The sensitivity of the skin of the penis to detect vibrations (biothesiometry) can be used as a simple office nerve function screening test. This involves the use of a small vibrating test probe placed on the right and left side of the penile shaft as well as on the head of the penis. The strength of the vibrations is increased until you can feel the probe vibrating clearly. Although this test does not directly measure the erectile nerves, it serves as a reasonable screening for possible sensory loss and is simple to perform. More formal nerve conduction studies are only performed in selected cases.
Erythrocytosis has been noted in men on TRT, and should be monitored every 6–12 months depending upon the patients’ response to changes in haematocrit levels. For mild elevations, the dosage of testosterone can be decreased or the interval of using the medication can be increased. With the haematocrit greater than 50%, decisions to temporarily discontinue the medication or periodic phlebotomy may be indicated.38
Modern drug therapy for ED made a significant advance in 1983, when British physiologist Giles Brindley dropped his trousers and demonstrated to a shocked Urodynamics Society audience his papaverine-induced erection. The drug Brindley injected into his penis was a non-specific vasodilator, an alpha-blocking agent, and the mechanism of action was clearly corporal smooth muscle relaxation. The effect that Brindley discovered established the fundamentals for the later development of specific, safe, and orally effective drug therapies.[better source needed][better source needed]
A CVA can occur anywhere through the brain, midbrain, brainstem and spinal cord leading to varying degrees of SD depending on location. A decline in libido, erection and ejaculation are frequent in men who have had a CVA, with a reported prevalence of ED that varies from 17% to 48% (28,29). Right hemispheric infarcts seem to affect erections more so than left-sided ones. The exact effects of CVA on sexual function are complex and multifactorial, as disability, psychological and emotional status can affect sexual function aside from the location of the CVA.
Performance anxiety can be another cause of impotence. If a person wasn’t able to achieve an erection in the past, he may fear he won’t be able to achieve an erection in the future. A person may also find he can’t achieve an erection with a certain partner. Someone with ED related to performance anxiety may be able to have full erections when masturbating or when sleeping, yet he isn’t able to maintain an erection during intercourse.
"The good news is, our study also found that a large proportion of men were naturally overcoming erectile dysfunction issues. The remission rate of those with erectile dysfunction was 29%, which is very high. This shows that many of these factors affecting men are modifiable, offering them an opportunity to do something about their condition," Professor Wittert says.
Nocturnal penile tumescence testing (NPT) may be useful in distinguishing mental from physical impotence. This test involves the placement of a band around the penis that you would wear during two or three successive nights. If an erection occurs, which is expected during rapid eye movement (REM) sleep, the force and duration are measured on a graph. Inadequate or no erections during sleep suggests an organic or physical problem, while a normal result may indicate a high likelihood of emotional, psychological, or mental causes.
Experts often treat psychologically based impotence using techniques that decrease anxiety associated with intercourse. The patient's partner can help apply the techniques, which include gradual development of intimacy and stimulation. Such techniques also can help relieve anxiety during treatment of physical impotence. If these simple behavioral methods at home are ineffective, a doctor may refer an individual to a sex counselor.
Both ED and low testosterone (hypogonadism) increase with age. The incidence of the latter is 40% in men aged 45 years and older.  Testosterone is known to be important in mood, cognition, vitality, bone health, and muscle and fat composition. It also plays a key role in sexual dysfunction (eg, low libido, poor erection quality, ejaculatory or orgasmic dysfunction, reduced spontaneous erections, or reduced sexual activity). 
You can also consider using a cock ring or penis ring if you are able to get an erection but have trouble maintaining it. Cock rings work by trapping the blood inside your penis so it remains harder for longer. You can purchase one in a sex store or online for about $5 and you put in on over your penis and testicles when you are half hard and keep it on until you are finished with sex. It’s a great cheap fix for erectile issues.
When sexually stimulated/aroused, the nerves supplying the penis release a chemical, nitric oxide (NO). Nitric oxide is important because it stimulates the production of a chemical called cyclic guanosine monophosphate (cGMP). cGMP causes the muscle in arteries of the penis to relax and increase blood flow into the penis. NO is broken down in the body by phosphodiesterase enzymes. PDE5 inhibitors thus prevent the breakdown of NO and thus promote increased blood flow into the penis.