A Vector Force Model of Upper Eyelid Position in the Setting of a Trabeculectomy Bleb.

PURPOSE: A vector force model for the determination of upper eyelid position in the setting of a trabeculectomy bleb is presented. The model is used to explain the clinical courses of 5 patients with bleb-induced upper eyelid malposition and the efficacy of modalities previously described for the treatment of bleb-induced upper eyelid retraction. The novel use of botulinum toxin in the treatment of bleb-induced eyelid retraction and unique surgical considerations in patients with trabeculectomy blebs undergoing upper eyelid surgery are discussed. METHODS: A vector force analysis was conducted and a force diagram constructed. The clinical and surgical courses of 5 patients with trabeculectomy blebs and upper eyelid malposition were reviewed. The vector force model was applied to these cases and the previously described treatment modalities for bleb-induced upper eyelid retraction. RESULTS: Vector force analysis demonstrates that in the case of trabeculectomy bleb-induced upper eyelid retraction, the net force vector, which represents the sum of all the individual forces acting on the eyelid, has a positive vertical component resulting in superior displacement of the eyelid. In contrast, bleb-induced ptosis results when the net force vector has a negative vertical component. In 3 patients, alterations in the bleb resulted in resolution of upper eyelid malposition. Botulinum toxin was used to achieve a normal upper eyelid position in 1 patient with lateral canthal tendon disinsertion and unilateral eyelid retraction and 1 patient with bilateral eyelid retraction. One patient developed unilateral ptosis in concert with the emergence of a large Tenon cyst that resolved with the treatment of the cyst via eyelid massage. One patient with unilateral ptosis and an ipsilateral bleb underwent external levator advancement but was unable to achieve the desired upper eyelid height as retraction over the bleb occurred with any attempt to elevate the eyelid above a marginal reflex distance of 1.5 mm. The efficacy of previously reported modalities for the treatment of trabeculectomy bleb-induced upper eyelid retraction can be explained by either a reduction in the positive vertical component of the net force vector or augmentation of the negative vertical component. CONCLUSIONS: A vector force model systematically accounts for the multiple determinants of upper eyelid position in the setting of a trabeculectomy bleb. This model provides a framework for the evaluation of bleb-induced upper eyelid malposition and offers a logical, mathematical explanation for the occurrence of bleb-induced upper eyelid retraction and the usefulness of previously reported treatment modalities for this clinical entity.