Stretching of Putative Mechanoreceptors in the Inferior Tarsal Muscle Regulates Tonic and Clonic Reflex Contractions of Slow-Twitch Fibers in the Palpebral Orbicularis Oculi Muscle Causing Apraxia of Eyelid Opening: A Case Series.

The levator palpebrae superioris muscle (LPSM) and facial muscles comprise both fast-twitch fibers (FTFs) and slow-twitch fibers (STFs). Still, they lack the muscle spindles to induce reflex contractions of STFs. Because reflex contractions of STFs in the LPSM and frontalis muscle, which are the major eyelid opening muscles, are induced by stretching of mechanoreceptors in the superior tarsal muscle, those in the palpebral orbicularis oculi muscle (POOM), which is the major eyelid closing muscle, should not be induced by stretching of the same proprioceptors but instead induced by the proprioceptors in the vicinity of the POOM. Apraxia of eyelid opening (AEO) after eyelid closure might be caused by prolonged POOM contraction. Most patients with AEO tend to stretch the upper and lower eyelids by applying contact lenses and eyedrops to disinsert the aponeurosis and retractor from the tarsi. They taught us that pulling down or raising the lower eyelid decreased or increased involuntary contraction of the POOM, which relieved or worsened AEO, respectively. Then, they asked us to have the lower eyelid lowered and the upper eyelid raised surgically. Whenever the upper eyelid is opened by contractions of the LPSM with the global layer of superior rectus muscle (GLSRM), the lower eyelid is concomitantly opened by contractions of the global layer of inferior rectus muscle (GLIRM), which counteracts the contraction of the GLSRM to maintain the visual axis. We hypothesized that patients with retractor disinsertion raise the lower eyelid by eyelid closure to stretch putative mechanoreceptors in the inferior tarsal muscle (ITM), which induces prolonged tonic and clonic reflex contractions of STFs in the POOM, resulting in AEO. To retrospectively verify the hypothesis, we report five cases with AEO. In the first case, AEO was induced by tight eyelid closure but was prevented by pulling down the lower eyelid during eyelid closure. Surgery to reinsert retractors into the tarsi cured AEO. In the second case, the patient sustained both severe aponeurosis-disinserted blepharoptosis and AEO. In this patient, the first surgery to reinsert aponeuroses to the the tarsi cured AEO, but a tight eyelid closure induced prolonged POOM contraction. The second surgery conducted to reinsert the retractors to the tarsi cured AEO. In the third case, with the entire eyelid AEO, surgery done to reinsert the retractors to the tarsi almost cured the entire eyelid AEO. In the fourth case, an increased clonic contraction of the POOM on the right eyelid after a tight eyelid closure was relieved by 4% lidocaine instillation to anesthetize the ITM. In the fifth case, downgaze induced clonic reflex contraction of the right POOM because of the right retractor disinsertion. Thus, prolonged tonic and clonic reflex contractions of STFs in the POOM appeared to be regulated by enhanced stretching of putative mechanoreceptors in the ITM in patients with retractor disinsertion due to increased contractions and microsaccades of FTFs in the GLIRM. Because reflex contractions of STFs in the POOM by stretching of putative mechanoreceptors in the ITM might essentially attach the upper and lower eyelids to the globe, AEO might simply be the increased reflex contraction of the POOM.