Genitourinary dysfunction in Parkinson's disease

Authors


  • Potential conflict of interest: Nothing to report.

Abstract

Bladder dysfunction (urinary urgency/frequency) and sexual dysfunction (erectile dysfunction) are common nonmotor disorders in Parkinson's disease (PD). In contrast to motor disorders, genitourinary autonomic dysfunctions are often nonresponsive to levodopa treatment. The brain pathology causing the bladder dysfunction (appearance of overactivity) involves an altered dopamine-basal ganglia circuit, which normally suppresses the micturition reflex. By contrast, hypothalamic dysfunction is mostly responsible for the sexual dysfunction (decrease in libido and erection) in PD, via altered dopamine-oxytocin pathways, which normally promote libido and erection. The pathophysiology of the genitourinary dysfunction in PD differs from that in multiple system atrophy; therefore, it might aid in differential diagnosis. Anticholinergic agents are used to treat bladder dysfunction in PD, although these drugs should be used with caution particularly in elderly patients who have cognitive decline. Phosphodiesterase inhibitors are used to treat sexual dysfunction in PD. These treatments might be beneficial in maximizing the patients' quality of life. © 2010 Movement Disorder Society

Parkinson's disease (PD) is a common movement disorder associated with the degeneration of dopaminergic neurons in the substantia nigra. In addition to the movement disorder, patients with PD often show nonmotor disorders. The nonmotor problems of PD include neuropsychiatric disorders, sleep disorders, sensory symptoms, and autonomic disorders.1 Genitourinary dysfunction is one of the most common autonomic disorders.2, 3 Studies have shown that the genitourinary dysfunction has great significance in relation to quality-of-life measures, early institutionalization, and health economics.4, 5 It is particularly important to note that, unlike motor disorder, genitourinary dysfunctions are often nonresponsive to levodopa, suggesting that they occur through a complex pathomechanism6; for this reason, add-on therapy is required to maximize patients' quality of life. This article reviews genitourinary dysfunction in PD, with particular reference to neural control of the bladder and genital organs, symptoms, objective assessment, and treatment.

BLADDER DYSFUNCTION IN PD

Neural Control of Micturition

Normal Micturition and Detrusor Overactivity

The lower urinary tract (LUT) consists of two major components, the bladder and urethra. The bladder has abundant muscarinic M2,3 receptors and adrenergic beta 3 receptors, and is innervated by cholinergic (parasympathetic) and noradrenergic (sympathetic) fibers for contraction and relaxation, respectively.7 The urethra has abundant adrenergic alpha 1A/D receptors and nicotinic receptors, and is innervated by noradrenergic (sympathetic; contraction) and cholinergic (somatic; contraction) fibers (Fig. 1). The LUT performs two opposite functions, storage and emptying of urine, both of which require an intact neuraxis that involves almost all parts of the nervous system.8 This is in contrast to postural hypotension, which arises due to lesions below the medullary circulation center in humans.9

Figure 1.

Neural circuitry relevant to micturition. PAG, periaqueductal gray; LC, locus ceruleus; NBM, nucleus basalis Meynert; PVN, paraventricular nucleus; MPOA, medial preoptic area; A, adrenergic/noradrenergic; ZI, zona incerta; VTA, ventral tegmental area; SNC, substantia nigra pars compacta; DLTN, dorsolateral tegmental nucleus; PBN, parabrachial nucleus; IML, intermediolateral cell column; GABA, γ-aminobutyric acid; T, thoracic; L, lumbar; S, sacral. See text.

Normal urinary storage is dependent on the sacral autonomic reflex.7, 10 The storage reflex is thought to be tonically facilitated by the brain, particularly the pontine storage center.11, 12 The pontine storage center lies just ventrolateral to the pontine micturition center (PMC). In addition to the pontine storage center, the storage function is facilitated by the hypothalamus, cerebellum, basal ganglia, and frontal cortex. These areas have been shown to be activated during urinary storage by functional neuroimaging in humans.13

In contrast, normal micturition is dependent on the spino-bulbo-spinal autonomic reflex,7 which particularly involves the midbrain periaqueductal gray matter (PAG)14–17 and the PMC.7, 11 The PAG is thought to be central in regulating micturition and has a range of inputs from the higher structures. The PMC is located in or adjacent to the locus coeruleus.18–20 The PMC is thought to activate the sacral bladder preganglionic nucleus by glutamate,21 while suppressing the sacral urethral motor nucleus (the Onuf's nucleus) by γ-amino-butyric acid (GABA) and glycine.22 The voiding function seems to be initiated and facilitated by the higher brain structures, e.g., the hypothalamus and prefrontal cortex, which seem to overlap in the storage-facilitating area.13, 23

Bladder (detrusor) overactivity (DO) is the major cause of urinary urgency/frequency and incontinence.24 In lesions above the brainstem, the micturition reflex arc is intact, where DO is considered an exaggerated micturition reflex.24, 25 This is in line with the fact that the DO appearing after experimental stroke requires mRNA synthesis in the PMC.26 The exaggeration of the micturition reflex might be brought about by more than simply the decreased inhibition of the brain, and might be further facilitated by glutamatergic and D2 dopaminergic mechanisms.27

Basal Ganglia Circuit and Dopamine

The net effect of the basal ganglia on micturition is thought to be inhibitory (Fig. 2).7, 28, 29 Functional neuroimaging during bladder filling results in activation in the globus pallidus of normal volunteers30 and in the putamen in patients with PD.31 In contrast, dopamine transporter imaging was lower in PD patients with urinary dysfunction than in those without it.32, 33 DO can be reproduced in experimental parkinsonism.29, 34 Electrical stimulation of the substantia nigra pars compacta (SNc) inhibited the micturition reflex,35, 36 and striatal dopamine levels in situ significantly increased in the urinary storage phase in experimental animals.37

Figure 2.

Possible relationship between basal ganglia circuit (left-side) and micturition circuit (right-side). (modified from Sakakibara et al., 200339). DA, dopamine; GABA, gamma-aminobutyric acid; SNc, substantia nigra pars compacta; GPi, globus pallidus internus; SNr, substantia nigra pars reticulate; STN, subthalamic nucleus; GPe, globus pallidus externus; VTA, ventral tegmental area; PMC, pontine micturition centre; Glu, glutamate; Black line, inhibitory neurons; White line, excitatory neurons; Hatched line, neurons of undetermined property. See text.

The micturition reflex is under the influences of dopamine (both inhibitory in D1 and facilitatory in D2) and GABA (inhibitory).7, 28 Both the SNc neuronal firing and the released striatal dopamine seem to activate the dopamine D1-GABAergic direct pathway (Fig. 2), which not only inhibits the basal ganglia output nuclei, but also may inhibit the micturition reflex via GABAergic collateral to the micturition circuit.37–39 Intracerebroventricularly administered dopamine inhibits the micturition reflex,35 and high-frequency stimulation in the subthalamic nucleus (STN) results in bladder inhibition in experimental animals.39, 40 In patients with PD, disruption of this pathway may lead to DO and resultant urinary urgency/frequency.

In addition to the nigrostriatal fibers, the ventral tegmental area (VTA)-mesolimbic dopaminergic fibers are thought to be involved in the control of micturition (Fig. 1). In animals, VTA lesions induced more severe bladder overactivity than SNc lesions did.41 Stimulation of VTA elicited both facilitation and termination of the micturition reflex.36, 42 Therefore, DO in PD might depend, at least in part, on the degeneration of the VTA-mesolimbic neurons in PD.

BLADDER DYSFUNCTION IN PD

Lower Urinary Tract Symptoms

The reported prevalence of LUT symptoms (LUTS) in patients with PD ranges from 38 to 71%.43–48 However, it has been difficult to determine to what extent PD contributes to LUTS. This is because not only PD patients, but also men older than 60 years of age may have an obstruction component to their urinary symptoms brought about by benign prostate hyperplasia. Women may have stress urinary incontinence. Similarly, “idiopathic DO”10 may occur in men and women older than 65 years due in part to latent brain ischemia.49 In the above studies, most of the data were collected from patients who visited a urology, gynecology, or internal medicine clinic because of their symptoms. Furthermore, some of the studies were published before the diagnosis of multiple system atrophy (MSA)50 was recognized.

In recent studies of PD patients who were diagnosed according to modern criteria,5, 51–53 the prevalence of LUTS was found to be 27 to 63.9%, using validated questionnaires,51–53 or 53% in men and 63% in women, using a nonvalidated questionnaire that includes a urinary incontinence category,5 with all of these values being significantly higher than the incidence rates in healthy controls. The majority of patients had onset of bladder dysfunction after appearance of motor disorder. Urinary incontinence in PD frequently occurred in conjunction with fecal incontinence, whereas no significant relation was observed between bladder and sexual dysfunction.5 Also, bladder dysfunction substantially affects the quality of life in patients with PD.5 Araki and Kuno have shown a correlation between bladder dysfunction in patients with PD and neurological disability.51 Correlations have been shown between bladder dysfunction in patients with PD and neurological disability, and bladder dysfunction and stage of disease,5 both suggesting a relationship between dopaminergic degeneration and LUTS. However, Campos-Sousa et al. did not find such a correlation.53 LUTS was more common in an older PD patient group than in a younger PD group, as is also the case in healthy (non-PD) populations.5 Storage symptoms are the most common of the LUTS symptom types. Storage symptoms include nocturia (nighttime urinary frequency), which is the most prevalent symptom reported by patients with PD (>60%).5, 51–53 Patients also complain of urinary urgency (33–54%) and daytime frequency (16–36%). Urinary incontinence was present in 26% of male and 28% of female patients with PD.5

Although less common than storage symptoms, voiding symptoms also occur in PD patients. In the study by Sakakibara et al., PD patients had significantly higher rates of retardation in initiating urination (44% of men only), prolongation/poor stream (70% of men only), and straining (28% of women only) compared with the control group.5 Araki et al. noted a correlation between voiding symptoms and stage of disease.54 However, despite the voiding symptoms, PD patients have low postvoid residuals.5 Therefore, it seems reasonable to say that overactive bladder (urgency/frequency syndrome) is a feature of bladder dysfunction in PD.

Videourodynamics, Pressure-Flow Analysis, and Sphincter Electromyography

Bladder (detrusor) Overactivity

The storage-phase urodynamic abnormalities in PD include reduced bladder capacity together with detrusor overactivity (DO), which is an involuntary phasic detrusor contraction,10 in 45 to 93%43, 44, 54–58 of patients, and uninhibited external sphincter relaxation, which is an involuntary decrement of the sphincter electromyogram activity usually accompanying DO, in 33%53 of patients (Fig. 3). These findings represent the suprasacral type of parasympathetic and somatic dysfunction. Therefore, DO can be the major contributing factor to overactive bladder in PD, which was equally found in both men and women. There is a correlation between DO and stage of disease.55 It is likely that these urodynamic abnormalities are relevant to nigrostriatal and VTA-mesolimbic lesions in PD.

Figure 3.

Detrusor (bladder) overactivity by urodynamic measurement.

Mild, Weak Detrusor and Sphincter Obstruction

Pressure-flow analysis10, 59, 60 of the voiding phase in PD has shown weak detrusor activity during voiding (40% of men; 66% of women).56 There is a correlation between a weak detrusor and the stage of the disease.55 A subset of PD patients had DO during storage but weak detrusor activity in voiding. This combination has recently been estimated to occur in 18% of patients with PD.61 DO during storage but weak detrusor activity in voiding seems to be caused by multiple factors rather than a single factor. One possible mechanism for DO during storage but weak detrusor activity in voiding in PD is that not only bladder-inhibitory, but also bladder-facilitatory brain regions, such as PMC, are affected in this disorder. Some older studies described detrusor-external sphincter dyssynergia or pseudo-dyssynergia in PD, and these findings were attributed to PD by analogy with bradykinesia of the limbs.62 However, in our patients with PD, detrusor-external sphincter dyssynergia was rare.56 In contrast, a pressure-flow analysis in PD revealed that half of the patients with PD showed mild urethral obstruction.56 Patients with PD are reported to have high resting urethral pressure, probably as a result of medication—i.e., levodopa and its metabolites, such as norepinephrine—which may contract the internal sphincter via alpha 1A/D-adrenergic receptors. Irrespective of voiding symptoms in PD, the average volume of postvoid residuals in PD was 18 mL and none of the patients had postvoid residuals greater than 100 mL.56

Differential Diagnosis of Parkinsonism by Bladder Dysfunction

DO is not disease-specific, and is commonly seen in other types of degenerative and cerebrovascular parkinsonism. In the differential diagnosis of PD and MSA, large postvoid residuals, open bladder neck, and neurogenic change in sphincter motor unit potentials are all common in MSA,56, 63 whereas they are rarely seen in clinically typical PD, suggesting relative sparing of the lumbosacral PGN and sacral sphincter motoneurons in PD. However, recent evidence suggests that PD with dementia, or dementia with Lewy bodies,64 may have large postvoid residuals and neurogenic change in the sphincter motor unit potentials,65 thereby mimicking MSA.

TREATMENT OF BLADDER DYSFUNCTION IN PD

Dopaminergic Drugs

It is possible that levodopa and other antiparkinson medication may affect bladder function in PD. Aranda and Cramer66 studied the effects of 3 to 8 mg apomorphine injection on the storage function in two de novo PD patients, and found that the bladder capacity increased. They gave oral levodopa to one of the patients, and the bladder capacity increased. We compared the frequency of bladder dysfunction in de novo PD and PD with levodopa. In that study, LUTS was less frequent than in the treated group.59 In another study, after 3 months of treatment with levodopa, the storage urodynamic parameters were slightly improved in de novo PD.67

In contrast, in non-de novo patients, studies concerning the effect of dopaminergic drugs on micturition have produced conflicting results. Regarding overactive bladder, some reports have shown a storage-facilitating effect of dopaminergic drugs.5 In contrast, Kuno et al. showed that a change in medication from bromocriptine (D2 selective agonist) to pergolide (D1<2 agonist) brought lessening of nocturia,68 and Yamamoto described improvement of DO by pergolide.69 Benson et al.70 gave 2,000 mg of levodopa in two longstanding PD patients, and bladder capacity increased in both patients. After discontinuation of levodopa, the bladder capacity further increased in one of the patients, but decreased in the other. Other reports have shown a voiding-facilitating effect of dopaminergic drugs.71 Fitzmaurice et al.72 have described that, in advanced PD with the on-off phenomenon, DO worsened with levodopa in some patients and lessened in others. Winge et al.73 found that the effect on micturition of treatment with dopaminergic drugs in PD was unpredictable. Recent studies have shown that in early PD74 and advanced PD with the on-off phenomenon,6 a single dose of levodopa exacerbates DO in the filling phase. We still do not know the exact reasons for the discrepancy.

There are several factors underlying the complex bladder behavior in non-de novo PD patients. In animal studies, externally (intravenous/intraperitoneal) administered levodopa is known to facilitate a micturition reflex in rats,75 which may act on the brain, spinal cord, and periphery. Post-synaptic dopamine D1 (excitatory) and D2 (inhibitory) receptors have a millimolar affinity to dopamine, whereas dendritic D2 (inhibitory) autoreceptors have a picomolar affinity to dopamine.76 Therefore, when levodopa is administered externally, it may first stimulate dendritic D2 autoreceptors, which might suppress the NSDN and facilitate the micturition reflex. In cases of PD under long-term treatment with levodopa, dopamine receptors are downregulated and potential hypersensitivity might occur.77 The A11 dopaminergic cell group lies in the dorsal-posterior hypothalamus, which is affected in marmosets with MPTP-induced parkinsonism.78 This cell group descends as the sole source of spinal dopamine mainly through the dorsolateral funiculus, connecting to the superficial dorsal horn and the sacral preganglionic neurons.79 Bladder hyperactivity might also involve an activation of D2 receptors in the spinal cord.80 Peripheral dopamine D1 and D2 receptors also exist in the bladder,81 although their exact role has not been delineated.

Cholinergic Drugs

Anticholinergics82 are generally used as a first-line treatment for overactive bladder. However, it is important to balance the therapeutic benefits of these drugs with their potential adverse effects. When the dose of drug increases, postvoid residuals may appear.75 Dry mouth and constipation are common.83 Cognitive adverse events by anticholinergics are a concern particularly in the elderly. For example, trihexyphenidyl (for PD) and oxybutynin (for overactive bladder) have been shown to have central side effects.84, 85 Factors contributing to the central effects of drugs may include receptor subtype-selectivity and blood-brain barrier (BBB) penetration.86 The cerebral cortex has abundant muscarinic M1 receptors. Whereas most anticholinergics are receptor-nonselective, darifenacin has selective M3 agonistic action and is less apt to block central M1 receptors. Among the factors of BBB penetration, diffusion is facilitated by smaller molecular size (<450–500 KDa), neutrality or smaller polar surface area (<90A), and lipophilicity or a smaller partition coefficient of water versus oil (log P<3).87 Although most anticholinergics have neutrality, trospium is ionic and less apt to penetrate the BBB. Particularly in elderly patients who have hallucinations or cognitive decline (PD with dementia/ dementia with Lewy bodies),64, 65 anticholinergics should be used with extreme caution.

Other Treatments

When a first-line treatment fails or is contraindicated, a second-line treatment might be considered. The main action of central 5-hydroxytryptamine (5-HT, or serotonin)-ergic neurons on the LUT is facilitation of urine storage.88 In PD, neuronal cell loss in the raphe nucleus has been documented.89 Therefore, serotonergic drugs, such as duloxetine and milnaciplan90 (both, serotonin and norepinephrine reuptake inhibitors), can be a choice to treat overactive bladder in PD,87 although clinical effectiveness of serotonergic drugs on the bladder awaits further clarification. When prescribing these drugs, their GI and sedative actions should be considered.

Nocturnal polyuria (nighttime urine product >33% of whole day) is a factor in geriatric nocturia, which should be distinguished from overactive bladder. In patients with PD, the imbalance between diurnal and nocturnal production of urine can be observed in the course of the disease. It is reported that the circadian arginine-vasopressin rhythm is lost in experimental parkinsonism.91 Treatment with desmopressin, a potent analogue of arginin-vasopressin, proved to be effective in reducing nocturia in PD.92 Hyponatremia and water retention should be checked when using this drug. In general, this medication should only be used with extreme caution for nocturia in PD.

The subthalamic nucleus (STN) is regarded as the key area in the indirect pathway, which is dominant in the parkinsonian state.93 Deep brain stimulation (DBS) in the STN inhibits many cells within the STN, probably due to depolarization block and release of GABA from activation of inhibitory afferent terminals.94 In the STN, neuronal firings related to the micturition cycle have been observed in cats.39 DBS in the STN proved to have an inhibitory effects on the micturition reflex in animals39, 40 and in patients with PD.95–97 DBS in the STN also increased bladder capacity and facilitated bladder afferent pathways in the brain of PD patients.98, 99

SEXUAL DYSFUNCTION IN PD

Neural Control of Erection

Normal Erection

Sexual dysfunction is not uncommon in PD.5, 100–104 Studies have shown that sexual dysfunction has great significance in relation to quality-of-life measures. However, the detailed mechanism of sexual dysfunction in PD has not been well-known.

The genital organ primarily shares lumbosacral innervation with the lower urinary tract. Erection is a vascular event105; occurring secondarily after dilatation of the cavernous helical artery and compression of the cavernous vein to the tunica albuginea.105 Helical artery dilatation is brought about by activation of cholinergic and nitrergic nerves; this activation facilitates nitric oxide secretion from the vascular endothelium. Ejaculation is brought about by contraction of the vas deferens and the bladder neck, in order to prevent retrograde ejaculation, by activation of adrenergic nerves (Fig. 4). Sexual intercourse in healthy men can be divided into three phases106: (1) desire (libido), (2) excitement and erection, and (3) orgasm, seminal emission from the vas deferens, and ejaculation from the penis. Erection can be further classified into three types by the relevant stimulation: (1) psychogenic erection (by audiovisual stimulation), (2) reflexive erection (by somatosensory stimulation), and (3) nocturnal penile tumescence (NPT) (associated with rapid eye movement [REM]-sleep). ‘Morning erection’ is considered the last NPT in the nighttime.

Figure 4.

Neural circuitry relevant to erection. PAG, periaqueductal gray; LC, locus coeruleus; NBM, nucleus basalis Meynert; PVN, paraventricular nucleus; MPOA, medial preoptic area; A, adrenergic/noradrenergic; ZI, zona incerta; VTA, ventral tegmental area; SNC, substantia nigra pars compacta; DLTN, dorsolateral tegmental nucleus; PBN, parabrachial nucleus; IML, intermediolateral nucleus; GABA, γ-aminobutyric acid; T, thoracic; L, lumbar; S, sacral; NA, noradrenaline; Ach, acetylcholine; NO, nitric oxide. See text.

Hypothalamic Neurons and Dopamine

Among the three types of erection, reflexive erection requires an intact sacral cord, particularly the intermediolateral (IML) cell columns. Pathology studies have shown that involvement of the IML nucleus is common in MSA, whereas it is uncommon in PD. Therefore, reflexive erection can be affected in patients with MSA. In patients with a supra-sacral spinal cord lesion, reflexive erection might be preserved, whereas psychogenic erection is severely disturbed because of a lesion in the spinal pathways to the sacral cord. Libido and erection are thought to be regulated by the hypothalamus; particularly the medial preoptic area (MPOA) and the paraventricular nucleus (PVN) (Fig. 4).107, 108 Electrical or chemical stimulation in the MPOA/PVN evoked erection and mating behaviors in experimental animals, both of which were abolished by destruction of these areas. Somatosensory inputs from the genitalia ascend in the anterior spinal cord, and project to the MPOA/PVN via the thalamic nuclei. Erotic visual inputs from the retina are thought to reach the MPOA via the mamillary body. Recent neuroimaging studies have shown that penile stimulation or watching pornography activated these areas in humans.109 NPT110 seems to be regulated by the hypothalamic lateral preoptic area.111 The raphe nucleus and the locus ceruleus are candidate areas participating in the regulation of NPT. Oxytocinergic neurons in the hypothalamic PVN are thought to facilitate erection by projecting directly to the sacral cord, and by projecting to the midbrain periaqueductal gray and the Barrington's nucleus (identical to the PMC). Serum oxytocin concentration increases during masturbation in healthy men.

In experimental animals, dopamine is known to facilitate erection and mating behaviors. The MPOA/PVN receives projections from the nigral dopaminergic neurons.112 A microdialysis study showed that the dopamine concentration in the MPOA was increased by sexual stimulation. It is reported that dopamine D1/D2 receptors in the hypothalamus participate in erection whereas only D2 receptors participate in ejaculation. Pathology studies have shown that the hypothalamus is affected in PD.112 Recently, polymorphism in the dopamine D4 receptor gene is shown to contribute to individual differences in human sexual behavior.113 Prolactinergic neurons are thought to be inhibitory in sexual function. Serum prolactin levels increase after orgasm in healthy men. Prolactin-producing pituitary tumors often cause gynecomastia and erectile dysfunction in male patients. Hyperprolactinemia occurs after the use of sulpiride, metoclopramide, and chlorpromazine (all dopamine receptor antagonists). Therefore, dopaminergic neurons seem to facilitate oxytocinergic neurons whereas they inhibit prolactinergic neurons. Some de novo PD patients have hyper prolactinemia,114 which may contribute to erectile dysfunction in those patients.

SEXUAL DYSFUNCTION IN PD

Sexual Symptoms

The reported prevalence of sexual symptoms in patients with PD ranges from 37 to 65%.115–121 Only few previous studies have looked at sexual symptoms in PD and control subjects. Jacobs et al.118 studied 121 men with PD (mean age 45 years) and 126 age and sex matched community derived controls. Patients were more dissatisfied with their present sexual functioning and relationship, whereas no differences were found for the frequency of sexual intercourse itself. Erection and ejaculation were not inquired. Sakakibara et al.5 analyzed sexual function of 84 PD patients (46 men, 38 women, age 35–70-years old) and 356 healthy control subjects (258 men, 98 women, age 30–70-years old).5 As compared with the control group, the frequency of dysfunction in PD patients was significantly higher for decrease of libido (84% men, 83% women), decrease of sexual intercourse (55% men, 88% women), decrease of orgasm (87% men), and decrease of erection (79%) and ejaculation (79%) in men. Therefore, sexual dysfunction is significant in PD. The majority of patients had onset of the sexual dysfunction after the appearance of the motor disorder. This is in contrast to patients with MSA, who often have sexual dysfunction before the onset of motor disorder.

Comparing the results between four age subgroups (subjects in their 30's, 40's, 50's, and 60's) in the control group, the frequencies of sexual intercourse and of orgasm were significantly lower in older individuals.5 In the PD group, only the frequency of orgasm was lower in older men (P < 0.05). Comparing the results between both sexes in the control group, decrease of libido and orgasm were more common in women (P < 0.01). In the PD group, there was no significant difference in sexual function items. Bronner et al.121 reported that use of medications (selective serotonin reuptake inhibitors used for comorbid depression), and advanced PD stage contributed to the development of ED.

Rigiscan

In healthy men, sexual intercourse is thought to be carried out by integrating affective, motor, sensory, autonomic, and other factors. In male patients with PD, depression, motor disorder, and pain inevitably lead to sexual dysfunction. In contrast, it has been difficult to determine to what extent autonomic factors contribute to the sexual dysfunction in PD. However, erectile dysfunction often precedes motor disorder in MSA, and abnormal NPT is not uncommon in PD. These findings strongly suggest that the disorder does in fact contribute to sexual dysfunction in PD. Rigiscan is an objective measure for erectile dysfunction, which allows both tumescence and rigidity measurement; and is suitable for assessing NPT.

Only few data have been available concerning the relationship between NPT and dopamine. However, in experimental animals, administration of levodopa elicited erection and yawning together. Animals with experimental parkinsonism showed fewer REM stages during sleep than control animals did.

TREATMENT OF ERECTILE DYSFUNCTION IN PD

Dopaminergic Drugs

It is possible that levodopa and other antiparkinson medication may affect sexual function in PD. However, it is not entirely clear to what extent levodopa ameliorates sexual dysfunction in PD. In contrast, subcutaneous apomorphine injection is used to ameliorate fluctuating symptoms in PD. It has also been used to treat erectile dysfunction in the general population122 and in patients with PD,123 although the dose is different (general population, initial 2 mg and up to 3 mg,122 PD, 4 mg124). Apomorphine is thought to stimulate dopamine D2 receptors, and activate oxytocinergic neurons in the PVN. Nausea is a common side effect of this drug. Cabergoline125 and pergolide126 are also reported to improve sexual dysfunction in PD. In contrast, pathological hypersexuality may occur together with127 or without delirium,128 which is attributed to the dopamine dysregulation syndrome in this disorder. DBS in the STN has produced either improved sexual well-being129 or transient mania with hypersexuality130 in patients with PD.

Phosphodiesterase-5 Inhibitors

When dopaminergic drugs did not help, phosphodiesterase-5 inhibitors, e.g., sildenafil, vardenafil, etc., become the first line treatment in PD.131, 132 These drugs inhibit nitric oxide degradation and facilitate smooth muscle relaxation in the cavernous tissue. When treating PD patients with postural hypotension, these drugs should be prescribed with extreme caution.132

CONCLUSIONS

This article reviewed the current concepts of genitourinary dysfunction in PD. Central nervous system pathology is clearly associated with bladder (urinary urgency/frequency) and sexual dysfunction (decrease in libido and erection) in PD. Anticholinergic agents are generally used to treat bladder dysfunction, while phosphodiesterase inhibitors are used to treat erection dysfunction. These treatments are beneficial in maximizing patients' quality of life.

Author Roles: Ryuji Sakakibara made all parts of this article, with kind supervision by doctors Tomoyuki Uchiyama, Tomonori Yamanishi, and Masahiko Kishi.

Ancillary