Iatrogenic hypotension can occur in men in neurodegenerative disease using sildenafil (49). Hussain et al. placed men with PD and MSA on sildenafil and recorded blood pressure before and after. Half of the 12 MSA patients developed postural hypotension, while none of the twelve PD patients did. Since MSA can be difficult to distinguished diagnostically from PD, baseline blood pressure measurements prior to prescribing the medication and seeking medical assistance if symptomatic hypotension occurred was recommended for all patients with PD, and MSA. Of note, none of the men with MSA who developed hypotension discontinued sildenafil use due to its effectiveness at improving their erections.
ED is defined as the inability to achieve a full erection or the inability to maintain an erection adequate for sexual intimacy. Other types of sexual dysfunction such as premature ejaculation and low libido may occur; however, the most common and disruptive problem in men is ED. Although most men will experience periodic episodes of ED, these episodes tend to become more frequent with advancing age.
If you have a neurological disorder or spinal cord injury and other erectile dysfunction treatments aren’t effective, two types of surgical implants could offer solutions to your ED. “An implantable pump can be used to manually create an erection by pumping fluid into cylinders placed inside the penis,” explains Feloney. “The other option is a malleable prosthesis that works like a gooseneck lamp to direct the penis into position for intercourse." Risks for these ED treatments include infection and mechanical breakdown.
The penis contains three cylinders, the two corpora cavernosa, which are on the top of the penis (see figure 1 below). These two cylinders are involved in erections. The third cylinder contains the urethra, the tube that the urine and ejaculate passes through, runs along the underside of the penis. The corpus spongiosum surrounds the urethra. Spongy tissue that has muscles, fibrous tissues, veins, and arteries within it makes up the corpora cavernosa. The inside of the corpora cavernosa is like a sponge, with potential spaces that can fill with blood and distend (known as sinusoids). A layer of tissue that is like Saran Wrap, called the tunica albuginea, surrounds the corpora. Veins located just under the tunica albuginea drain blood out of the penis.
Alprostadil is a potent vasodilator and smooth muscle relaxant identical to the naturally occurring PGE1. PGE1 binds with specific receptors on smooth muscle cells and activates intracellular adenylate cyclase to produce cAMP, which in turn induces tissue relaxation through a second messenger system (96). PGE1 is the only FDA approved form of intracavernosal therapy and is available commercailly as EDEX, or Caverject. Its efficacy was demonstrated in several clinical trials where the rate of responders ranged from 40% to 80% (97,98). The most common adverse event is penile pain, which is not related to the injection of the medication itself. In men with prolonged use the pain is usually self-limited (99).
The primary nerve fibers to the penis are from the dorsal nerve of the penis, a branch of the pudendal nerve. The cavernosal nerves are a part of the autonomic nervous system and incorporate both sympathetic and parasympathetic fibers. They travel posterolaterally along the prostate and enter the corpora cavernosa and corpus spongiosum to regulate blood flow during erection and detumescence. The dorsal somatic nerves are also branches of the pudendal nerves. They are primarily responsible for penile sensation. 
NO is produced by the enzyme NO synthase (NOS).  NOS plays many roles, ranging from homeostasis to immune system regulation. To date, 3 subtypes have been identified: nNOS, iNOS, and eNOS, which are produced by the genes NOS1, NOS2, and NOS3, respectively. This nomenclature is derived from the sources of the original isolates: neuronal tissue (nNOS), immunoactivated macrophage cell lines (iNOS), and vascular endothelium (eNOS). The subtypes are not, however, limited to the tissues from which they were first isolated.
Inside the cell, NOS catalyzes the oxidation of L-arginine to NO and L-citrulline. Endogenous blockers of this pathway have been identified. The gaseous NO that is produced acts as a neurotransmitter or paracrine messenger. Its biologic half-life is only 5 seconds. NO may act within the cell or diffuse and interact with nearby target cells. In the corpora cavernosa, NO activates guanylate cyclase, which in turn increases cyclic guanosine monophosphate (cGMP). Relaxation of vascular smooth muscles by cGMP leads to vasodilation and increased blood flow.
A physical exam checks your total health. Examination focusing on your genitals (penis and testicles) is often done to check for ED. Based on your age and risk factors, the exam may also focus on your heart and blood system: heart, peripheral pulses and blood pressure. Based on your age and family history your doctor may do a rectal exam to check the prostate. These tests are not painful. Most patients do not need a lot of testing before starting treatment.