The Fencing Response (FR) is an involuntary posturing of the limbs that occurs immediately following a sudden, forceful impact or trauma to the head. This temporary, reflexive movement is considered a significant observable sign in the assessment of a traumatic brain injury (TBI), such as a concussion. It is a clear neurological indicator resulting from the mechanical forces transmitted to the brain upon impact.
Defining the Posture
The physical presentation of the Fencing Response is quite distinct and gives the phenomenon its name because it closely mimics the en garde stance of a classical fencer. The posturing is characterized by an asymmetrical movement: one arm extends stiffly, often held straight out or upward, which can be referred to as the “foil arm.” Simultaneously, the opposite arm flexes sharply at the elbow and wrist, resembling the position of the “mask arm.”
This distinct, temporary position is a tonic reaction, meaning it is a sustained muscular contraction rather than a convulsive movement. The entire response is very brief, typically lasting only a few seconds immediately after the head trauma occurs. Since the response is so rapid, documentation often relies on sideline observers or video evidence, particularly in contact sports where it is frequently observed.
The occurrence of this immediate, involuntary limb movement is considered a form of tonic posturing, which is distinct from the more delayed and generally more severe decerebrate or decorticate posturing. In a review of high-impact video footage, the Fencing Response was documented in approximately 66% of knockout incidents, highlighting its commonality with moderate-force head impacts.
Neurological Mechanism
The Fencing Response is believed to be triggered by a sudden, temporary disruption within the brainstem, specifically involving the vestibular nuclei. The vestibular system is a complex network responsible for maintaining balance, spatial orientation, and coordinating movement, and the nuclei are located near the junction of the brain and spinal cord. The sudden rotational forces and torque associated with a head impact are thought to momentarily stretch or impact this delicate region of the brainstem.
This transient mechanical insult causes a functional lesion, which temporarily releases the brainstem from the inhibitory control of the higher brain centers, such as the cerebral cortex. When this occurs, primitive reflexes that are normally suppressed in adults are briefly reactivated. The Fencing Response closely resembles the Asymmetrical Tonic Neck Reflex (ATNR), a normal, temporary reflex seen in human infants.
Central to this process is the Lateral Vestibular Nucleus (LVN), also known as Deiter’s nucleus, which is highly vulnerable to the mechanical forces of a rotational head injury. The LVN has direct connections to motor neurons in the spinal cord. The rotational force appears to unilaterally activate this nucleus, leading to the asymmetrical posturing of one arm extending and the other flexing.
Clinical Significance
The presence of the Fencing Response following a head injury serves as an immediate indicator of a neurological disturbance. Observing this posturing suggests that the injury involved significant rotational forces, which are known to cause more widespread brain tissue damage than purely linear forces. The response is evidence that the impact was forceful enough to mechanically affect the brainstem, specifically the midbrain region.
Historically, the Fencing Response has been considered a “red flag” sign, correlating with a more severe traumatic brain injury or concussion. This posturing is often used in immediate sideline assessments, particularly in contact sports, as a clear signal that the athlete has sustained a concussion and requires immediate removal from play. The observation of this specific tonic posturing is now included in assessment protocols for potential concussions.
Recent research has nuanced the interpretation of this clinical sign. While the presence of the Fencing Response indicates a substantial biomechanical force to the brain, its presence alone is not necessarily a predictor of longer recovery times or worse long-term clinical outcomes. Studies involving professional athletes who exhibited the response found no clinically meaningful difference in their time to return to play or in their cognitive and symptom recovery. This suggests that while it is a strong indicator of the impact’s force and a brainstem event, it does not predetermine the severity of the long-term clinical injury.