What is the success rate of MUSE?
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In the initial studies of MUSE's effectiveness, the success rate was 64%. More recent studies have demonstrated its efficacy to be only 30%, however. Attempts to increase this success rate via the use of the ACTIS venous
constrictor, a constricting band that is placed at the base of the penis, have helped some men. In some men, an erection rigid enough for penetration may occur in the standing position; however, when these individuals change to a supine position, the erection may decrease. In these men, changing the position used for intercourse or using the constricting band has proved helpful.
It is difficult to predict who will and who will not respond to MUSE. The patient's age and the cause of the erectile dysfunction, for example, are not predictive of response. Nevertheless, MUSE is unlikely to be effective in men who have not responded to intracavernous injection therapy.
What are the side effects of MUSE?
The most common side effect, occurring in one-third to one-half of all men who take MUSE, is pain. This pain may be present in the penis, urethra, testis, or perineum. The intensity of the pain varies according to the dose taken. Thus, as the dose increases, the intensity of the pain may likewise increase. Hypotension and syncopal episodes (temporary loss of consciousness caused by decreased blood flow to the brain) have been reported in 1.2 to 4% of men who took MUSE, with their frequency depending on the dose used. Other side effects include urethral bleeding (in 4 to 5% of men who took MUSE), dizziness (in 1%), and urinary tract infection (in 0.2%). Prolonged erections and penile fibrosis (scarring) rarely occur. Ten percent of female partners experience vaginal irritation or vaginitis.
What is penile injection therapy?
Intracavernous injection therapy is the process whereby a small amount of a chemical is injected directly into the corpora cavernosa. These chemicals relax smooth muscle, thereby helping to increase blood flow into the penis.
The major advantage of injection therapy is that it does not depend on oral absorption, as pills do, or on absorption through the tissues, as MUSE does. The disadvantage is that it requires a small penile injection. Most men are anxious when they initially start with injection therapy but find that the procedure itself is not especially uncomfortable. In most patients who do not respond to first-line oral therapy or who are not candidates for oral therapy, injection therapy provides satisfactory erections.
The only FDA-approved chemicals for intracavernous injection therapy are Caverject (Pharmacia & Upjohn) and Edex (Schwarz Pharma) (Table 21). Both of these agents consist of prostaglandin E1. Other agents used alone or in combination include papaverine and phentolamine. All three medications—prostaglandin E1, papaverine, and phentolamine—may be used in combination, in which case the combination is referred to as triple P or trimix. Prostaglandin E1 and triple P are the two most common forms of injection therapy used, and each offers a unique set of advantages and disadvantages.
How do I perform penile injection therapy?
Before you start to use intracavernous injection therapy at home, you will receive a test dose in the physician's office. Of all of the therapies available, intracavernous injection therapy carries the highest risk of priapism, with as many as 2% of patients experiencing this side effect. Most cases of priapism occur with first use, during the test dosing. This fact is important because if you return to your urologist's office within 3-4 hours, the erection can easily be brought back down with an injection of another chemical. If your urologist is concerned about priapism, he or she may choose to terminate your erection by injecting you with a chemical to stop the erection before you head home. Thus, test dosing minimizes your risk of having a case of priapism at an inopportune time.
Your urologist can also use the test dosing session as a time for hands-on instruction. That is, you can learn how to inject yourself and actually perform your first self-injection with the physician's guidance in the office. This consideration is very important because the first time you perform the injection therapy at home, you will probably be nervous. Remembering that you performed the injection in the office may help you relax.
The needle that you use to inject the medication is quite short and small. It is short because it does not need to pierce deeply into the penis, just into the corpora on one side, for the therapy to be effective (Figure 34). It is small because you will be injecting only 1 cc or less
Figure 34. Injection therapy: Proper location of injection.
Used with permission from Pfizer. Inc.
Table 21. Dosage and Volume Calculations for Injection Therapy Using Prostaglandin E1 (Caverject, Edex)
Source: Ellsworth, P. 100 Questions and Answers About Erectile Dysfunction, 2e. Jones and Bartlett Publishers, LLC, 2008.
of medication. After your initial test dose, your urologist will decide on a dose that you will try initially at home. The volume you inject will vary with the amount of prostaglandin E1 you need to achieve an adequate erection and the concentration of the solution (Table 21). Do not get discouraged if this initial dose is not adequate. Most physicians would prefer to prescribe a dose that is too small and then increase it as needed to avoid priapism.
When using injection therapy at home, you should keep several points in mind:
• Look where you are going to inject the syringe to make sure that no superficial veins are in the area.
• Gently wipe the area with an alcohol swab.
• Always inject the medication on the side of the penis toward the base. The needle should be inserted straight into the penis at a 90-degree angle to the penis.
• Apply pressure to the injection site for a minute or two. If you see any bleeding from the injection site, maintain the pressure for about 5 minutes. Men taking blood thinners should apply pressure to the injection site for about 5 minutes.
• Never reinject the medication once you have made the initial injection, even if you fear that you have not injected yourself properly.
• Alternate sides with each injection.
• Do not inject medication more frequently than every 48-72 hours.
• If your erection lasts longer than 3 hours, call the urologist on call. Do not wait—a delay in seeking care will just make it more difficult to treat the prolonged erection.
• If you are having difficulty with performing the injections, talk with your urologist. Perhaps getting more instruction or an autoinjector (e.g., the PenInject 2.25 autoinjector) or teaching your partner would be helpful.
• Remember that with Edex and Caverject, once the medication has been reconstituted (i.e., once the powder is dissolved in the sterile water), it must be refrigerated. The solutions tend to lose their efficacy after 7 days.
• Make sure that the volume of the medication and the dose of medication that you are injecting are consistent (see the calculations in Table 21).
• Do not reuse needles and carefully dispose of used needles.
• Remember that your erection may persist after you climax and ejaculate, but will go down when the medication wears off and exits from your system.