Prostate Cancer is the most commonly diagnosed visceral malignancy in men in the United States.  In 1998, estimates indicated that 184,500 new cases of prostate cancer would have been diagnosed and that approximately 39,200 men would die from the disease by the end of the year.  One of the more common treatments for the localized prostate cancer involves surgical extirpation of the prostate during radical prostatectomy.  Although this surgical procedure offers some survival advantage with long term follow-up, the occurrence of complications such as incontinence and erectile dysfunction has prompted concern in both patients and physicians.

To address this issue, Walsh and colleagues initially studied the preservation of the postoperative erectile function by defining the usual location of the cavernous nerves in terms of their relation to prostate and urethra (Figures 1A and 1B).  Using this knowledge, Walsh developed and described the anatomic radical retropubic prostatectomy, a procedure specifically designed to preserve the cavernous nerves while still allowing surgical extirpation of the cancerous prostate. 

Background

The appeal of the anatomic approach to radical prostatectomy results from the preservation of the cavernous nerves and their stimulatory signals to the penis. Reported rates of potency following radical prostatectomy range widely from 11% to 86%, however.  This variation may result from a number of factors, including differences in surgical technique, discrepancies in preoperative erectile status, age variation within study populations, and variation in the methods used to determine postoperative potency.

In general, success rates are reportedly higher in studies from larger university centers when compared with general Medicare population.  In addition, complete return of erectile function to preoperative levels is less likely in patients who are older and have more advanced disease. 

The number of neurovascular bundles preserved during radical prostatectomy is another factor that appears to correlate with postoperative erectile status.  Specifically, higher rates of potency are generally associated with bilateral nerve preservation compared with the unilateral preservation of bilateral nerve sacrifice.  Therefore, newer techniques that refine the anatomic “nerve-sparing” radical prostatectomy would theoretically improve postoperative quality of life for many patients. 

In particular, more accurate intraoperative identification of the neurovascular bundles may be helpful.  Even well intentioned attempts to preserve the neurovascular bundles and cavernous nerves are sometimes unsuccessful because locating these structures proves difficult.  This problem may result from variation in the location of the neurovascular bundles; presence of blood in the operative field, which makes visualization of the bundles difficult; or even poor exposure due to body habitus. 

A recently developed device helps urologic surgeons identify and map the neurovascular bundles so they may attempt to improve postoperative potency.  Patient selection for use of this surgical apparatus follows conventional criteria for radical retropubic prostatectomy with localized prostate cancer. 

Equipment

The new device, called CaverMap Surgical Aid (Blue Torch Corporation, Norwood, Mass), combines a nerve stimulator with an erectile response detection system for intraoperative use.  This system consists of three major components (Figure 2).  The control unit contains the digital and analog electronics, adjustable controls, user interface, and connectors for the probe handle and disposable kit.  The probe handle, which is sterile and reusable, is the component that allows the surgeon to control the device intraoperatively.  The disposable kit, which contains items for onetime use, is made up of the probe tip that attaches to the probe handle, the tumescence sensor, and the lead that connects the sensor to the control unit.  The electrode containing probe tip emits the electrical current. 

General Use

The probe tip is placed on tissues that are suspected to contain sensitive nerves.  When the tissue is stimulated, erectile responses are detected in the form of minute changes in penile circumference in a previously placed tumescence sensor.  A full erection is not achieved for two reasons:  (1) only external nerve stimulation is used, (2) other coordinated signals and changes in vasculature are usually required for a normal erection.  Rather, a small erectile response in the form of an increase or decrease in penile girth is produced. 

Based on the presence or absence of a confirmed response, the course of the cavernous nerves can then be mapped, and informed decisions regarding the dissection pattern to be used can be made for each individual patient.  The CaverMap Surgical Aid can also be used to assess the functional state of the cavernous nerves at the end of the procedure. 

Stimulation is accomplished by inducing a biphasic current pulse train for up to 80 seconds with a controlled current intensity (8 to 20 mA) and pulse duration of 800 microseconds.  The device is programmed to provide a gradual increase in current (from 8 to 20 mA) in 20-second increments. 

Changes in penile circumference are electronically detected and displayed as relative change, with both visible and audible displays (light-emitting diode [LED] scale and changes in a tone, respectively).  The penile sensor contains a small amount of mercury that allows measurement of electric resistance to the supplied current.  As the penile circumference changes, so does the length of mercury in the sensor loop, which is reflected in minute changes in resistance.  Minimal changes in tumescence of 0.5% are considered positive responses to nerve stimulation.  To avoid exhausting the erectile response, stimulation is discontinued once an unequivocal response is reached, with no attempt to determine maximal responses with higher current intensity. 

Operative Technique

Step 1: Anesthesia and preliminary dissection

After prophylactic antibiotics are given and sequential intermittent compression devices are placed and activated, endotracheal anesthesia is administered.  The patient is prepped and draped according to surgeon preference.  Prior to placement of the initial Foley catheter, the tumescence sensor is placed around the base of the penis (Figure 3).  The sensor, a thin, elastic, circular piece, should fit snugly around the base of the penis.  This strain gauge is connected to the control unit so that the system is able to sense changes in penile diameter in response to nerve stimulation. 

A lower midline incision is used to enter the space of Retzius, and appropriate retraction is placed for adequate surgical exposure.  Following bilateral pelvic lymphadenectomy (with or without pathologic analysis of frozen sections), the periprostatic fat is removed and the superficial brand of the dorsal vein is divided.  The puboprostatic ligaments are divided or preserved according to surgeon preference, and the endopelvic fascia is incised. 

Step 2: Determining baseline nerve status and function

Baseline responses to nerve stimulation should be established either at this time or following division of the dorsal vein complex and urethra.  The CaverMap Surgical Aid is used to stimulate the neurovascular bundles bilaterally as they pass through the tissue posterolateral to the urethra (Figure 4).  The probe tip may be bent to various angles to facilitate placement into the desired position along the prostate.  If no tumescence response is obtained, the stimulation procedure is discontinued, and the procedure proceeds as the surgeon would ordinarily.  If a response is obtained, further stimulation is then performed.

Step 3: Mapping neurovascular bundles

Stimulation proceeds sequentially along the lateral aspect of the prostate at the prostatic apex, the mid region, and the base of the prostate.  By noting where the probe tip is located when it elicits a tumescence response, the course of the neurovascular bundles becomes apparent and provides an anatomic “map” for the surgeon interested in their preservation.   

The degree of stimulation is controlled by the probe handle, which has both “coarse” and “fine” switches (Figure 5).  When the “coarse” switch is activated, the signal is delivered along eight electrodes across the tip to locate the general vicinity of the neurovascular bundles.  When the “fine” switch is activated, the output is changed so that the stimulatory signal is delivered along only four, two, or one electrode(s) at the probe tip for more precise, accurate localization of the neurovascular bundles.

Step 4: Dividing the lateral pedicle                                                

Beginning at the apex of the prostate, the lateral attachments are divided and controlled according to surgeon preference.  If a tumescence response is noted when stimulating  tissue, that tissue is spared.  The lateral tissues are mobilized closer to the prostate, and special attention should be paid to the remaining extracapsular tissue during this dissection.  This should not be preserved at the expense of transecting the prostatic capsule or a palpable neoplasm.  In that case, the neurovascular bundle should be sacrificed and included with the specimen to perform the proper cancer operation. 

Stimulation is then performed sequentially in cephalad direction.  The “coarse” and “fine” switches may be used for precise localization of neurovascular bundles.  If stimulation produces a tumescence response, tissue is preserved; this tissue overlies the neurovascular bundles.  Dissection continues in a cephalad direction to the level of the bladder neck. 

Step 5: Completing the procedure

 Following complete removal of the prostate and seminal vesicles with ligation of the vasa deferentia, hemostatis is achieved, and the bladder neck is reconstructed according to the surgeon preference with eversion of the bladder mucosa.  Before vesicourethral anastomosis is performed, the CaverMap Surgical Aid can be used to stimulate the neurovascular bundles once more to confirm unilateral or bilateral nerve function.  The anastomosis is then completed, a final Foley catheter and drains are placed, and the wound is closed in layers.

Clinical Experience

Initial clinical experience with the CaverMap Surgical Aid appears promising.  In the first published study to examine the clinical usefulness of this device, Klotz and Herschorn reported a 84% success rate preserving erectile function in 17 patients who underwent intraoperative dissection guided by the nerve stimulator.  However, on a more cautionary note, these investigators also reported a 12% rate of positive margins at either the apical or lateral regions.  Whether this might have been prevented without using the CaverMap Surgical Aid is not known. 

Our clinical experience with the CaverMap Surgical Aid so far has been positive.  The easy-to-use device is now in routine use at our institution for those patients who undergo radical retropubic prostatectomy and who are also candidates for nerve-sparing procedures. 

Summary and Conclusions

Erectile dysfunction following radical prostatectomy results from a number of factors, and preservation of the neurovascular bundles contributes to better postoperative potency.  The CaverMap Surgical Aid uses a nerve stimulation device to identify and map the route of the neurovascular bundles and the cavernous nerves during radical prostatectomy.  This information allows the surgeon to perform the nerve-sparing prostatectomy with improved results, particularly in cases where the anatomy may not be obvious. 

This technology must be used judiciously.  Although it provides much more anatomic information that ever before, the surgeon must balance the sometimes incompatible goals of a negative margin of resection with preservation of the neurovascular bundles.  No direct evidence suggests that the CaverMap Surgical Aid contributes in any way to increases in the rates of margin positivity, but knowledge of the precise location of the neurovascular bundles may increase the temptation to remove a smaller than usual margin.  This may occur in an effort to preserve a bundle that is closely apposed to the prostate and that may have ordinarily been divided during the dissection.

Another benefit of using the CaverMap Surgical Aid involves helping establish baseline and postdissection continuity and determining efficacy of the cavernous nerves.  In particular, a tumescence response following removal of the surgical specimen is reassuring in demonstrating a physical neural connection between the area receiving stimulation and the penis. 

Investigators at our institution and others are now performing controlled studies with longer periods of follow-up.  When the resulting data are available, they will be stratified and analyzed to determine the true benefit of the CaverMap Surgical Aid.  With confirmation of the promising initial results, cavernous nerve mapping may become a standard part of anatomic radical prostatectomy.     

Suggested Reading:

Hendrick JG, Kaplan SA.  What the literature reveals about the complications of radical retropubic prostatectomy. Contemp Urol 1997; 1:13-22.

Klotz L, Herschorn S. Early experience with intraoperative cavernous nerve stimulation with penile tumescence monitoring to improve nerve sparing during radical prostatectomy.  Urology 1998; 52: 537 – 542.

Quinlan DM, Epstein JI, Carter BS, et al.  Sexual function following radical prostatectomy: Influence of preservation of neurovascular bundles.  J Urol 1991; 145-998-1002.

Walsh PC.  Anatomic radical retropubic prostatectomy.  In: Walsh PC, Retick AB, Vaughan ED, et al, eds.  Campbell’s Urology.  Toronto, Ontario: WB Saunders; 1998: 2569-2585

Walsh PC, Lepor H, Eggleston C. Radical Prostatectomy with preservation of sexual function: Anatomical and pathological considerations.  Prostate 1993: 4:473-485

 Copyright ã 1998 by MPI.