Botox is associated with those seeking beauty, but an academic says it has important medical benefits too. Besides smoothing out wrinkles, Botox can treat strabismus, or crossed eyes. Flora Lau Hoi-shan, clinical assistant professor in ophthalmology and visual sciences at Chinese University, says Botox has been used to treat patients with crossed eyes in Hong Kong since 2009, and 71 injections have been given to 44 patients at the Hong Kong Eye Hospital. Botox works by blocking nerve endings and paralysing muscles. While the effects are temporary in most patients and repeat injections are usually needed to keep the eye straight, the success rates are about 80 per cent for near vision and 60 per cent for distant vision. Lau encourages Botox use for treating patients with convergent strabismus caused by treatment for nasopharyngeal cancer, one of the city's top seven cancers. Radiotherapy is one of the main treatments for the cancer, but one of its key complications is sixth-nerve palsy, which can lead to crossed eyes when the muscles that move the eyeball outwards lose the strength to do so. The deficiency leads to double vision and poor depth judgment. To correct this, Botox is injected into the medial rectus, the muscle used for inward eye movement. As this weakens, the two muscles become balanced and the eyeball returns to the middle position. An electromyography machine, which detects the electrical potential generated by muscle cells, is used to help guide the needle to the right spot. "We put the needle through the conjunctiva [tissue covering the eye's surface] then we ask the patients to roll the eyeball towards the nose," Lau said. "The muscles [of the medial rectus] will then contract and the machine will sound an alarm." Patients are only partly anaesthetised and have to fully co-operate with medical staff while feeling the sensation of a needle moving in round their eyes. The injections - costing HK$100 for adults at the Eye Hospital - take effect within a week. The effects wear off in two to three months, but in some patients the eyes will stay straight after several operations.