Epidemiology

The incidence of neuropathic pelvic pain from peripheral nerves, other than the pudendal nerve is unknown. In a systematic review of epidemiological studies, it was estimated that 6.9% to 10% of patients with chronic pain have neuropathic pain symptoms [1]. Peripheral neuropathies may result after major gynecological surgery in 1.9% of patients. Affected nerves included the obturator (39%), iliohypogastric/ilioinguinal (21%), genitofemoral (17%), femoral (7.5%), and lumbosacral plexus (0.2%) [2].

Etiology

Neuropathic pain often arises as a direct consequence of a lesion or disease that affects the sensory component of the nervous system [3]. Peripheral nerves may be injured as a result of direct trauma, stretch, crush, fibrosis with entrapment, suture ligation, and repetitive low-impact forces. Patients may also develop peripheral neuralgias from peripheral sensitization from repetitive painful stimuli or from central sensitization. In addition, a referral pattern from visceral pain via visceral somatic convergence can lead to pain along certain dermatomes [4] (Table 16.1).

Therefore, it is important to consider visceral etiologies when diagnosing a patient with peripheral neuralgias. Other causes of specific peripheral nerve injuries are organized by nerve later in this chapter.

History and Physical Examination

Patients with peripheral neuropathic pain may associate the start of their pain with a specific event such as blunt trauma, a fall, surgery, obstetrical procedure, or radiation. Repetitive activities such as sitting, exercising, and sports may also cause nerve injury. Pain is experienced either as dysesthetic pain or nerve trunk pain. Dysesthetic pain is usually described as a constant or intermittent searing, burning, or icy-cold pain. Skin and subcutaneous structures along the distribution of the nerve are affected. On the other hand, nerve trunk pain is experienced as a constant or paroxysmal deep-seated, sharp, knife-like pain. It is well localized at a specific point along the nerve, usually at the site of injury or impingement. Nerve trunk pain improves with rest or optimal position and is aggravated by movement. Symptoms may be localized in the lower abdomen, thighs, genitoanal region, and buttocks. Asking the patient to demonstrate or verbalize the location of the pain is the first step to identifying the affected nerve. In patients with pain in the genitoanal region, peripheral nerves other than the pudendal nerve should be considered. The ilioinguinal, genitofemoral, obturator, and posterior femoral cutaneous nerves may supply innervation of the skin of the vulva, perineum, and anus (Figure 16.1).

Figure 16.1 Innervation of the perineal area.

The goal of clinical examination is to discriminate between visceral and somatic pain and to localize the pain anatomically to a specific dermatome. Pain can be localized by testing the patient’s response to light touch with a cotton swab. Sites where pain is produced either on the abdomen or perineal region should be documented and can be marked on the skin. A trigger point may be identified in the muscles within or surrounding the affected area. Indeed, the presence of allodynia and trigger points greatly increases the probability of neuropathic pain [5]. Special attention should be placed on scars and prior surgical incisions, as these may be sites of nerve injury. Please refer to Chapter 3 on physical examination.

Diagnostic Procedures

To diagnose peripheral neuralgia as a cause of chronic pelvic pain, alleviation of pain must be demonstrated by a nerve block. There are two objectives when performing a nerve block: confirm the diagnosis and manage pain. It is important that the patient understands that the primary objective is diagnostic and that the procedure may not yield long-term pain relief. To accurately perform a nerve block, image guidance should be used to localize the nerve and to demonstrate site-specific infiltration of the analgesia.

In this chapter, we focus on the technique of performing ultrasound-guided nerve blocks. Some descriptions of how to perform selective nerve blocks are based on the authors’ experiences and have not been published in peer-reviewed journals. References are given when available.

For a nerve block to provide diagnostic information, pain must be present at the time of the procedure. Evaluation of the analgesic effect should take place immediately following the block to allow for accurate interpretation of the results. Prior to performing a nerve block, informed consent should be obtained. Contraindications include coagulopathy, allergies to analgesic agent, and infection at the site of the injection. Patients with severe allodynia and/or central sensitization may require general anesthesia or conscious sedation, but this may complicate the interpretation of the results. Pain may be measured by a visual analog scale (VAS) or numerical rating before the injection and used to compare post procedure. If patients are not having any pain, the injection will not be useful for diagnosis, and patients should return when they are having pain. If pain results from a specific activity or position, patients may induce pain in order to proceed with the block [6, 7].

The neurovascular bundle of peripheral nerves can be effectively visualized via ultrasound. Clinicians must understand the physics of ultrasounds in order to select the most effective ultrasound probe to visualize specific anatomical locations. For instance, to visualize the abdominal wall and the ilioinguinal nerve, a 14 MHz linear probe may provide appropriate penetration and resolution of superficial structures in a thin patient. However, in obese patients, a 9 MHz or lower probe may be required for deep penetration to visualize the same structures at a cost of resolution. When performing ultrasound-guided blocks, it is best to work with an ultrasonographer who is skilled in musculoskeletal imaging.

After the probe is selected, the procedure begins by visualizing the neurovascular bundle in the expected anatomical location. Color flow can be used to help visualize the artery and the vein. Next, a needle is advanced under ultrasound guidance and is placed adjacent to the neurovascular bundle within the fascial plane. It is important not to place the needle within the vessel, as this can lead to vascular or neurological injury if infiltration is performed within the nerve. The needle needed to perform the block should be long enough to reach the target and may be “etched” to help visualization on ultrasound. Twenty-two-gauge needles are useful because they are small, yet still provide the haptic feedback that aids in identification of fascial planes and muscles. It is best to use an in-plane technique when advancing the needle under ultrasound guidance so that the needle can be completely visualized throughout its path. The clinician must be aware of surrounding anatomy, particularly vascular structures, through which the needle may be traveling. Once the needle is at its target, a small volume of local anesthetic (1–3 mL) should be used to minimize spread of analgesia to surrounding structures and to specifically select for the nerve. Separation of the fascial plane should be visualized; infiltration of the anesthetic into the muscle should be avoided. Lidocaine and bupivacaine are popular choices, but any local anesthetic can be used. Clinicians must be familiar with the time of onset and duration of the anesthetic used so that patients are evaluated in the appropriate timeframe post procedure.

After the injection, a technical evaluation should be performed while the anesthetic is in effect. Analgesic effect should be demonstrated in the distribution of the target nerve. A cotton swab or alcohol swab can be used to demonstrate decreased sensation along the specific dermatome. If the analgesic effect is localized beyond the dermatome of the target nerve, then the block is not diagnostic and should be repeated. Absence of analgesia may be secondary to technical difficulty and the area surrounding the nerve may have not been infiltrated correctly.

If the block was technically successful, pain level is next evaluated. Absence of pain or reduction in pain by more than 50% is diagnostic. Patients with pain provoked by specific activity should be asked to perform the task that triggers their pain, such as sitting, bending, and stretching. In patients for whom the block improved but did not eliminate pain, clinicians should evaluate for other sources of pain such as a visceral component to pain or involvement of additional peripheral nerves. If patients have no reduction of pain but are experiencing numbness in the targeted dermatome, it is unlikely that the target nerve is causing their pain. Repeat examination for other visceral or somatic sources of pain post procedure may help in the diagnostic process.

There are important limitations to ultrasound-guided nerve blocks. Patients with visceral-somatic convergence may respond well to a nerve block even though their pain is secondary to underlying pathology. Local infiltration may relieve pain from muscular trigger points and not from neuralgias. Patients who receive sedation or general anesthesia may be responding to systemic medications and would have both numbness in the distribution of the targeted nerve and pain relief. Lastly, patients may experience a placebo effect or have a strong psychological need for the procedure to work. Complications are rare and include nerve or vascular injury, infection, bleeding, pain at the site of injection, and exacerbation of chronic pain.

Nerve Blocks as Treatment

The mechanism of how nerve blocks may provide long-term relief is poorly understood [8]. Nerve blocks may provide suppression of ectopic discharge in the neural membranes and have an antiinflammatory reaction. It may interrupt the “vicious cycle” of self-sustaining pain and central sensitization. No quality randomized studies exist for the evaluation of nerve blocks in long-term relief, but on average, about 30%–60% of patients can be helped with repetitive nerve blocks [8]. Addition of corticosteroids may have added benefits, but this has not been clearly demonstrated in prospective studies. Addition of dexamethasone or other corticosteroids can prolong the analgesic effect of regional anesthesia; however, it is unknown whether this positively impacts long-term outcomes. In the experience of these authors, a series of nerve blocks can reduce or eliminate pain in select patients. Performing a nerve block three to four times a year is a low-risk intervention that can effectively reduce pain and the need for medications.