Phrenic Nerve Block and Ablation

The relation of the phrenic nerve to hiccup was recognized at least as early as 1833 by Shortt who recommended blistering the surface of the neck over the origin and course of the phrenic nerves for intractable hiccups. A number of reports of freezing, transecting, crushing, and injecting lidocaine into the phrenic nerve to relieve intractable hiccups were published in the 1920s and 1930s.

Each phrenic nerve controls the movement of the hemidiaphragm and provides the sensory innervation to the middle of the diaphragm, some parts of the pleura and pericardium. The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 3rd and 5^th cervical nerves (C3-C5) in humans. The three nerves join at the upper lateral portion of the anterior scalene, forming the phrenic nerve. The nerve descends obliquely across the anterior scalene, through the gap between sternocleidomastoid and omohyoid muscles, and deep into the thorax. The right phrenic nerve follows the course of the vena cava to provide motor innervation to the right hemidiaphragm. The left phrenic nerve descends to provide motor innervation to the left hemidiaphragm in a course parallel to that of the vagus nerve.

The phrenic nerve is sonographically visualized as a solitary round hypoechoic structure of about 0.76 mm. However, visualization of the phrenic nerve can be considered as time-consuming even in expert hands. The transverse cervical artery that crosses superficial to the phrenic nerve in 95% cases can help the operator in identifying the phrenic nerve.

The hiccup center is located in the spinal cord between the third and fifth cervical segments. The afferent limb of the hiccup reflex consists of the vagus and phrenic nerve with contributions of the sympathetic chain arising from the 6th to the 12th thoracic segments, whereas the efferent limb consists mainly from the phrenic nerve. The proposed mechanism attributable for persistent relief from hiccups after resolution of the phrenic nerve block is a neural pathway interruption. The phrenic nerve is part of the afferent pathway and the main efferent part of the hiccup reflex.

A phrenic nerve block is useful in the diagnosis and treatment of intractable hiccups. It is also occasionally useful as both a diagnostic and therapeutic manoeuvre to determine whether pain from subdiaphragmatic processes, including abscess and malignancy, is being mediated via the phrenic nerve. A phrenic nerve block with local anesthetic is also used in a prognostic manner before ablation of the phrenic nerve for treatment of intractable hiccups.

Phrenic nerve block guided by ultrasound is known to be safer than the one solely depending on surface anatomy. The reason is that operators can identify the anatomical structures of nerves, muscles and blood vessels in real time while approaching a needle to target nerves. The phrenic nerve should appear as a 2- to 3-mm hypoechoic oval monofascicular structure with a hyperechoic perineurium lying on top of the anterior scalene muscle as it exits beneath the posterior margin of the sternocleidomastoid muscle.

Local anaesthetic and steroid can be injected under real-time ultrasound guidance targeting the phrenic nerve on top of the anterior scalene muscle as it exits beneath the posterior margin of the sternocleidomastoid muscle.

Pulsed radiofrequency ablation can be applied to the phrenic nerve for patients who get a positive response to local anaesthetic block. Pulsed radiofrequency can provide sustained relief in patients with persistent or intractable hiccups.

Injections are generally avoided in patients with systemic infection or skin infection over puncture site, bleeding disorders or coagulopathy, allergy to local anaesthetics or any of the medications to be administered.

A phrenic nerve block is relatively contraindicated in patients with respiratory disease and unilateral diaphragmatic palsy.

The proximity to the external jugular vein and other large vessels suggests the potential for inadvertent intravascular injection or local anesthetic toxicity from intravascular absorption.

The proximity of the phrenic nerve to the central neuraxial structures and the spinal accessory nerve also can result in side effects and complications.

The phrenic nerve block involves a partial, unilateral diaphragmatic paralysis with negligible impairment of the vital functional capacity in patients with no pulmonary pathologies; however, patients with an advanced restrictive pulmonary disease or deterioration of the contra lateral mobility of the diaphragm can experience significant respiratory symptoms.

Blockade of the recurrent laryngeal nerve with its attendant vocal cord paralysis, combined with paralysis of the diaphragm, may make the clearing of pulmonary and upper airway secretions difficult.