How does asymmetric tonic neck reflex (ATNR) manifest in a stroke patient?

Asymmetric Tonic Neck Reflex (ATNR) Manifestation in Stroke Patients

The asymmetric tonic neck reflex manifests in stroke patients as increased muscle tone and strength in the limbs toward which the head is rotated (face side) and decreased tone in the opposite limbs (skull side), representing a reemergence of this primitive brainstem reflex that indicates increased reliance on reticulospinal pathways following stroke. 1

Clinical Presentation

Static Manifestations

• When the patient's head is rotated to one side, the ipsilateral arm (face side) demonstrates increased extensor tone while the contralateral arm (skull side) shows increased flexor tone 1
• The classic "fencer's position" may be observed, though this is typically subtle in stroke patients rather than the dramatic posturing seen in infants 2, 1
• Neck receptor activation from head rotation enhances excitability of contralateral soleus motoneurons and depresses ipsilateral ones, affecting lower extremity tone 3

Dynamic Manifestations During Movement

• During reaching tasks, ATNR significantly affects reaching distance, particularly when shoulder abduction loading is required at approximately 25% of maximum ability 2
• Stroke patients demonstrate larger center of pressure and center of mass excursions during voluntary head motions compared to healthy controls 4
• Disrupted weight-bearing patterns emerge, with patients avoiding loading of the paretic limb during head movements 4

Relationship to Flexion Synergy

Torque Generation Patterns

• Contralateral head rotation (face pointing away from paretic arm) increases maximum voluntary torque in three flexion synergy components: elbow flexion, shoulder abduction, and shoulder external rotation 1
• This modulation occurs only in stroke patients, not in healthy controls, indicating pathological reliance on ipsilateral reticulospinal pathways 1
• The effect is most pronounced during maximal effort tasks, suggesting brainstem pathway dominance when cortical control is insufficient 2, 1

Clinical Significance

• ATNR reemergence and flexion synergy share a common neuroanatomical mechanism: increased dependence on ipsilateral reticulospinal pathways following corticospinal tract damage 1
• This represents loss of independent joint control and impaired sensorimotor integration 4

Assessment Considerations

Examination Technique

• Test the patient with head rotated both ipsilateral and contralateral to the paretic limb during strength testing 1
• Observe for asymmetric changes in muscle tone and strength generation between the two head positions 1
• Evaluate during functional reaching tasks, as ATNR effects may be more apparent during dynamic movements than static positioning 2

Quantifiable Changes

• Head rotation velocity is typically reduced in stroke patients while maintaining similar range of motion compared to healthy subjects 4
• Surface electromyography may reveal hypertonic activity in neck muscles and altered activation patterns in limb muscles during head rotation 5

Common Pitfalls

• Do not confuse ATNR with voluntary movement patterns—the reflex is involuntary and modulates existing motor output rather than producing isolated movements 1
• ATNR effects may be subtle and require comparison between head positions to detect, rather than being obvious in neutral positioning 2, 1
• The reflex is velocity-dependent and effort-dependent, so testing during minimal effort tasks may fail to elicit the response 2, 1
• Altered postural adjustments during head motions reflect impaired sensorimotor integration beyond simple reflex reemergence 4