Questions about sex life are often overlooked during the clinical interview with Parkinson's disease (PD) patients and their caregivers. Consequently, in patients with PD the sexual experience is often poorly evaluated and research in this area is scarce. However, as corrections in drug therapy can be made and effective pharmacological and behavioral therapies are available, sex life in PD should be proactively investigated .
Sexual experience is a complex process that requires coordinated functioning of the person's mental, autonomic, sensory, and motor systems. It relies on the proper functioning of the neurological, vascular and endocrine systems and a healthy emotional state . Three main factors could be involved in altering the sexual health of patients with PD .
Both the disease and the medications can directly cause sexual dysfunction; indirectly, the sexual function can be modified by maladaptive psychological mechanisms due to suffering from a life-changing and progressive disease or disease-related changes in the relationship between the patient and their caregiver .
Concerning this, psychobiology studies show that bonding between two people involves various neurotransmitters and hormones such as dopamine, serotonin, oxytocin and vasopressin and the bond between two individuals activates brain areas involved in reward and satisfaction mechanisms  that are also involved in the neuropathology of Parkinson's disease (PD).
On the other hand, sexual desire in dyadic relationships also involves the so-called mirror mechanisms  that are activated in individuals when they observe the actions and emotions of others. This sensorimotor transformation affects the content of actions and emotions and their emotional form or quality . It allows individuals to relate to another’s state quickly and is essential in species that provide extensive parental care .
On the basis of these premises, there could be an intriguing relationship between sexual activity, mirror mechanisms and PD.
The disease and its medications
The clinical picture of PD comprises both progressive characteristics motor features and highly prevalent and diverse non-motor symptoms (NMS) ,  and sexual dysfunction is considered a non-motor feature of the disease. The motor symptoms of PD may make the act of sexual intercourse difficult. Diminishing physical capacity may necessitate the patient taking on a more passive role, thus altering the couple’s equilibrium. Further, if the patient’s movement disorder is disruptive at night, the couple may sleep in separate beds or even separate rooms, thus decreasing the opportunity for spontaneous sexual contact. Autonomic nervous system involvement may be a cause of sexual dysfunction; thus the latter is closely related to a combination of NMS and motor dysfunction.
Highly effective medications are available for the treatment of Parkinson's disease. They are essentially based on dopaminergic replenishment and can affect sexual behavior through direct stimulation of the D2 receptor in the medial preoptic area; thus, they inhibit prolactin secretion. This hormone is known to control sexual desire in both men and women and, indeed, inhibits sexual functions, exerting an inhibitory action on sexual functions , . A decrease in the level of prolactin interferes with the inhibitory effect on sexual function and increases sexual behavior . On the other hand, Nitkowska et al.  reported that in men with PD higher prolactin levels and low levels of sex steroids can alter cognition, mood and QoL. Therefore, also these changes in mental status can influence the sexual behavior of individuals with PD . Furthermore, dopaminergic drugs can increase the plasma level of oxytocin, by increasing its release from the paraventricular nucleus of the hypothalamus . Oxytocin produces erectogenic effects in the lumbosacral spinal cord , , thus enhancing sexual responses.
Politis et al.  demonstrated increased sexual behavior in PD subsequent to greater activation of the limbic, paralimbic, temporal, occipital, somatosensory and prefrontal cortex that was related to emotional, cognitive, autonomic, visual and motivational processes. Increased sexual desire was also seen in the group using levodopa  to the point of compulsive sexual behavior, which is considered an impulse control disorder ,  and is mainly associated with the use of dopaminergic agonists and high doses of levodopa , .
Psychological mechanisms and the role of caregiving
Depression and anxiety are common in patients with PD . An increase in psychological morbidity can also be found in those living and caring for these patients. Anxiety and depression, either in the patient or their partner, can affect libido and sexual performance. Performance anxiety and fear of failure can also come into play, further worsening the sexual problems, and avoiding either or both partners (and leading to avoidance in either or both partners).
Other factors that might indirectly explain the modification in sexual behavior in PD are related to the change in roles in the couple with one of the partners becoming a caregiver. The patient might have to receive more care than they have been used to or are willing to accept, possibly leading to lower self-esteem. The extra demands on the time and energy of the partner might require significant adjustments in the caregiver’s work commitments and leisure time. The impact of these factors on the couple’s sex life and, in general, on the marriage will depend partly on their ability to discuss their problems. However, interpersonal communication might also be compromised by PD, with some patients having a reduced range of verbal and nonverbal emotional expression. Therefore, further stress can be created if the partner misinterprets the patient’s communication.
It is necessary to consider separately males and females as well as patients and their partners. Testosterone acts on the male brain to promote sexual arousal and desire. Although taking levodopa does not result in significantly increased levels of testosterone with increasing age , , there are varying degrees of reduction of both free and total testosterone  and possibly the responsiveness of neurons in relevant areas of the brain such as the locus coeruleus, which is the brainstem center for testosterone-dependent arousal mechanisms. These changes contribute to age-related decreases in sexual interest and, to some extent, erectile function.
The role of hormones in the effects of ageing on women’s sexuality remains less clear. In women, levels of testosterone gradually decrease with age (starting in the mid-30s) independently of menopause. This decrease might contribute to an age-related decrease in sexual interest in some women. As women age, relationship factors and mental health are likely to be as important as or more important than physiological factors. Many women report a decrease in their sexual interest and responsiveness as they progress through midlife, and they are less likely to become distressed or worried about these changes as they get older. For many women who are in a relationship, the quality of the relationship and sexual problems are more important than their sexual responsiveness .
Based on their activities of daily living, men and women have different expectations and discrepant sexual needs can be observed in PD . In men with PD, feelings of the burden of sexual dysfunction are accompanied by fears, such as not being able to meet the expectations of their partners. Thus, they avoid sexual activities and have thoughts of separation and withdrawal from the relationship .