A Review by Antonio Mesa, DO
Reprinted from Miami Medicine
Pelvic outlet syndrome (POS) is a group of disorders that occur when blood vessels or nerves in the pelvic outlet  are compressed, causing pelvic pain and dysesthesias. It is analogous to the more commonly known thoracic outlet syndrome.  As with thoracic outlet syndrome, there are three main types of POS: neurogenic, venous, and arterial, depending on which structure is affected. Unlike thoracic outlet syndrome, however, neurogenic POS is less common. Although in both the cause is compression of a neurological structure, in POS the compression is of the sacral plexus or pudendal nerve branches. It is not a radiculopathy or a cauda equina syndrome, which are caused by more proximal lesions and have very different prognoses and treatments.
Venous Outlet Syndrome
Venous POS occurs when a pelvic vein is compressed or there is insufficiency of the valves of the pelvic veins [3,4,5] leading to chronic pelvic pain and even venous thrombosis. A common presentation of this condition is pelvic congestion syndrome, which can be difficult to diagnose, particularly in women. Many men with this condition present with painful varicoceles, which may facilitate diagnosis. Transcatheter venography is the definitive diagnostic study for pelvic congestion syndrome, but this is only performed if there is a high index of suspicion and noninvasive imaging studies such as ultrasound, CT scan, or MRI have been inconclusive.
Depending on the underlying cause, there are a variety of medical, surgical, and endovascular treatments available, including transcatheter pelvic vein embolization.
Arterial Pelvic Outlet Syndrome
Arterial POS occurs when a pelvic artery is compressed or stenotic, resulting in reduced blood flow to the pelvis or even the lower extremities. Unlike thoracic outlet syndrome, atherosclerosis plays a prominent role in arterial POS. [6,7,8] Women with endometriosis are at higher risk of developing this condition,  especially if they are on long-term oral contraceptive therapies. As with venous POS, depending on the underlying cause, there are a variety of medical, surgical, and endovascular treatments available.
Neurogenic Pelvic Outlet Syndrome
Neurogenic POS has a variable presentation, ranging from focal dysesthetic pain such as in localized provoked vestibulodynia (LPV) or orchialgia, to somatic pain such as with coccydynia or rectodynia, to more generalized pain involving the entire pelvic floor. The causes of neurogenic POS vary depending on the type, but some common factors include structural abnormalities of the pelvic bones, trauma or injury to the pelvis or hips, adhesion formation secondary to endometriosis or following surgery, [11,12] repetitive motions of the hips or lower back, poor posture, obesity, pregnancy, or tumors. Given the variety of causes, the prognosis of POS depends on the severity of the symptoms, the underlying cause of POS, and the response to treatment. Some people may experience mild or intermittent symptoms that do not interfere with their daily activities, while others may have chronic or disabling pain and weakness that limit their function and quality of life.
Vascular Pelvic Outlet Syndrome
As with vascular POS, diagnosis of neurogenic POS requires imaging studies such as ultrasound, CT, MRI, and/or x-ray. At times, neurophysiological studies such as nerve conduction studies, electromyography, or evoked potentials may be needed to confirm the diagnosis or rule out other conditions. A thorough neurological examination, with a focus on the pelvic region, is the basis for both diagnosing the condition and selecting the appropriate diagnostic testing.
Treatment options for neurogenic POS include conservative measures such as lifestyle modifications and physical therapy and/or pelvic floor therapy, with or without medications. Lifestyle modifications may include avoiding activities that trigger or worsen symptoms, maintaining a healthy weight, quitting smoking, or using ergonomic devices at work or home. The goal of therapy is to optimize pelvic muscle tone, improve range of motion of the back and hips, achieve proper balance and gait, as well as reduce inflammation and pain. Often, therapists will also help determine which exercises or activities might trigger symptoms and can develop plans to allow patients to remain physically active without causing additional problems.
If these do not provide adequate symptomatic relief, medications, blocks or injections can be added to the treatment plan. Medications may be oral or topical, such as intravaginal or rectal suppositories. Depending on the cause of the pain, medication options include non-steroidal anti-inflammatories, muscle relaxants, antiepileptics (particularly, but not limited to the gabapentinoids), tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors (SNRI’s), and even ketamine. 
Interventional treatments include trigger point injections with or without acoustic wave therapy to provide adequate release of entrapment neuropathy, [14,15] nerve or plexus blocks, and neurostimulation. In some cases, surgery may be needed to relieve the compression of the nerves or blood vessels. Surgery is usually reserved for patients who have severe symptoms that do not respond to conservative measures or who have complications such as thrombosis or nerve damage. The success rate of surgery for POS varies widely depending on the type of POS and the surgical technique used.
Pelvic outlet syndrome is a complex and challenging condition that requires an awareness of the various types of POS that can occur and how they present. A thorough history and physical examination are essential to identify the type and cause of POS, as well as to rule out other conditions that may mimic its symptoms. A variety of imaging and neurophysiological studies are used to confirm the diagnosis and assess the extent of the condition. Treatment should be individualized based on the patient’s symptoms, preferences, and goals. Early diagnosis and treatment are important to prevent permanent damage to the nerves or blood vessels and to improve the patient’s function and quality of life.
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