Sixth (abducent) cranial nerve palsy is a typical yet infrequent mononeuropathic complication of diabetes. It usually causes considerable diplopia, which can be debilitating and significantly impair the everyday and professional activity of afflicted individuals. In most cases, nerve function restores itself, although it usually takes several months or even over a year for the symptoms to resolve (1). No specific treatment of nerve palsy-induced diplopia in diabetic patients has been established (2,3). We report the successful use of botulinum toxin A in the early treatment of diplopia caused by sixth nerve palsy in two type 2 diabetic patients. In both patients, diplopia made it impossible for them to continue with their professional activities.
The first patient (female, computer operator, aged 52 years, with a 15-year history of type 2 diabetes and HbA1c 8.2%) complained of diplopia, which occurred several days earlier. Sixth nerve palsy in the left eye was diagnosed, and her squint angle was found to be +35Δ (prism dioptries) when measured with an orism cover test. After prompt injection of botulinum toxin A (15 units) into the medial rectus muscle in the left eye, her diplopia and squint resolved completely (Fig. 1). The second patient (male, taxi driver, aged 50 years, with an 8-year history of type 2 diabetes and HbA1c 8.7%) was referred with sixth nerve palsy in the left eye persisting for 3 months. His squint angle was +55Δ. The patient complained of severe diplopia, predominantly when looking left. Botulinum toxin A (15 units) was injected into the medial rectus muscle in the left eye, which resulted in the reduction of the squint angle to +15Δ. To achieve complete resolution of diplopia, prism correction was used (Fig. 2). Six months later, the vision of both patients remains stable, with no diplopia occurring while looking forward.
Injections of botulinum toxin A in the treatment of sixth nerve palsy have been used since the early 1980s (4) with a success rate of 15–100%, depending on the severity and duration of the nerve palsy (5–7). In general, early botulinum toxin use is recommended, as then the resolution of diplopia can be complete (8). In long-standing cases, successful treatment inadvertently requires ocular muscle surgery, which might be also associated with botulinum toxin injections (3,9,10).
However, despite a relatively long history of botulinum toxin treatment in ocular muscle paralysis (4,6), its use and investigations of treatment with it in diabetic mononeuropathy have been less than scarce. Concomitantly, no routine treatment is offered to patients, who are often left untreated until the nerve palsy itself subsides, which actually does happen in the majority of cases. However, during this period the patient is usually unable to work and is regarded a disabled person.
Our report is, to the best of our knowledge, the first one describing the effects of the early use of botulinum toxin A in type 2 diabetic patients, in whom diplopia caused by sixth nerve palsy made them unable to work. In agreement with the recommendations mentioned above, in type 2 diabetic patients the use of botulinum toxin A at the very beginning of diplopia and nerve lesion also seems to be more effective than in patients with a long duration of symptoms. We believe that botulinum toxin A, especially if used immediately after nerve palsy occurs, offers an attractive option in the treatment of cranial nerve palsy-induced diplopia, thus saving the patient from surgical treatment and assuring that quality of life and professional activity will not suffer due to significant disturbance of vision.