Abnormal impedances common with neurostimulation

Cleveland-If sacral neuromodulation has failed for your patient, look beyond the problem of lead position. Abnormal impedances are common and are frequently associated with revision, say researchers from Cleveland Clinic.

Despite the growing popularity of sacral neuromodulation for various refractory urinary and defecatory disorders and more, some 30% to 40% of sacral neuromodulator implantations have to be revised. Usually, lead position gets the blame, but problems with the lead occurred in nearly 13% of patients who received implants at Cleveland Clinic, said first author Andrew Lenis at the 2012 AUA annual meeting in Atlanta.

Measuring electrical impedances will indicate the source of the electrode abnormality. Normal values range from 400 ohms to 1,500 ohms. In a short circuit, which results from wires touching one another, sometimes because of a crush injury or fluid leak, impedance values will be low. With an open circuit, possibly due to lead fracture or a loose connection, impedance values tend to be high-greater than 4,000 ohms, said Lenis, a medical student at Case Western Reserve University in Cleveland, working with Raymond Rackley, MD, and colleagues.

Lenis and his colleagues retrospectively reviewed the records of all patients (some 600) who underwent sacral neuromodulation at Cleveland Clinic and selected those who had abnormal electrode impedance at any follow-up visit, those who underwent surgical revision, and those who underwent explantation. Patients before 2003 and patients in whom the devices were not originally implanted at Cleveland Clinic were excluded, leaving 72 patients to examine. The patients were 90% female, just below the obese category in body mass index, and had been followed for almost 4 years. The primary indications for InterStim (Medtronic, Minneapolis) lead placement were urgency/frequency and urgency incontinence, and most received a unilateral lead.

Majority of impedances were open circuits

There were 86 abnormal electrode impedance events in 72 patients, for a rate of 12.7%. Nearly two-thirds of the cases were open circuit, while a third were short circuits, and two patients (3.5%) had a combination of open- and short-circuit failures. Greater than 60% of the hardware-related electrode abnormalities resulted in surgical revision or explantation.

“All the open- and short-circuit failures were identified by measuring impedances in the office during follow-up visits,” Lenis explained.

The open circuits occurred much later than short circuits. The median time to an open circuit was 16 months. Fifty-four percent of these patients underwent a surgical revision or explantation, accounting for 18% of all (167) surgical revisions and explantations. About half the open circuits could be attributed to a single electrode, and one-third of those were from electrode 3 and the rest from 0, 1, and 2, although there were no significant differences in which electrode had an open circuit.

In contrast, short circuits took less time, occurring at a median of 3 months. Seventy-five percent of these patients underwent revision or explantation, accounting for 12% of the total revisions and explantations. There was no pattern in which the electrode tended to short circuit.

The two patients who had combined open- and short-circuit failures presented 19 and 81 months after implantation, and both had surgical revision, which accounted for only 1% of the total.

Acquired defects, potentially from shear and other forces like flexion or tension, may predispose a lead to develop microfractures that can result in impedance abnormalities and diminished battery life.

“There seems to be a good number of patients who had a history of fall or trauma before they presented with an electrode impedance abnormality,” said Lenis, but the authors are still analyzing this and hope to present that data in the future.

One of Lenis’ co-authors is a speaker and consultant for Medtronic.