Medway NHS Trust, Kent, UK
Chronic Pelvic Pain is classified as Chronic Primary Pelvic Pain with no obvious diagnosis & Chronic Secondary Pelvic Pain with well recognized pathology (ICD 11)
Pudendal neuralgia is classified under chronic secondary pelvic pain
The main indication for Pudendal nerve block is pudendal neuralgia. The diagnosis of pudendal neuralgia is primarily clinical in the absence of biochemical, imaging and electro diagnostic criteria
Aetiology of pudendal neuralgia include Pudendal nerve entrapment (most frequent)
The other causes described include Post-surgical neuropathy, Stretch neuropathy, Peripheral polyneuropathy & Postradiotherapy neuropathy
Pathogenesis of pudendal neuralgia involves compression of the pudendal nerve at the level of the sacrospinal/Sacro tuberous ligaments, possibly accounting for 42% of cases & within Alcock’s canal (medial to the obturator internus muscle, within the fascia of the muscle), possibly accounting for 26% of cases. Cycling, and to a lesser extent horse riding have been reported as the most common causes of PN with repeated impacts generate high perineal pressure
Essential Criteria (Nantes) for the diagnosis of Pudendal nerve entrapment syndrome or Compressive pudendal neuralgia, 5 criteria and they all must all be present. Pain in the distribution of the pudendal nerve from the anus to the penis or clitoris, Pain predominantly experienced while sitting, Pain does not wake the patient at night, No objective sensory impairment & Pain relieved by diagnostic pudendal nerve block
Anatomy Pudendal nerve is the main nerve of the perineum, the pelvic floor muscles and the external genital organs. The nerve arises from the primary ventral rami of s2, s3 & s4 sacral plexus. It consists of sensory, motor & autonomic fibres. somatic fibres. The nerve divides into three, inferior rectal nerve, perineal nerve & dorsal nerve of the penis/clitoris.
The perineal nerve emerges from the pelvis through the greater sciatic foramen in a caudal course and it re-enters the pelvis through the lesser sciatic foramen, between the sacrospinous & sacrotuberous ligaments. It is in the inter ligamentous part of its course where compressive nerve pathologies may be often found & also the course through Alcock’s canal has been described as one of the most susceptible areas for nerve entrapment. The internal pudendal blood vessels are also found along the course of the pudendal nerve & this is extremely helpful in identifying the nerve with colour doppler. At the level of the ischial spine the internal pudendal artery is found lateral to the nerves in majority of cases.
Technique Pudendal nerve blocks may be performed by two approaches: anterior-perineal or posterior-trans gluteal. The perineal approach is used for distal entrapments or for analgesia in gynaecological surgery. The posterior approach has been used for proximal nerve entrapment syndrome. Approaches for pudendal nerve blocks include Anatomical landmarks, Neurostimulation, Fluoroscopy, Tomography & Ultrasound guidance.
Ultrasound guided trans gluteal posterior approach: the protocol consists of informed consent, monitoring, asepsis, lateral decubitus or in prone position. Once the settings are adjusted and the image optimised, a curved transducer is placed initially in a transverse plane across the proximal gluteal area and then moved caudally.
The following structures need to be recognised, from proximal to distal
Transducer’s position 1: posterosuperior iliac spine & Ileum,
Transducer position 2: greater sciatic foramen, Ischium, piriformis muscle, sacral plexus, superior gluteal artery and deep down, bowel movement may be recognised.
Transducer’s position 3: the ischial spine must be recognised as a straight hyperechoic line, the sacrospinous ligament as a continuation of the hyperechoic line of the ischial spine & sacrotuberous ligament, superficial and parallel to the sacrospinous ligament, deep to the gluteus maximus. Laterally the superior gemellus muscle, the sciatic nerve and the inferior gluteal artery. With the transducer in this position, on the medial edge of the image, identify internal pudendal artery and the pudendal nerve, found in the inter fascial plane medial to the pudendal vessels in most cases. The transducer should be moved until the pudendal neurovascular bundle appears in the centre of the ultrasound image
Transducer in position 4: the transducer is moved more caudal to the ischial spine to enter the lesser sciatic foramen, at the level of obturator internus & ischial tuberosity for entry point to Alcock’s canal. Pudendal neuro vascular bundle is identified with colour doppler
In transducer positions 3 & 4 the puncture will be made in-plane from medial to lateral using a short-bevel 80–100mmx 22G needle, Sensory or motor neurostimulation may be used in the proximity of the nerve in order to determine the presence of paraesthesia/motor contraction in the territory of the pudendal nerve. Local anaesthetic & corticosteroid is used in a volume ranging from 4 to 6 mis with negative aspiration
Complications Although the reported rate of complications is low, they may still occur, and they include pudendal nerve injury, vascular injury, intravascular injection, muscle weakness in the sciatic nerve territory, muscle pain, urinary or faecal incontinence and numbness in the pudendal nerve area.
Conclusions A detailed knowledge of the pudendal nerve anatomy and its variants is essential for the use of ultrasound guided pudendal nerve block. Ultrasound with use of colour deep has shown to be of significant help in performing this procedure safely
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