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Moro Reflex
Definition/Introduction
The Moro reflex, also known as the startle reflex, is a normal, primitive reflex observed in infants. The reflex is an involuntary, protective motor response triggered by sudden disruptions in body balance or extreme stimulation.[1] The Moro reflex was first described by Ernst Moro in 1918.[2] The reflex can be observed as early as 25 weeks post-conceptional age and is typically present by 30 weeks post-conceptional age.[3] The reflex is present in full-term infants and begins to disappear by 12 weeks, with complete disappearance by 6 months of age.[4] The exaggeration or retention of the Moro reflex may indicate a neurodevelopmental concern.[4][5]
The reflex is elicited by pulling up on the infant's arms while in a supine position and then releasing them, simulating the sensation of falling.[1] The reflex is triggered by the suddenness of the stimulus and not the distance of the drop. There is no need to lift the infant's head off the bed to elicit this reflex. The normal Moro reflex starts with the abduction of the upper extremities and extension of the arms. The fingers extend, and there is a slight extension of the neck and spine. After this initial phase, the arms adduct, and the hands come to the front of the body before returning to the infant's side.[4]
Issues of Concern
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Issues of Concern
The Moro reflex is particularly weak in preterm newborns due to lower muscle tone, inadequate resistance to passive movements, and slow arm recoil, compared to full-term newborns at the same post-conceptual age.[6] The absence of the Moro reflex during the neonatal period and early infancy is highly diagnostic, indicating a variety of compromised conditions.[4] Conversely, an exaggerated reflex response is frequently observed in newborns experiencing withdrawal due to maternal drug use.[7]
Clinical Significance
The absence or premature disappearance of the Moro reflex can result from a birth injury, severe asphyxia during the birthing process, intracranial hemorrhage, infections, brain malformations, or general muscular weakness due to various causes.[1][8] An asymmetrical Moro reflex can be due to a local injury. Common causes include damage to a peripheral nerve, brachial plexus injury, or clavicle fracture, which can inhibit the reflex on the affected side.[4][9]
Prolonged retention of the Moro reflex can also be a sign of spastic cerebral palsy.[8] An exaggerated Moro reflex can also be observed in newborns with hyperekplexia, a rare genetic disorder marked by an excessive startle response and increased muscle tone. The condition can follow either autosomal dominant or autosomal recessive inheritance patterns, depending on the specific gene mutation, and is manageable with appropriate treatment.[10][11][12]
In one study, the presence or absence of the Moro reflex was more closely related to the infant's development and less likely to be associated with pathogenic conditions.[6] Another study demonstrated a clear association between retained primitive reflexes and delayed motor development in very-low-birth-weight infants.[13] Training clinicians on how to perform the Moro reflex correctly is essential and can improve the yield of the neonatal physical examination.[14]
Nursing, Allied Health, and Interprofessional Team Interventions
The Moro reflex is a normal reaction to certain stimuli in infants, and the interprofessional healthcare team should understand what a typical response looks like and when to be concerned. Families often express concerns about their child's development. Although clinicians play a key role in providing reassurance and guidance, it is often frontline nursing staff in hospitals or clinics who are first available to address parental concerns and recognize atypical findings. Early recognition of abnormal findings allows for timely referral and the involvement of specialists, such as physical and occupational therapists, neurologists, and developmental pediatricians.
The interprofessional team must recognize that an asymmetric Moro reflex at birth can be indicative of neuronal damage or a clavicular fracture.[9] Additionally, it is essential to note that the Moro reflex typically disappears by 6 months of age, and the retention of this reflex should raise suspicion for possible spastic cerebral palsy or other neurodevelopmental concerns.[8][13]
Media
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Moro Reflex.
Contributed by O Chaigasame, MD
References
Zafeiriou DI. Primitive reflexes and postural reactions in the neurodevelopmental examination. Pediatric neurology. 2004 Jul:31(1):1-8 [PubMed PMID: 15246484]
Brigo F, Porro A, Trinka E. The Moro reflex: insights into the pathophysiology of generalized tonic-clonic seizures and infantile spasms. Epileptic disorders : international epilepsy journal with videotape. 2022 Oct 1:24(5):952-956. doi: 10.1684/epd.2022.1471. Epub [PubMed PMID: 35904039]
Allen MC, Capute AJ. The evolution of primitive reflexes in extremely premature infants. Pediatric research. 1986 Dec:20(12):1284-9 [PubMed PMID: 3797120]
Futagi Y, Toribe Y, Suzuki Y. The grasp reflex and moro reflex in infants: hierarchy of primitive reflex responses. International journal of pediatrics. 2012:2012():191562. doi: 10.1155/2012/191562. Epub 2012 Jun 11 [PubMed PMID: 22778756]
Pina D, Roubertie A, Spitz MA, Ravelli C, Bahi-Buisson N, Gheurbi F, Buchy M, Loppinet T, Chemaly-Perin N, Nougues MC, Heron B, Lopez R, Anheim M, Fradin M, Cances C, Avez-Couturier J, Dalmon F, Lesca G, Des Portes V, Lion-François L. STARDEV Study: Neurodevelopmental Trajectory and Long-Term Outcomes of Patients with Startle Disease/Hyperekplexia. Movement disorders clinical practice. 2025 Apr 7:():. doi: 10.1002/mdc3.70071. Epub 2025 Apr 7 [PubMed PMID: 40192101]
Sohn M, Ahn Y, Lee S. Assessment of Primitive Reflexes in High-risk Newborns. Journal of clinical medicine research. 2011 Dec:3(6):285-90. doi: 10.4021/jocmr706w. Epub 2011 Nov 10 [PubMed PMID: 22393339]
Chasnoff IJ, Burns WJ. The Moro reaction: a scoring system for neonatal narcotic withdrawal. Developmental medicine and child neurology. 1984 Aug:26(4):484-9 [PubMed PMID: 6479468]
Zafeiriou DI, Tsikoulas IG, Kremenopoulos GM, Kontopoulos EE. Moro reflex profile in high-risk infants at the first year of life. Brain & development. 1999 Apr:21(3):216-7 [PubMed PMID: 10372911]
Level 3 (low-level) evidenceReiners CH, Souid AK, Oliphant M, Newman N. Palpable spongy mass over the clavicle, an underutilized sign of clavicular fracture in the newborn. Clinical pediatrics. 2000 Dec:39(12):695-8 [PubMed PMID: 11156066]
Aglave NR, Sontakke RA, Bokade C, Jhunjhunwala K. Hyperekplexia: Unveiling a Rare Neurological Condition With a Treatable Solution. Cureus. 2024 Jun:16(6):e61770. doi: 10.7759/cureus.61770. Epub 2024 Jun 5 [PubMed PMID: 38975479]
Santos BSCD, Mattos JPG, Juliano LS, Souza RR, Marinho CAF. The implications of hyperekplexia on children's quality of life: a report on two cases. Revista paulista de pediatria : orgao oficial da Sociedade de Pediatria de Sao Paulo. 2025:43():e2024189. doi: 10.1590/1984-0462/2025/43/2024189. Epub 2025 Mar 24 [PubMed PMID: 40136121]
Level 2 (mid-level) evidenceFalsaperla R, Sortino V, Giacchi V, Saporito MAN, Marino S, Tardino LG, Marino L, Gennaro A, Ruggieri M, Barberi C, Polizzi A. Neonatal Hyperekplexia: Is It Still a Diagnostic Challenge? Evidence From a Systematic Review. Journal of child neurology. 2024 Oct:39(11-12):415-424. doi: 10.1177/08830738241273425. Epub 2024 Sep 2 [PubMed PMID: 39223854]
Level 1 (high-level) evidenceMarquis PJ, Ruiz NA, Lundy MS, Dillard RG. Retention of primitive reflexes and delayed motor development in very low birth weight infants. Journal of developmental and behavioral pediatrics : JDBP. 1984 Jun:5(3):124-6 [PubMed PMID: 6736257]
Pavageau L, Sánchez PJ, Steven Brown L, Chalak LF. Inter-rater reliability of the modified Sarnat examination in preterm infants at 32-36 weeks' gestation. Pediatric research. 2020 Mar:87(4):697-702. doi: 10.1038/s41390-019-0562-x. Epub 2019 Sep 7 [PubMed PMID: 31493776]