A 3-day-old term newborn is examined in the postnatal ward. When the examiner allows the infant's head to drop a few centimetres into the examining hand, the response marked **A** in the diagram is elicited. However, the infant's left arm fails to abduct and extend during the initial phase of this reflex, while the right arm responds normally. The left arm remains adducted and internally rotated. What is the most likely diagnosis?

Explanation

## Why Erb-Duchenne brachial plexus injury (C5-C6) is right An asymmetric Moro reflex — specifically, failure of the affected arm to abduct and extend while the contralateral arm responds normally — is the hallmark clinical sign of unilateral brachial plexus injury, most commonly Erb-Duchenne palsy (C5-C6 injury). This injury typically occurs from shoulder dystocia or forceps delivery and presents with the classic "waiter's tip" deformity (adduction and internal rotation of the affected arm, exactly as described in this case). The Moro reflex is the most sensitive primitive reflex to detect this asymmetry at birth. Nelson 21e emphasizes that asymmetric Moro reflex is a critical red flag for unilateral pathology, with Erb-Duchenne palsy being the most common cause in the immediate postnatal period. ## Why each distractor is wrong - **Clavicular fracture**: While clavicular fracture is the most common birth injury overall and may occur with shoulder dystocia, it does not typically produce an asymmetric Moro reflex. The infant may show pain with arm movement, but the reflex arc itself (mediated by C5-C6 spinal nerves) remains intact. Clavicular fracture presents with crepitus, swelling, or asymmetric arm movement during handling, not a selective failure of abduction/extension during Moro testing. - **Hemiplegic cerebral palsy**: Cerebral palsy may present with asymmetric tone or posture, but it is not the typical cause of asymmetric Moro reflex in a 3-day-old. Cerebral palsy is a disorder of movement and posture due to cortical/subcortical injury, and while it may eventually cause asymmetry, the acute asymmetric Moro reflex in the immediate newborn period is far more suggestive of peripheral nerve injury (brachial plexus). Additionally, cerebral palsy would more likely present with other signs (e.g., asymmetric tone, delayed postural reflexes) rather than isolated asymmetric Moro. - **Intracranial hemorrhage with diffuse neurological depression**: Intracranial hemorrhage typically causes an ABSENT or globally depressed Moro reflex (generalized CNS depression), not an asymmetric one. The presence of a normal right-sided Moro response argues against diffuse CNS pathology. Asymmetry points to a focal, peripheral nerve lesion rather than central injury. **High-Yield:** Asymmetric Moro reflex = unilateral pathology; Erb-Duchenne (C5-C6 brachial plexus injury) is the most common cause, presenting with "waiter's tip" (adduction + internal rotation) of the affected arm and failure to abduct during Moro testing. [cite: Nelson Textbook of Pediatrics, 21st edition, Chapter 17 — Neurological Examination and Assessment of the Newborn]