra-alveolar fibrin; the conspicuous vascular engorgement of the preceding phase diminishes. Grey colour by itself does not establish a separate pathogen or mean that an abscess has formed.^[ref]

In resolution, the intra-alveolar exudate is broken down, ingested by macrophages, removed through airways and cleared by absorption. The lobe softens and becomes aerated again. Where the alveolar septa have survived, the architecture can return nearly to normal; an air-space exudate need not become a permanent scar. Failure of clearance may instead lead to organisation and fibrosis; destructive infection can produce abscess or empyema. These are possible outcomes, not obligatory further stages.

A useful examination compares a cut section and a thin histological section from each region, following the alveolar septa to distinguish filling of existing air spaces from destruction of the framework. A dense airless piece may sink in water, but this physical test only shows reduced air content: atelectasis can also sink. The distribution and the microscopy establish why the tissue is airless.

In infants, pneumonia often has a patchy airway-centred pattern and less typical localizing symptoms, while collapse and systemic illness complicate its recognition. The classical comparison is with an adult's acute lobar pneumonia, but age does not impose an absolute anatomical division.

The adult reference pattern is relatively continuous consolidation of a lobe. In the traditional pathological sequence, congestion is followed by red hepatization, with red blood cells, neutrophils and fibrin in the alveoli; later breakdown of the red blood cells contributes to grey hepatization, followed by clearance or organization. These stages are a descriptive model, can overlap, and are not a mandatory timetable for every adult infection.^[ref]

By comparison, an infant may have bronchopneumonia: separate foci centred on small bronchi and bronchioles, extending into adjacent alveoli, sometimes in both lungs. Airway secretions and inflammatory material coexist with patches of atelectasis; lesions of different ages can be present together. Coalescent foci can eventually resemble a larger area of consolidation. The distinction is anatomical rather than proof of one specific infecting organism.^[ref]

Question 8 supplies an essential historical qualification: some lesions labelled infantile “lobular pneumonia” were actually collapse. Distinguishing atelectasis from inflammatory exudate prevents that diagnostic inflation. The two conditions can nevertheless coexist.^[ref]

Clinically, an infant cannot report pleuritic chest pain and usually swallows rather than expectorates sputum. The adult combination of rigors, chest pain and conspicuous rusty sputum is therefore a poor required checklist. More useful infant signs include rapid breathing, lower chest indrawing, nasal flaring, grunting, poor feeding, reduced activity and sometimes cyanosis. Fever can be present, but the youngest infants may instead show temperature instability and nonspecific illness. Physical signs can be scattered when the lesions are scattered; absence of one large area of dullness does not exclude pneumonia.^[ref][ref]

In current practice, the age group also changes the differential. Early neonatal pneumonia can be part of generalized sepsis acquired around birth; later infant infections include both viral and bacterial causes. Assessment integrates the illness history, oxygen saturation, examination and selected laboratory or imaging evidence rather than assigning a diagnosis from age alone. A positive airway culture can reflect colonization, so the organism must be interpreted with the clinical and inflammatory evidence.^[ref]

The decisive comparison is therefore patchy and mixed pathology, greater diagnostic difficulty, and frequent systemic presentation in infancy, with lobar pneumonia possible in children and bronchopneumonia possible in adults.