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Editor,—Modern inferior oblique surgery is considered to be a safe procedure with little chance of serious complications.1 2 Monopolar diathermy is known to be a potentially dangerous tool if used inappropriately or injudiciously.3 There have been cases of fires, explosions caused by igniting bowel gas during laparoscopic surgery,3and patient death caused by interference with a cardiac pacemaker.4 Our case demonstrates once again that an inadequate understanding of equipment or its inappropriate use can cause significant injury to the patient.
A 22 year old woman presented herself to Moorfields eye casualty department with a 2 day history of seeing a blob in front of her left eye. Her acuity was 6/6 right eye and 6/12 left eye corrected with no improvement with pinhole. Examination revealed left vitreous haemorrhage with bilateral chorioretinal scars in the temporal periphery slightly below the horizontal in each eye (Fig 1, top). The scars roughly corresponded to the possible insertion of the inferior oblique muscles. She had no medical problems, was on no medication, but had a family history of retinal detachment. She had had bilateral inferior oblique myectomy 6 years earlier. Her acuity improved to 6/6 bilaterally when the vitreous haemorrhage had cleared 4 months later but she was left with bilateral scotomas. There was evidence of anomalous vessels in the large scar in the left eye but no leakage was seen on fluorescein angiography (Fig 1, bottom). These vessels have not been treated with laser therapy but are being observed regularly in clinic. She had normal pupillary responses to light. The surgeon who performed the inferior oblique myectomy was contacted and confirmed that it was their normal practice to use monopolar diathermy to cut the muscle during this type of surgery.
Monopolar diathermy works by forming an electrical current from the indifferent electrode plate (usually attached to the patient’s thigh) to the forceps or blade held by the surgeon. Current flows through the body creating thermal damage in the tissues near the forceps or knife resulting in coagulation. There have been case reports of current flowing in this way causing unexpected damage proximal to the forceps particularly when operating on vascular structures in which a lot of current must flow through a small amount of tissue. This situation has occurred during circumcision operations and in some cases total ablation of the penis has resulted.5 Similar damage may occur during bowel surgery resulting in pedicle injury.6Current flows particularly well through blood vessels which means that vascular structures are at particularly high risk of damage from this mechanism. Bipolar diathermy is generally speaking a much safer way to cauterise tissue. Current only flows between the tips of the diathermy forceps rather than through the rest of the patient. This makes collateral damage much less likely to occur.
The normal insertion of the inferior oblique muscle is oblique with the convexity upwards and is predominantly below the horizontal meridian, with which it may make an angle of from 15o to 20o. The main variation in insertion is in the degree of obliquity and convexity of the curve but the insertion may often show gross irregularities such as angular serrations or dehiscences (Fig 2). The insertion is usually between 8 and 10 mm long. The macula is usually 1–2 mm posterior and 1 mm superior to the posterior tip of the inferior oblique insertion.7 Although at first sight our patient’s scars do not seem to directly correspond to this position there are a number of possible explanations. Firstly, this patient required vertical squint surgery so it is possible that her inferior oblique muscles did not have a normal site of insertion. Another factor might be the angle of traction on the muscle when the diathermy was being applied. The forward pull on the muscle would have brought it into close contact with the globe along some of its length anteriorly and this may have resulted in the somewhat peripheral site of the scarring. Finally, there are other structures in close proximity to the muscles such as blood vessels (vortex veins), nerves, and their accompanying fibrous tissue all of which could have played a part in conducting current and causing a burn. The muscle belly where diathermy is applied has a larger cross sectional area than the insertion or place of approximation of the tendon to the globe. This results in a relatively large current flowing through a small cross sectional area which can easily result in a burn as it sometimes does in circumcision surgery.5
A variety of techniques are used for weakening or severing the inferior oblique muscle in the treatment of vertical gaze deviations; these include myotomy, myectomy, disinsertion, denervation, and recession.1 Myotomy and denervation using cutting diathermy has been described1 and in one series of 86 patients who underwent thermoelectric weakening there was no mention of any visual complications.8 This procedure was carried out using an “electric knife” but it is not specified whether this was a monopolar or dipolar device.
Internal ophthalmoplegia has been described as a complication of inferior oblique myectomy using what was probably bipolar cautery and the most likely cause of this was thought to be excessive stretching of the nerve to the inferior oblique with secondary trauma to the ciliary ganglion.9
In our case we believe that the potential of the monopolar diathermy to cause damage may have been exacerbated by stretching the inferior oblique muscle while using the cutting diathermy on it. This would have reduced the cross sectional area causing greater resistance to flow consequently generating more heat in the muscle including its site of origin which is usually in close proximity to the macula. Although difficult to prove this we have no other explanation for the development of bilateral symmetrical chorioretinal scars in this otherwise healthy young woman.
Monopolar electrocautery has been used successfully for many years by ophthalmologists particularly in the field of oculoplastics10 and should not be abandoned. However, we believe that monopolar diathermy should not be used during inferior oblique surgery as monopolar cutting seems to offer little advantage over the use of bipolar cautery and a conventional blade and has the potential to cause significant retinal injury.