Mechanoreceptor Stretching in the Superior Tarsal Muscle Triggering Limb and Craniocervical Dystonia: A Report of Two Cases

Voluntary upper eyelid opening stretches mechanoreceptors in the superior tarsal muscle (STM), serving as an analog to a serial muscle spindle, inducing reflex contractions of slow-twitch fibers in the levator palpebrae superioris and facial expression muscles via the mesencephalic trigeminal nucleus and rostral locus coeruleus (LC). Because the LC projects widely to the brainstem and spinal cord, we hypothesized that excessive STM mechanoreceptor stretching could induce dystonic contractions across broader muscle groups. This observational case report describes two unique patients presenting with multifocal and limb dystonia. Diagnostic mechanical testing (eyebrow elevation and cheek depression via taping) and sensory masking (high-frequency focal vibration) were utilized to evaluate the immediate modulation of proprioceptive inputs. Therapeutic interventions included oculoplastic surgical reconstruction with longitudinal follow-up intervals ranging from 1 week to 18 months. Case 1 involved a 29-year-old man with severe levator aponeurosis disinsertion presenting with task-specific limb flexion dystonia. Mechanical reduction of the STM stretch, initially via taped eyebrow elevation and subsequently through corrective surgery, completely resolved the dystonic movements. Case 2 involved a 38-year-old man who developed left-predominant craniocervical and limb dystonia following lower eyelid surgery, driven by increased reflex contractions of the palpebral orbicularis oculi muscle opposing eyelid opening. Symptoms were significantly alleviated either by facial taping to reduce STM stretching or by applying high-frequency focal vibration to trigeminal nerve branches to mask the abnormal proprioceptive input. Secondary metabolic, drug-induced, and primary genetic etiologies were formally ruled out in both cases. These cases suggest a novel, hypothesis-generating model wherein excessive proprioceptive stimulation from STM mechanoreceptors hyperactivates the rostral LC, potentially exerting downstream tonic influences on spinal and brainstem motor pathways. While these striking clinical outcomes highlight a plausible ocular-proprioceptive trigger for craniocervical and peripheral dystonias, prospective controlled studies are required to validate this pathophysiological mechanism.