Prostatitis is an infection or inflammation of the prostate gland that presents as several syndromes with varying clinical features. The term prostatitis is defined as microscopic inflammation of the tissue of the prostate gland and is a diagnosis that spans a broad range of clinical conditions.
The National Institutes of Health (NIH) has recognized and defined a classification system for prostatitis in 1999.  The 4 syndromes of prostatitis are as follows:
III - Chronic prostatitis and chronic pelvic pain syndrome (CPPS; further classified as inflammatory or noninflammatory)
IV - Asymptomatic inflammatory prostatitis
Acute prostatitis and chronic bacterial prostatitis are defined by documented bacterial infections of the prostate and are treated with antibiotic therapy and supportive care (see Treatment).
CPPS is characterized primarily by urological pain complaints in the absence of urinary tract infection. This syndrome excludes the presence of active urethritis, urogenital cancer, urinary tract disease, significant urethral stricture, or neurological disease affecting the bladder. It is subdivided into inflammatory and noninflammatory subtypes. Inflammatory CPPS is defined by the presence of white blood cells in the semen, expressed prostatic secretions, or voided bladder urine after prostatic massage (see Workup). Noninflammatory CPPS is defined by the absence of white blood cells. 
Asymptomatic inflammatory prostatitis is characterized by the incidental discovery of prostatic inflammation without genitourinary complaints.  This condition is diagnosed during a workup for infertility or elevated prostate-specific antigen (PSA) level. This disease entity can produce elevated white blood cells in the ejaculate (leukocytospermia) and can cause male infertility but is usually otherwise left untreated. See the following for more information:
In bacterial prostatitis, sexual transmission of bacteria is common, but hematogenous, lymphatic, and contiguous spread of infection from surrounding organs must also be considered. Although various routes have been postulated, none has been firmly substantiated.
A history of sexually transmitted diseases is associated with an increased risk for prostatitis symptoms.
The presence of acute inflammatory cells in the glandular epithelium and lumens of the prostate, with chronic inflammatory cells in the periglandular tissue, characterizes prostatitis (see the image below). However, the presence and quantity of inflammatory cells in the urine or prostatic secretions does not correlate with the severity of the clinical symptoms.
Chronic pelvic pain syndrome is diagnosed based on pain in the setting of negative cultures of urine and prostatic secretions. Neuromuscular dysfunction or congenital reflux of urine into the ejaculatory and prostatic ducts may be a precipitating factor.
Viral and granulomatous prostatitis may be associated with HIV infection and is another cause of culture-negative disease. A common viral pathogen of prostatitis in HIV-infected patients is cytomegalovirus (CMV).  Mycobacteria, such as Mycobacterium tuberculosis, and fungi, such as Candida albicans, have also been associated with culture-negative disease in this population. 
Acute bacterial prostatitis may be caused by ascending infection through the urethra, refluxing urine into prostate ducts, or direct extension or lymphatic spread from the rectum. Approximately 80% of the pathogens are gram-negative organisms (eg, Escherichia coli, Enterobacter, Serratia, Pseudomonas, Enterococcus, and Proteus species). [6, 7] Mixed bacterial infections are uncommon. One case report of prostatitis caused by methicillin-resistant Staphylococcus aureus was documented in a diabetic patient.
Consider Neisseria gonorrhoeae and Chlamydia trachomatis infection in any male younger than 35 years presenting with urinary tract symptoms.
Nursing home patients with indwelling urethral catheters may also be at increased risk of acute bacterial prostatitis. Sclerotherapy for rectal prolapse may also increase risk. 
Chronic bacterial prostatitis may be due to the following:
A primary voiding dysfunction problem, either structural or functional
E coli is responsible for 75-80% of chronic bacterial prostatitis cases. Enterococci and gram-negative aerobes such as Pseudomonas are usually isolated in the remainder of cases.
C trachomatis, Ureaplasma species, Trichomonas vaginalis
Uncommon organisms, such as M tuberculosis and Coccidioides, Histoplasma, and Candida species , must also be considered. Tuberculous prostatitis may be found in patients with renal tuberculosis
Human immunodeficiency virus
Inflammatory conditions (eg, sarcoidosis)
The etiology of chronic prostatitis and chronic pelvic pain syndrome is poorly understood but may involve an infectious or inflammatory initiator that results in neurologic injury and eventually in pelvic floor dysfunction in the form of increased pelvic tone.  About 5-8% of men with this syndrome eventually have a bacterial pathogen isolated from urine or prostatic fluid.
Causes of chronic prostatitis and chronic pelvic pain syndrome may include the following:
Functional or structural bladder pathology, such as primary vesical neck obstruction, pseudodyssynergia (failure of the external sphincter to relax during voiding), impaired detrusor contractility, or acontractile detrusor muscle
Ejaculatory duct obstruction
Increased pelvic side wall tension
Nonspecific prostatic inflammation
Causes of asymptomatic inflammatory prostatitis are similar to those of chronic inflammatory prostatitis without symptoms.
United States statistics
Prostatitis is one of the most common diseases seen in urology practices in the United States, accounting for nearly 2 million outpatient visits per year. [9, 10, 11] The diagnosis is made in approximately 25% of male patients presenting with genitourinary symptoms. Autopsy studies have revealed a histologic prevalence of prostatitis of 64-86%.
Approximately 8.2% of men have prostatitis at some point in their lives.  Among the 4 categories of prostatitis, the most common is chronic prostatitis/chronic pelvic pain syndrome, accounting for 90-95% of prostatitis cases. Acute bacterial prostatitis and chronic bacterial prostatitis each make up another 2-5% of cases.
The incidence of mycobacterial prostatitis, concomitant with disseminated disease, is increasing in underdeveloped countries. Areas with widespread sexually transmitted disease (STD) rates and prostitution have a higher incidence of acute bacterial prostatitis.
In patients younger than 35 years, the most common variant of the syndrome is acute bacterial prostatitis. HIV-related disease is also predominantly seen in younger patients.
Among older patients, nonbacterial prostatitis (National Institutes of Health [NIH] types II and IV) are the most common. Of importance, rare causes of prostatitis should be sought during evaluation. According to case reports of Wegener granulomatosis in the fourth and fifth decades of life, prostatitis can be a presenting feature of Wegener granulomatosis and a clinical manifestation of relapse. [12, 13, 14] Fungal infection with C albicans and Coccidioides immitis and mycobacterial infection with M tuberculosis have also been reported.
The prognosis in patients with the first occurrence of acute bacterial prostatitis is good, with aggressive antibiotic therapy and good patient compliance. In patients with recurrent chronic prostatitis who may present with acute exacerbations, causative underlying factors affect outcome.
Prostatitis may lead to urosepsis with significant associated mortality in patients with diabetes mellitus, patients on dialysis for chronic renal failure, patients who are immunocompromised, and postsurgical patients who have had urethral instrumentation.
Chronic prostatitis and asymptomatic inflammatory prostatitis have not been definitively linked to the development of prostate cancer.
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