Hypothesis / aims of study
Sacral neuromodulation is a longstanding treatment for patients with overactive bladder (OAB) or non-obstructive urinary retention (NOUR), refractory to conservative treatments. A tined lead with 4 electrodes is placed through the third or fourth sacral foramen. Which electrode settings are used for the therapeutic response is usually defined based upon the sensory response on stimulation. This sensory response consists of the sensory threshold (ST) and the perceived location of stimulation (PLoS) upon stimulation of different electrode configurations.
In general, a ST <2mA and a perianal or genital PLoS are considered optimal response and a ST >4mA and/or PLoS at another location than perianal or genital are considered a bad response.
The hypothesis of this study is that the use of the curved lead delivers more optimal sensory responses upon stimulation of different electrode configurations, due to a better alignment with the target nerve.
Study design, materials and methods
Single tertiary center, prospective study (August 2013 - June 2015) involving 40 patients with overactive bladder dry (OABD) and wet (OABW) and 15 with non-obstructive urinary retention (NOUR) refractory to first-line treatment.
To map the sensory responses a pelvic dermatome chart was developed with 1cm² squares with coordinates. The following 16 electrode settings were tested: C+0-/1+0-/2+0-/+3+0-/C+1-/0+1-/2+1-/3+1-/C+2-/0+2-/1+2-/3+2-/C+3-/0+3-/1+3-/2+3-. Square wave pulses with pulse width of 210µsec and 14 Hz frequency were delivered at the respective electrodes with increasing amplitudes up to the sensory threshold (ST). The amplitude was noted as well as the point that the patient marked on the dermatome chart.
To determine the ST, the method of limits was used, 4 mA was the upper limit. If no sensation was reported at 4 mA, this configuration was noted as “not perceived”, since these high thresholds jeopardize battery longevity. Based upon expert opinion (since no structured data are available), perianal and genital sensation were considered prognostically good, whereas “other” or “not perceived” were considered bad.
Success to treatment was defined as more than 50% improvement in relevant parameters on 3-days voiding diaries.
Results
Gender: 42 female (76%); 13 male (24%). Mean age: 51.7 +/- 17.0 years (range 18-79 years). Indications: OABD: 6 (11%); OABW: 34 (62%); NOUR 15 (27%). Baseline voiding parameters: OABD: 16.5 +/- 3.6 voids/24h; OABW: 4.4 +/- 2.7 urgency incontinence episodes/24h; NOUR: 3-5x self-catheterizations/24h.
33/35 (94%) patients implanted with the curved lead and 13/20 (65%) implanted with the straight lead had a successful tined lead procedure and proceeded to get an IPG implanted. In these patients, the sensory responses on all electrode configurations were tested.
All patients found it easy to mark specific spots on the dermatome chart. All 736 records could be used.
The mean amplitude of the ST was not different between the straight (1.9 +/- 1.2 mA) and curved group (1.8 +/- 1.0 mA) (MWU; p=0.826). The PLoS of the different electrode configurations were different between both groups (Chi²; p=0.003). In the curved group 53% of the PLoS were at the perianal region, compared to 40% in the straight group. Genital sensation was reported by 23% in both the curved and straight group. However, in the curved group only 19% was perceived at the “other” location (leg, toe, lower back), compared to 31% in the straight group.
From a clinical perspective, the most ideal stimulation would be perceived at the perianal or genital region with a stimulation amplitude <2mA; suboptimal would be anal or genital perception and stimulation amplitude >=2mA but <=4mA; bad stimulation would be >4mA or perception at “other” locations. Comparing these clinical relevant parameters showed significant differences in favour of the curved group (Chi²; p<0.001). The data are presented in the table.
During SNM treatment, patients may experience episodes of reduced/lack of efficacy, which can be solved by troubleshooting. The first step is changing the electrode configurations, assuming the patient has several “optimal configurations”. The table lists the percentage of patients broken down in relation to the number of possible “optimal configurations” and shows that more patients with a curved lead have more backup configurations. The table indirectly also shows that 38% in the straight group and 15% in the curved group have less than 4 optimal configurations.
Interpretation of results
More optimal electrode configurations were seen in the group implanted with the curved lead in comparison to the group implanted with the straight lead. This can be explained by the theory the curved lead more adequately follows the curvilinear course of the sacral root (which runs medial to lateral along the fascia of the piriformis muscle anterolateral to the sacrum).