What is Moro reflex

Moro reflex is often called a startle reflex, is a type of involuntary (without trying) response to stimulation where a baby throws back his or her head, extends out the arms and legs, cries, then pulls the arms and legs back in when a baby is startled by a loud sound or movement. A baby’s own cry can startle him or her and trigger the Moro reflex. The Moro reflex is one of many reflexes (involuntary movements or actions) that are seen at birth.

Your baby’s doctor will check for the Moro reflex right after birth and during well-child visits. To see the Moro reflex, the child will be placed face up on a soft, padded surface. The head is gently lifted with enough support to just begin to remove the body weight from the pad. (Note: The infant’s body should not be lifted off the pad, only the weight removed.) The head is then released suddenly, allowed to fall backward for a moment, but quickly supported again (not allowed to bang on the padding). The normal response is for the baby to have a startled look. The baby’s arms should move sideways with the palms up and the thumbs flexed. The baby may cry for a minute. As the Moro reflex ends, the infant draws its arms back to the body, elbows flexed, and then relaxes.

Moro reflex normally goes away after 3 or 4 months, but can lasts about 5 to 6 months. The absence of the Moro reflex during the neonatal period and early infancy is highly diagnostic, indicating a variety of compromised conditions. Based on the findings in normal infants, the absence or diminution of the Moro reflex within 2 to 3 months of age and the persistence of the response beyond 6 months of age can be regarded as abnormal ¹⁾.

Absence of the Moro reflex in an infant is abnormal.

• Absence on both sides suggests damage to the brain or spinal cord.
• Absence on only one side suggests either a broken shoulder bone or an injury to the group of nerves that run from the lower neck and upper shoulder area into the arm may be present (these nerves are called brachial plexus).

A Moro reflex in an older infant, child, or adult is abnormal.

If the reflex is absent or abnormal, further tests may need to be done to examine the child’s muscles and nerves. Diagnostic tests, in cases of decreased or absent reflex, may include:

• Shoulder x-ray
• Tests for disorders associated with brachial plexus injury

When does the Moro reflex disappear?

The Moro reflex can be elicited in all infants during the first 12 weeks of age ²⁾. After the neonatal period, however, the response becomes increasingly less typical with age, eventually consisting only of abduction and extension of the upper limbs. Beyond 12 weeks of age, the proportion of infants exhibiting a negative response rapidly increases, reaching about 80% at 20 weeks of age ³⁾. The Moro reflex usually disappears by 6 months of age ⁴⁾. Based on the findings in normal infants, the absence or diminution of the Moro reflex within 2 to 3 months of age and the persistence of the response beyond 6 months of age can be regarded as abnormal ⁵⁾.

Moro reflex test

In his original method, Ernst Moro in 1918 ⁶⁾ elicited the reflex by hitting the pillow on either side of an infant’s head with his hands. Later a variety of methods for eliciting the Moro reflex were devised including hitting on the table surface, warm or cold application to the chest or stomach, and a tap on the abdomen ⁷⁾. Nowadays, however, the head drop method is the most common, because a slight drop of the infant’s head relative to the body axis in the supine position is generally accepted as the most effective technique for eliciting the Moro reflex ⁸⁾. For elicitation in this method, the infant is held suspended in a symmetrical supine position with one of the examiner’s hands behind the chest and the other supporting the head, and the head being held in a midline position and then dropped back a few cm. It is important to ensure that both the subject’s hands are open at the moment of elicitation of the reflex so as not to provoke an asymmetrical response ⁹⁾. Infants should be tested while they are awake, but not crying ¹⁰⁾.

The drop of the baby method is an alternative one for eliciting the Moro reflex: the infant is suspended horizontally, as in the head drop method, and then the examiner lowers his or her hands rapidly about 10 to 20 cm and brings them to an abrupt halt. There is no dorsiflexion of the neck with this technique ¹¹⁾. On the other hand, Lesný ¹²⁾ reported that a nociceptive stimulus applied to the infant’s skin and subcutaneous tissue of the epigastrium by pinching was effective for eliciting the reflex.

The initial phase of the response comprises abduction of the upper limbs at the shoulders and extension of the forearms at the elbows, with slight extension of the spine and retraction of the head. The forearms are supinated and the digits extended, except for the semiflexed index fingers and thumbs, forming the shape of a “C” ¹³⁾. There is sometimes a slight tremor or clonus-like rhythmic movements of the limbs. Subsequently the arms adduct at the shoulders and the forearms flex at the elbows: the upper limbs describe an arc-like movement, bringing the hands in front of the body, which finally return to the original position ¹⁴⁾. The responses of the lower limbs are usually eliminated from the evaluation, because they show wide variability among the normal population ¹⁵⁾. With Moro reflex, habituation develops only on an experimental basis with intensively repetitive trials, that is, not in the clinical setting ¹⁶⁾. No significant difference in the response has been reported at birth or through the first 5 months of age between the term cephalic-presenting and breech-presenting infant groups ¹⁷⁾.

The Moro Reflex and Startle Reaction

Although there has been much confusion regarding the Moro response and the startle reaction in the past, most authors agree today that they are different entities ¹⁸⁾. The startle reaction, the response to a sudden stimulus, is one of the defensive reactions and consists essentially of flexion movements. It differs considerably from the Moro response primarily characterized by extension. Detailed observations with video recording ¹⁹⁾ or film ²⁰⁾ and an electrophysiological study with surface electrodes ²¹⁾ demonstrated the differences in motor behavior between them. Katona ²²⁾ found that the startle reaction induced by an auditory stimulus showed clear habituation in premature infants, whereas the Moro reflex did not, and that the startle reaction could not be elicited in anencephalic newborns, while the Moro reflex was always elicited in these infants. Pucher et al. ²³⁾ investigated the Moro reflex using a tilt table with simultaneous monitoring of autonomic parameters including respiration, heart rate, and transcutaneous pO2 and pCO2 and concluded that the Moro reflex was not the result of a startle reaction, because of no significant alteration in these parameters.

Moro reflex clinical significance

Moro reflex responses in Normal Infants

The study of the Moro reflex in normal term infants has been undertaken by many authors. The results obtained with the head drop method are well consistent. The Moro reflex can be elicited in all infants during the first 12 weeks of age. After the neonatal period, however, the response becomes increasingly less typical with age, eventually consisting only of abduction and extension of the upper limbs. Beyond 12 weeks of age, the proportion of infants exhibiting a negative response rapidly increases, reaching about 80% at 20 weeks of age ²⁴⁾. The Moro reflex usually disappears by 6 months of age ²⁵⁾. Several authors compared the Moro reflex in preterm infants tested at 40 weeks postconceptional age or at 4 months of corrected age with that in term infants. Although none of the studies confirmed the outcome in the subjects, there is a general agreement as to the similarity in the response between the two groups, especially when only infants with no or low perinatal risk factors are compared ²⁶⁾.

Moro reflex Abnormal Responses

Based on the findings in normal infants, the absence or diminution of the Moro reflex within 2 to 3 months of age and the persistence of the response beyond 6 months of age can be regarded as abnormal ²⁷⁾. The absence of the response during the neonatal period and early infancy is of especial clinical significance and may indicate a compromised condition or disorder including birth injury, severe birth asphyxia, intracranial hemorrhage, infection, brain malformation, general muscular weakness of any cause, and cerebral palsy of the spastic type ²⁸⁾. On the other hand, a hyperactive response of the Moro reflex is a common feature of neonatal withdrawal from maternal drug abuse including volatile substances, heroin, and opioids ²⁹⁾. An exaggerated response may also be detected in infants with a severe bilateral intrauterine disturbance such as hydranencephaly ³⁰⁾.

Asymmetry of the response is usually a sign of local injury. Damage to a peripheral nerve or cervical cord or a fracture of the clavicle may inhibit the reflex on the affected side. However, it should be noted that Dubowitz ³¹⁾ demonstrated an asymmetrical response in normal infants. Their responses appeared to be related to the clenching of one fist during the procedure, and he considered that this might be caused by inhibition of the response on one side due to contraction of the finger flexors. Because Reiners et al. ³²⁾ found, in a prospective study, that none of 22 infants with a clavicular fracture exhibited an asymmetrical Moro response, the diagnostic value of the Moro reflex for the detection of such fractures should not be overestimated. Retention of the Moro reflex is common in children with mental retardation (intellectual disability) without motor disturbance including Down’s syndrome and in children with cerebral palsy of the athetoid type ³³⁾. It is also sometimes observed in children with a severe brain malformation or with cerebral palsy of the spastic type ³⁴⁾.

Moro reflex neural mechanism

The Reflex Center

Katona ³⁵⁾ reported that the Moro reflex could be elicited in anencephalic newborns with a nervous system that had developed only to the rostral level of the pons. In a study involving analysis of the relationship between the morphological structures of the rudimentary brain and primitive reflexes in six anencephalic newborns, Hanabusa ³⁶⁾ found that the Moro reflex could be elicited only when the vestibular nuclei were preserved. This finding indicates that the reflex is principally mediated by the vestibular nuclei. Rönnqvist et al. ³⁷⁾ reported that the average latency of the Moro reflex in 15 term neonates in the quiet awaking state detected with an optoelectronic device was 117.0 ms on the right arm and 129.2 ms on the left. These latencies are much longer than those of the spinal reflexes, clearly indicating that the reflex is mediated in the brain stem, not at the level of the spinal cord ³⁸⁾. Thus, the center of the Moro reflex seems to be in the lower region of the pons to the medulla.

Afferent and Efferent Pathways

The origin of afferent pathways for the Moro reflex, whether it is primarily vestibular, proprioceptive, or exteroceptive, has been a main subject of discussion. The head drop, the most common way of eliciting the Moro reflex, stimulates both the vestibular system and the proprioceptive receptors in the neck. Rönnqvist ³⁹⁾ investigated the reflex by tilting the table without extension of the infants’ neck to eliminate the proprioceptive inputs from the cervical vertebrae and neck muscles. The response could be elicited in 225/250 trials (90%), and twenty-one of the 25 negative trials were made while the infants were sleeping or crying. Prechtl ⁴⁰⁾ also demonstrated that sudden raising or dropping of the infants, whose head, neck, and trunk were fixed in a plaster cast, could elicit the response. Bloomfield et al. ⁴¹⁾ reported an infant with CHARGE syndrome who exhibited a persistent complete absence of the Moro reflex with preservation of other primitive reflexes. Moro’s original method cannot yield a satisfactory response if the table is too stable to produce a change in position on striking and necessitates jolting movement of the table to elicit a response ⁴²⁾. These findings support the view that this reflex is principally mediated by the vestibular system. In contrast to the grasp reflex, the Moro reflex has not been observed in a fetus, which is also in agreement with its vestibular origin, because fetuses are protected from acceleration or shaking in intrauterine life ⁴³⁾.

On the other hand, Parmelee Jr. ⁴⁴⁾ found that vestibular stimulation was not sufficient for a good Moro response when neck movement was prevented. Prechtl ⁴⁵⁾ described an infant with bilateral absence of the inner ears who had exhibited a normal Moro response to a head drop, which was reported by Karlsson in an address to a study group in Oxford. These observations suggest that the proprioceptive inputs from the neck also contribute to elicitation of the reflex. There are direct and indirect, via the cervical cord, ascending pathways that originate in the proprioceptive receptors in the neck and connect with vestibular nuclei. These pathways originally send signals to the brain stem to regulate the neck righting ⁴⁶⁾. The signals generated by the head drop may travel via these routes to reach and activate the reflex mechanism in the brain stem. The head drop method is most effective, because it produces a large number of ascending signals to the target through the two pathways and thereby induces a high level of neural excitation that acts on the reflex center.

Although pinching of an infant’s epigastrium has been reported to be effective for eliciting the reflex ⁴⁷⁾, a nociceptive stimulus is generally ineffective ⁴⁸⁾. Besides the primary somatosensory afferents, there is a path taken by nociceptor axons that reaches the pontine reticular formation, which has close interconnections with vestibular nuclei ⁴⁹⁾. The nociceptive signals may travel via this pathway toward the reflex center in the brain stem. However, the level of neural excitation generated by nociceptive stimulation appears to be usually low, and the response can be elicited only when it infrequently exceeds the threshold of the reflex.

The reflex center probably contains a number of interneurons, because of the relatively long latency. The routes of afferent pathways can be multiple, and the efferent pathways of the response seem to originate in the vestibulospinal and/or reticulospinal neurons, because the response can even be obtained in anencephalic newborns devoid of both corticospinal and rubrospinal neurons ⁵⁰⁾. Thus, the reflex movement is generated by the subcortical structures without cortical participation, which explains why focal cerebral injury does not cause distinct disturbance of the Moro reflex ⁵¹⁾. It is also noteworthy that no asymmetrical response is sometimes detected in neonates and young infants who later develop spastic hemiplegia. The primary motor cortex and nonprimary motor areas project a lot of neurons to different motor centers in the brain stem including vestibulospinal and reticulospinal neurons. The brain stem also receives inputs from the basal ganglia and cerebellum. The Moro reflex in infants disappears with age, due to the increased inhibition of these upper brain structures.