Moro reflex

The Moro reflex is a primitive reflex observed in newborns that serves as a protective startle response. It is an important indicator of neurological function and developmental maturity in infants. Evaluating this reflex helps clinicians assess the integrity of the central nervous system in early life.

Definition and Concept

The Moro reflex is an involuntary motor response elicited in infants, typically in response to a sudden loss of support or abrupt change in position. The reflex involves rapid extension and abduction of the arms followed by flexion and adduction, often accompanied by crying. It is one of the key primitive reflexes assessed during neonatal neurological examinations.

This reflex is distinct from other primitive reflexes such as the grasp reflex or tonic neck reflex because it is primarily a protective startle mechanism rather than a response to tactile stimulation or head position changes.

Anatomy and Physiology

Neural Pathways

The Moro reflex is mediated through the brainstem, particularly the vestibular nuclei, which detect sudden changes in position or balance. Afferent signals from the vestibular system and peripheral nerves are transmitted to the central nervous system, which then coordinates a rapid motor response through efferent pathways to the muscles of the upper limbs.

Muscles Involved

The primary muscles involved in the Moro reflex include:

• Deltoid muscles, responsible for arm abduction.
• Biceps brachii, contributing to arm flexion during the recovery phase.
• Trapezius, assisting in shoulder elevation and movement coordination.
• Other upper limb and scapular muscles that facilitate extension and adduction of the arms.

Development and Maturation

The Moro reflex begins to emerge in utero, typically around 28 weeks of gestation. It is fully developed and consistently observable at birth. The reflex peaks in responsiveness during the first few months of life and gradually diminishes as the infant’s central nervous system matures.

Normally, the Moro reflex disappears by 4 to 6 months of age as voluntary motor control and postural reflexes develop. Persistence beyond this age may indicate underlying neurological conditions or developmental delays.

Function

The primary function of the Moro reflex is protective. It enables the infant to respond to sudden loss of support or unexpected stimuli, effectively allowing the arms to extend and retract rapidly to prevent injury. This reflex also plays a role in early neurodevelopment by helping establish coordination and motor pathways in the central nervous system.

In addition to its protective role, the Moro reflex contributes to the integration of other primitive reflexes and early sensory-motor development, forming a foundation for more complex voluntary movements later in infancy.

Clinical Assessment

Testing Procedure

The Moro reflex is typically assessed during a neonatal neurological examination. The standard procedure involves:

• Placing the infant in a supine position with the head supported.
• Suddenly lowering the infant’s head or allowing a slight drop to create a sensation of falling.
• Observing the infant’s response, which should include rapid abduction and extension of the arms followed by adduction and flexion, often accompanied by crying.

Interpretation

A normal Moro reflex is symmetrical and consistent in its presentation. Variations may occur, but the reflex should be present bilaterally. Strength, symmetry, and duration are important parameters to assess. Abnormal responses may suggest neurological deficits, musculoskeletal injuries, or developmental issues.

Abnormal Findings

Exaggerated or Persistent Reflex

An exaggerated Moro reflex may indicate central nervous system irritability, hypertonia, or neurological disorders. Persistence of the reflex beyond 6 months of age can be associated with developmental delays, cerebral palsy, or other neurological conditions. Monitoring the reflex over time is important for early detection of potential issues.

Asymmetrical or Absent Reflex

An asymmetrical or absent Moro reflex may suggest:

• Brachial plexus injury, often occurring during difficult deliveries.
• Clavicular or humeral fractures affecting one limb.
• CNS pathology such as cerebral hemorrhage, hypoxic-ischemic injury, or congenital malformations.

Such findings warrant further neurological assessment and imaging if necessary to identify the underlying cause.

Clinical Significance

The Moro reflex is a critical component of neonatal neurological screening. Its presence, symmetry, and intensity provide valuable information about the integrity of the central nervous system and peripheral nerves. Assessment of this reflex helps in early detection of neurological disorders, birth injuries, and developmental abnormalities.

Regular monitoring of the Moro reflex during the first few months of life allows healthcare providers to track neurodevelopmental progress and implement early interventions if abnormalities are detected, improving long-term outcomes for the infant.

Associated Reflexes

The Moro reflex is one of several primitive reflexes observed in newborns. It is closely related to other reflexes that contribute to early motor development:

• Tonic neck reflex: Involves extension of the arm on the side the head is turned and flexion of the opposite arm, aiding hand-eye coordination.
• Grasp reflex: Involuntary flexion of fingers when the palm is stimulated, contributing to early object interaction.
• Rooting reflex: Turning of the head and mouth toward a tactile stimulus near the cheek, supporting feeding behavior.

The integration and eventual suppression of these primitive reflexes are essential for the development of voluntary motor control and coordinated movements.

References

1. Prechtl HF. The Neurological Examination of the Full-Term Newborn Infant. 2nd ed. London: MacKeith Press; 2000.
2. Als H. Developmental Care in Neonatal Intensive Care. Pediatr Clin North Am. 1986;33(3):529-547.
3. Brazelton TB, Nugent JK. Neonatal Behavioral Assessment Scale. 3rd ed. London: MacKeith Press; 1995.
4. Heineman KR, Prechtl HF. The Moro reflex and other neonatal reflexes. Dev Med Child Neurol. 1991;33(9):805-813.
5. McGraw MB. Neurological Development in the Newborn. Pediatr Clin North Am. 1965;12(1):1-18.
6. Oberg KC, et al. The clinical significance of primitive reflexes in neonates. J Pediatr. 2002;141(2):207-212.
7. Pinel JPJ, Barnes S. Biopsychology. 10th ed. Boston: Pearson; 2017.
8. Volpe JJ. Neurology of the Newborn. 6th ed. Philadelphia: Elsevier; 2018.
9. Sherrington CS. The Integrative Action of the Nervous System. New Haven: Yale University Press; 1906.
10. Hoppenfeld S. Physical Examination of the Spine and Extremities. 2nd ed. Philadelphia: Appleton & Lange; 1976.