Neonatal anaemia, polycythaemia and thrombocytopenia: SAQ

Part A (10 marks)

a) What is the most likely diagnosis, and what is the underlying pathophysiology? (3 marks) [2]

The most likely diagnosis is neonatal alloimmune thrombocytopenia, suggested by severe isolated thrombocytopenia at birth in an otherwise well term infant, a normal maternal platelet count, and a previous affected sibling. The pathophysiology involves maternal immunoglobulin G antibodies against paternally inherited human platelet antigens (most commonly HPA-1a) crossing the placenta and destroying fetal platelets, while the maternal platelet count remains normal because the antibodies target an antigen the mother herself lacks. [2]

b) What immediate investigation is required given the platelet count of 14 times 10 to the 9 per litre, and why? (2 marks) [1]

A cranial ultrasound is required urgently, because a platelet count below 50 times 10 to the 9 per litre carries a significant risk of intracranial haemorrhage, which may be clinically silent or present with seizures, apnoea, or a bulging fontanelle. Early detection guides the aggressiveness of platelet replacement therapy. [1]

c) Outline the immediate management of this infant's thrombocytopenia, including the specific blood product of choice. (3 marks) [1]

The immediate management is urgent platelet transfusion at a dose of 10 to 15 mL per kilogram of platelet concentrate. Human platelet antigen 1a-negative (HPA-compatible) platelets are the product of choice. If these are not immediately available, intravenous immunoglobulin at 0.5 to 1 g per kilogram combined with random donor platelets is given. The target is above 50 times 10 to the 9 per litre, or above 100 times 10 to the 9 per litre if intracranial haemorrhage is confirmed. [1]

d) What is the recurrence risk of this condition in future pregnancies, and what antenatal intervention should be offered? (2 marks) [2]

The recurrence risk in subsequent pregnancies where the fetus inherits the paternal antigen is approximately 85 to 90 per cent. Antenatal intravenous immunoglobulin should be offered to the mother, typically starting at 12 to 20 weeks gestation, to prevent fetal thrombocytopenia and intracranial haemorrhage. [2]

Part B (10 marks)

A separate 28-week gestation preterm infant, now 5 weeks old and weighing 1100 g, has a haemoglobin of 78 g per litre with a reticulocyte count of 2 per cent. The infant is clinically stable on low-flow nasal cannula oxygen and is tolerating full enteral feeds. [3]

e) What physiological process best explains the haemoglobin of 78 g per litre, and what are three contributing factors? (3 marks) [3]

The physiological anaemia of prematurity best explains this pattern, producing a haemoglobin nadir at 4 to 8 weeks of age (70 to 90 g per litre). Three contributing factors are the preterm infant missing the bulk of third-trimester iron transfer, a shortened red cell lifespan of 60 to 80 days, and iatrogenic blood loss from phlebotomy exceeding 10 mL per kg per day compounded by a blunted erythropoietin response. [3]

f) What iron supplementation regimen should be initiated, and at what dose and timing? (2 marks) [3]

Iron supplementation should be initiated at 2 to 4 weeks of age at a dose of 2 to 4 mg per kg per day of elemental iron, continued through the first year of life once the infant is on stable enteral feeds. [3]

g) At what haemoglobin threshold would you transfuse this stable preterm infant, and what trial evidence supports a restrictive strategy? (3 marks) [3]

For a stable preterm infant on low-flow oxygen, a restrictive transfusion threshold of approximately 97 to 100 g per litre is appropriate. The multicentre trial comparing higher versus lower haemoglobin transfusion thresholds in extremely low birth weight infants found no significant difference in death or major morbidity between strategies, supporting a restrictive approach. [3]

h) Name two long-term neurodevelopmental concerns associated with early iron deficiency in preterm infants. (2 marks) [3]

Two long-term neurodevelopmental concerns are impaired cognitive function that may persist despite later iron repletion, and behavioural difficulties, because the developing brain is particularly vulnerable to iron deficiency during periods of active myelination and neurotransmitter synthesis. [3]