The Asymmetrical Tonic Neck Reflex (ATNR), often shortened to the Tonic Neck Reflex, is an early involuntary movement pattern present in newborns. This primitive reflex is controlled by the brainstem and precedes the development of conscious, voluntary motor control. The timely presence and disappearance of the ATNR is an important indicator of a developing nervous system, which is why pediatricians assess it during early well-child exams.
The Mechanics of the Tonic Neck Reflex
The Tonic Neck Reflex is most easily observed when an infant is lying on their back and their head is gently turned to one side. This automatic movement is sometimes called the “fencing reflex” because the baby’s posture resembles a fencer’s stance. When the head is turned, the arm and leg on the side the face is pointing toward will automatically extend or straighten.
Simultaneously, the limbs on the opposite side of the body will flex, or curl inward. This whole-body response is a direct result of sensory input from the neck muscles communicating with the brainstem and is typically strongest in the first month to two months of life. Pediatricians test the ATNR by gently rotating the baby’s head to check for the characteristic extension and flexion of the limbs.
Developmental Timeline and Purpose
The Tonic Neck Reflex begins developing in utero and is thought to help the baby “corkscrew” through the birth canal during delivery. Following birth, the reflex is typically present up to about six months of age.
The reflex plays a significant role in early development by preparing the infant for later voluntary movements. When the baby turns their head and sees their extended hand, it fosters the first stages of hand-eye coordination and visual tracking. This association between head movement and limb extension is fundamental for developing an awareness of both sides of the body.
The ATNR must integrate, or disappear, to allow for more complex, intentional movements to emerge. The typical window for integration is between four and seven months of age, making way for skills like bringing the hands together, rolling over, and crawling. Once integrated, the baby’s movements are no longer dictated by the turning of the head.
Recognizing Persistent Tonic Neck Reflex
If the Asymmetrical Tonic Neck Reflex remains fully active beyond the six to eight-month mark, it is considered a retained or persistent reflex and may signal developmental delays. Retention of the ATNR interferes with the development of higher-level motor skills and coordination because the automatic link between head position and arm movement continues to override conscious control.
One of the most common difficulties seen with a retained ATNR is trouble crossing the body’s midline, the imaginary line dividing the body into left and right halves. This can make simple tasks like reaching across the body or using both hands together challenging. It also affects bilateral coordination, making activities like tying shoes or catching a ball difficult.
In older children, a persistent reflex can manifest as issues with fine motor control, particularly handwriting. Turning the head to look at the paper can cause a reflexive extension of the writing arm, leading to struggles with holding a pencil correctly or writing that trails off the page.
Common Signs of Retained ATNR
- Inconsistent hand dominance, where the child switches hands for the same task.
- Difficulties with balance, particularly when turning the head side-to-side.
- Problems with visual skills, such as smoothly tracking a line of text when reading.
- Struggles with fine motor control and handwriting.
If parents notice these signs in an older infant or toddler, a professional assessment by a pediatrician or an occupational therapist can help determine the next steps. Targeted movement programs can often help the nervous system integrate the reflex and support the development of more mature motor patterns.