The “fencer’s response” describes an involuntary body posture that can appear after certain neurological events. It is a transient phenomenon, typically lasting only a few seconds.
Understanding the Fencer’s Response
The fencer’s response is characterized by a specific positioning of the arms and head. One arm extends outward or upward, while the other arm flexes at the elbow, often with the head turned towards the extended arm. This unconscious physical position means the individual cannot control it.
The name “fencer’s response” comes from its visual resemblance to a fencer’s “en garde” stance, where one arm is outstretched as if holding a sword and the other is bent. This arm and head orientation is an observable indicator seen immediately after a head impact. It is distinct from other abnormal posturing that may develop later.
The Neurological Basis
The fencer’s response is a manifestation of the Asymmetrical Tonic Neck Reflex (ATNR). This primitive reflex is coordinated by the brainstem, a lower part of the brain responsible for basic life functions and involuntary movements. The ATNR involves an interplay between the brainstem, the vestibular system (which helps with balance), and sensory input from neck muscles.
When the head turns to one side, sensory information triggers an automatic motor response: the arm and leg on the side the head is facing extend, while the limbs on the opposite side flex.
Clinical Observations
The fencer’s response is observed in two contexts: as a normal developmental reflex in infants and as an indicator of neurological disturbance after a brain injury.
In Infants
The ATNR, or “fencing reflex,” is a normal primitive reflex present in newborns. It begins developing in the womb around 18 weeks of gestation and is observable at birth. This reflex plays a role in early motor development, helping infants with head control, muscle tone, and hand-eye coordination. It can facilitate a baby’s ability to reach for objects and aids in visual tracking.
The ATNR typically integrates, or disappears, as the infant’s brain matures, usually between 4 to 6 months of age. Its disappearance allows for more voluntary and coordinated movements to develop, such as rolling over, bringing hands together, and eventually sitting and crawling. If the ATNR persists beyond this timeframe, it can be associated with developmental delays, affecting skills like bilateral coordination, balance, and fine motor skills.
In Brain Injury
The fencer’s response can reappear or persist abnormally after certain types of brain injury, such as a concussion or traumatic brain injury (TBI). It occurs immediately after head trauma and typically lasts for several seconds. This response is thought to be activated when moderate forces are applied to the brainstem, momentarily reactivating the primitive ATNR.
The presence of the fencer’s response after head injuries serves as a visible sign of neurological disturbance. It can indicate damage to the brainstem or temporary disruption of higher cortical control. This posturing is an overt indicator of the injury’s force magnitude and can help classify the severity of the brain injury. Its observation often prompts immediate medical attention, as it can be followed by other signs of serious brain injury, including seizures, vomiting, or loss of consciousness.