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A 69 year old woman with nuclear sclerotic cataracts was examined. She was awaiting neurosurgery for treatment of drug refractory titubation (head tremor). Before cataract surgery, she underwent successful neurosurgery. The implanted Medtronic deep brain stimulation device rendered her asymptomatic of tremors. At the cataract preoperative clinic she showed the device identification card that stated “ultrasound diathermy … anywhere on your body … can result in severe injury or death.”1 Following confirmation from Medtronic that it was safe to proceed, the patient had an uneventful left phacoemulsification performed under general anaesthesia with the neurostimulator turned “off.” Seven months later she underwent a similar successful right phacoemulsification.
Deep brain neurostimulation of the thalamus is the treatment of choice for drug refractory essential tremor.2 Indications for its use are widening and include use in multiple sclerosis, advanced Parkinson’s disease, and movement disorders such as dystonia.3 The deep brain neurostimulator has three implanted components. The electrodes are implanted into the subthalamic nucleus or the globus pallidus interna,1,2,4 then an insulated lead is placed subcutaneously from the burr hole to a sealed neurostimulator device beneath the clavicle. The neurostimulator electrical stimulation pulses can be adjusted from external devices.4
There are two recorded fatalities in patients with implanted deep brain neurostimulation devices who had received short wave diathermy “as used by physiotherapists.”5 Medtronic also report two similar patients who were still comatose.1 Another report described permanent, severe central nervous system injury following stimulation of a spinal cord neurostimulator by a radiofrequency antitheft device.6
Brain damage results from heating of the implanted electrodes.7 Heat energy released directly to the body from an external source can be conducted via the insulated lead of an implanted neurostimulation device, raising the temperature at the electrode. Furthermore, if an external source generates an electric current in the insulated lead, this will result in a rise in the temperature at the electrode. Ultrasound diathermy transfers heat directly to the body where short wave diathermy results in an induced electrical current.8 One study calculated a potential rise of 9.76°C at the deep brain neurostimulator electrode when short wave diathermy was used.7 For phacoemulsification to be safe in patients with deep brain neurostimulators, it must not produce significant heat or generate an electric current.
The phacoemulsification hand piece uses the piezoelectric effect to drive the phacoemulsification needle tip in a linear jackhammer-like movement, physically cutting the lens.9 Acoustic cavitation results from an explosive collapse of vacuoles formed in fluid around the swiftly moving phacoemulsification needle tip.9 A study showed a maximum temperature rise of 3.5°C in the anterior chamber during routine phacoemulsification.10 The risk of this generated heat spreading to the implanted electrodes must be low. The phacoemulsification tip does not generate an oscillating magnetic field that might induce an electrical current. Theoretically, this should render ultrasound phacoemulsification safe in the presence of implanted deep brain neurostimulators.
With expanding technology, there will naturally be situations with the potential for interactions between equipment from different specialties. Consent should include the possibility of heat conduction to the implanted neurostimulation device. The use of local anaesthesia may allow early detection of discomfort or neurological sequelae. The surgeon should make use of all techniques to reduce the heat generated during phacoemulsification. Medtronic advise turning the neurostimulator off and not placing any cables over the patient’s chest and neck (R Coffey, 3 February 2005, personal communication). There are various neurostimulators, including cortical devices, that may have increased sensitivity to localised temperature increases. Heat formation at the phacoemulsification needle tip has been analysed; however, further research on the extent of heat dissipation is required.