The interscalene block numbs the shoulder by reaching the brachial plexus high in the neck, where its nerve roots run between two scalene muscles. Ultrasound shows those roots as a stack of dark circles, the landmark the whole block is built on. The probe finds the stack, the needle is laid beside it, and the local anesthetic spreads around the roots that carry the shoulder’s nerves. That same spread also reaches the nerve to the diaphragm, which runs close by in the neck. That nerve governs which patients can safely be given the block.
The block for the shoulder
The interscalene block is the block for the shoulder. It reaches the brachial plexus at its highest point, the roots and upper trunks, where the nerves to the shoulder and the upper arm gather before they branch. A shoulder arthroscopy, a rotator cuff repair, a shoulder replacement, a break of the upper humerus all sit squarely in its territory.
The block sits high, at the roots. The lowest roots often escape it, the ones that become the ulnar nerve and supply the little finger and the inner forearm. An interscalene block leaves the hand and the forearm half-awake, so it is a weak choice for surgery there. It suits the shoulder and the arm above the elbow. The hand is served by a block placed lower in the arm.
The shoulder draws its nerves from the upper roots the block reaches, the suprascapular and the axillary nerves among them, which carry both the deep sensation of the joint and the feeling of the skin over it. A single block at the roots covers the lot, the reason one well-placed injection can carry a shoulder operation. The cape of skin over the top of the shoulder comes from the neck above the plexus, a small area a root block can leave half-covered.
Positioning and the first look
The patient is set up before the probe goes down. The head of the bed is raised a little and the head turned away from the side to be blocked, which opens the neck and brings the scalenes forward. A pillow under the shoulders helps in a patient with a short, thick neck. The arm rests at the side, and the skin is cleaned and draped for a sterile field, since the needle breaks the skin. A high-frequency linear probe gives the fine detail the shallow plexus needs, the same probe used for the rest of the neck.
The first pass with the probe is a survey of the ground. The carotid is found, the scalenes and the stack identified, the depth of the roots read off the screen, and the path the needle will take is pictured before it is opened. A sweep with color Doppler lights the vessels in the field, the small branches that cross the groove along with the carotid and the jugular. A minute spent reading the anatomy first saves a longer search with the needle in hand.
The stoplight between the scalenes
Where the interscalene block sits. On the left, the entry point above the collarbone is marked with an X on the side of the neck. On the right, the brachial plexus is drawn with the interscalene target circled, between the scalene muscles. The surface mark gives the rough spot; ultrasound shows the roots themselves. Image: David Shankbone and Gray’s Anatomy, CC BY-SA 3.0.
On the screen the plexus has a look that names itself. Laid across the neck, the probe shows two muscles side by side, the anterior and the middle scalene, and between them a short stack of dark, round shapes. These are the nerve roots, three or so, piled one on the next like the lamps of a traffic light. The picture is steady enough that the stack is known at a glance, the upper roots of the plexus, counted at the bedside as C5, C6, and C7. The lowest of the three is often the C6 root split in two. The drug is laid around the whole stack, and the needle is kept out of any apparent gap between the circles.
The roots sit shallow, a centimeter or two under the skin, behind the sternocleidomastoid muscle that crosses the front of the neck. The carotid artery and the jugular vein lie to the inner side, the lung deeper and further down. Reading the two scalenes and the stack between them is the whole of finding the plexus here.
A nerve root at this level is darker than the fascicular nerves further out in the limb. It has not yet gathered the bright tissue that fills a peripheral nerve, so it reads as a plain dark circle with a thin rim. The stack of these dark circles, set between the two scalenes, is the mark the block aims for.
The roots dim and brighten as the probe is tilted, the same angle effect that touches any nerve, milder here than on a tendon. A small rock of the probe sharpens the dark circles against the muscle around them. The clearest picture of the stack comes from squaring the beam to the roots and centering them on the screen before the needle is brought in.
Finding the plexus: the scan down the neck
There are two ways to arrive at the stack, and both start from a landmark that stands out. The first lays the probe across the neck at the level of the cricoid cartilage, finds the pulsing carotid artery, and slides outward across the sternocleidomastoid until the two scalenes and the roots between them come into view. The carotid points the way to the scalenes, and the stack of roots sits in the groove between them.
The second starts lower, at the collarbone, where the plexus bunches above the subclavian artery in a tight cluster, and traces it upward into the neck. As the probe climbs, the cluster sorts itself into the separate roots of the interscalene groove. Tracing the plexus from the clear picture at the base of the neck up to the roots is the surer route when the groove is hard to read on its own.
Either way, the roots are confirmed by following them. The probe slides up and down the neck, and the dark circles hold their place between the scalenes, running as cords the length of the scan. A small vessel can sit among them, and it is cleared with a touch of color Doppler before any needle moves. The dorsal scapular nerve and the long thoracic nerve pass through the middle scalene close by, a reason to keep the needle clear of the muscle’s depth.
The level in the groove is set with care, because it decides the block. High in the groove the block covers the shoulder. Nearer the collarbone it reaches further down the arm. The spot is chosen for the surgery the block has to cover.
Bringing the needle in
The needle comes in from the side, in plane with the probe, so its whole length shows on the screen as it travels to the plexus. It is aimed at the groove between the scalenes, reaching the edge of the stack of roots. The tip stops there, clear of the roots themselves. It is watched the entire way, the more closely here for the vessels and the lung that lie near.
A test dose tells the placement. Local anesthetic spreading through the groove, lifting the roots and surrounding them, is the sign of a needle in the right plane. The drug is fed in slowly, the spread watched as it opens around each root. A spread that gathers on one side of the stack is a cue to move the tip across, so the whole plexus is bathed.
The aim is the drug in the groove, around the roots, drawn back from any root it might enter. A root that swells as the drug goes in, or a jolt of pain down the arm, calls the tip back at once. The block is laid around the nerves, in the sheath that holds them. The local anesthetic soaks into that sheath. No root is pierced.
The path runs through the middle scalene muscle to reach the groove, which keeps the needle clear of the carotid and the deeper vessels on the inner side. A small injection of fluid ahead of the drug opens the groove and confirms the tip sits in the right plane, lifting the roots into clearer view before the local anesthetic follows. Each push is read on the screen before the next is given.
The phrenic nerve next door
The brachial plexus in the neck and shoulder, drawn in yellow. The roots run down past the scalenus anterior muscle toward the subclavian artery and the clavicle. The phrenic nerve, labeled at the right, runs on the front of the scalenus anterior a short way from the plexus, the reason an interscalene block so often reaches it. Plate from Gray’s Anatomy, public domain.
The interscalene block has one effect that sets it apart from every other block. It happens in close to every block placed. The phrenic nerve, which drives the diaphragm on that side, runs down the front of the anterior scalene muscle, a finger’s breadth from the roots the block is aimed at. Local anesthetic spread in the groove reaches it almost every time. A standard interscalene block, at the volumes long used, stills the diaphragm on that side in close to every patient, and the lung above it loses about a quarter of its working capacity for as long as the block lasts. The stilled half of the diaphragm can be seen on ultrasound, no longer sweeping down with each breath, so a patient short of breath after the block is checked this way. The effect comes on within the half-hour the block takes to set, and it lasts for as long as the block holds the arm numb. Whether the loss matters depends on the lungs underneath. A fit patient still has the other diaphragm. The breath shortens only a little. Severe chronic lung disease leaves a patient leaning on both diaphragms to breathe, so the loss of one can tip them into distress. The same holds for a patient with a single working lung, or a diaphragm already paralyzed on the far side. For them an interscalene block can take away breath they cannot spare, so it is avoided or swapped for a block placed lower and further from the phrenic nerve. A block on both sides at once is off the table for the same reason, two stilled diaphragms leaving no breathing in reserve. This single fact shapes the whole use of the block. The way to soften it is to use less drug. A small volume, five to ten milliliters, is placed to the outer side of the groove. It reaches the shoulder. It touches fewer of the phrenic fibers. Even then the effect is only made smaller, so the lungs are weighed before every block. Ultrasound is what makes the smaller volume work, since the spread is watched and the drug laid only where it is needed, a control the blind techniques never had. The paralysis lifts as the block fades, the diaphragm returning with the feeling in the arm, no lasting harm done where the patient could spare the hours. The phrenic nerve next door is why the interscalene block is matched to the patient as closely as to the surgery.
The other neighbors
The phrenic nerve is not the only neighbor the block can reach. The sympathetic nerves to the face run nearby, and local anesthetic that drifts onto them brings a drooping eyelid and a small pupil on that side, a harmless sign called Horner syndrome. It clears when the block wears off. The nerve to the voice box can take a little of the drug as well, leaving the voice hoarse for a few hours.
None of these does lasting harm. Each passes when the block fades. They are useful to recognize, so they are not mistaken for something worse, and so the patient can be told what to expect. A hoarse voice and a drooping eyelid after a shoulder block are the block doing a little more than meant, harmless and short-lived.
Deeper neighbors carry the real danger. The vertebral artery climbs the neck close to the roots. The lung sits below the plexus. These are the structures the in-plane needle and the watched tip are there to avoid, and they are the reason the block is done with the anatomy in full view.
Sparing the diaphragm
The phrenic effect can be lowered in several ways. Ultrasound is behind each of them.
The first is volume. Less local anesthetic spreads less far, so it reaches fewer of the phrenic fibers on the anterior scalene. A dose well short of the volumes once used lowers the rate of diaphragm paralysis. Careful placement keeps the shoulder covered. Too little drug can leave the block short or weak, so the volume is trimmed with care.
The second is placement. The drug is laid to the outer side of the groove, away from the front of the anterior scalene where the phrenic nerve runs, and a little lower in the neck. A spread kept lateral and low reaches the roots the shoulder needs and spares more of the nerve to the diaphragm. The watched spread is what lets the drug be steered this way.
The third is a different target. A block placed a step lower, on the superior trunk a little below the roots, covers the shoulder and sits further from the phrenic nerve, so it spares the diaphragm more often than the classic interscalene does. The superior trunk block is a phrenic-sparing answer for a patient who needs the shoulder covered and the breath protected.
None of these makes the effect vanish. Each lowers the odds and the degree of diaphragm paralysis. The choice among them is made with the patient’s lungs in view. A patient with poor lungs may need all three, or a block of another kind entirely.
Among the blocks of the neck
The interscalene block has a close cousin a little lower, the supraclavicular block, set out on its own page. Both blocks reach the same plexus. The interscalene catches it high, at the roots, the level that covers the shoulder best. The supraclavicular catches it lower, at the trunks above the collarbone, where the whole arm below the shoulder comes under one tight cluster.
The choice between them follows the surgery and the patient. A shoulder operation leans toward the interscalene. An operation on the arm or the hand leans toward the supraclavicular. A patient who cannot lose a diaphragm is steered to the supraclavicular, since it spares the phrenic nerve more often. The same skill of reading the plexus and watching the spread carries from one to the other.
Lower still, a block can be placed below the collarbone or in the armpit, where the nerves have spread into cords and branches around the axillary artery. A block placed further down the arm covers the hand well and leaves the diaphragm alone. The block is picked for where the surgery is and for how much breathing the patient has to spare.
Staying out of trouble
Three deeper structures set the real limits of safety.
The vertebral artery runs up the neck near the roots, and local anesthetic driven into it reaches the brain in seconds. Color Doppler marks the artery before the needle moves, the drug is given in small divided doses, and the syringe is drawn back before each to check for blood. A tip kept in plane and in view stays clear of the vessel.
The lung sits below the plexus, nearer here than the needle ever needs to go, and a tip driven too deep can reach it and drop the lung. The block is laid in the shallow groove, the depth read off the screen, the needle never pushed past the roots. The lung sits closer at the supraclavicular block lower in the neck, so the risk there runs higher.
Local anesthetic can also track inward along a root toward the spinal cord. This spread is rare. It can reach the covering of the cord. A slow, watched injection guards against it, the spread seen to open in the groove where it belongs. Each of these risks is held off the same way, by keeping the needle and the drug in sight from the first pass to the last.
After the block takes hold
The block sets in over a quarter of an hour or so. The shoulder and the upper arm grow warm, then numb, and the arm hangs heavy and useless for the hours the block lasts. A long-acting local anesthetic holds the shoulder pain-free well past the surgery, often through the first night, the stretch of recovery where the pain is worst. A shoulder freed of pain for that first day lets the patient sleep and move early, which helps the recovery that follows.
A numb arm needs looking after. It is held in a sling so it is not left to dangle or to lie trapped, since the patient cannot feel it to protect it. The patient is told that the heavy, dead arm is the block at work, and warned of the hour it should start to wear off, so a block that outlasts its expected span is noticed and checked. A patient sent home with the block still working is told to guard the arm and to take an oral painkiller before the numbness fades, ahead of the pain it covers.
For pain that runs on for days, a fine catheter is threaded into the groove beside the roots and left in place, feeding local anesthetic for as long as it is needed. The same scan that places the single shot guides the catheter, the spread checked through it to confirm it still sits where it should. The catheter extends the block from hours to days.
The nerve next door
The interscalene block numbs the shoulder and the upper arm. It is the block for a shoulder operation, a rotator cuff repair, a fracture set at the top of the arm. The roots of the plexus stack between the scalene muscles in the neck, a short way under the skin, where a linear probe reads them as a stoplight stack of dark rounds.
The phrenic nerve runs close by, on the front scalene muscle. The drug spreading to the roots reaches it in nearly every block. That nerve drives the diaphragm, so one half of it is stilled for the hours the block lasts. A fit patient breathes through this without trouble. A patient whose lungs are already poor may not.
So the block is chosen with the lungs in mind, and placed with a small volume kept low on the roots. Five to ten milliliters set in the right place spares the phrenic more than the large doses of the past. The picture is what lets the dose go small. The roots, the needle, and the spread are all in view, so a little drug, well placed, does the work.
Common questions about the interscalene block
What is an interscalene block used for?
It numbs the shoulder and the upper arm for surgery such as a shoulder arthroscopy, a rotator cuff repair, a shoulder replacement, or a break of the upper humerus. It reaches the brachial plexus high in the neck, at the roots. It often spares the lowest nerves, so it is a weak choice for the hand or the forearm.
What does the brachial plexus look like on ultrasound here?
At the interscalene level the nerve roots show as a short stack of dark, round shapes between the anterior and middle scalene muscles, a picture likened to a traffic light. The roots sit shallow, behind the sternocleidomastoid. They read darker than a peripheral nerve, plain circles with a thin rim.
Why does an interscalene block affect breathing?
The phrenic nerve, which drives the diaphragm, runs on the anterior scalene muscle a short way from the roots, and local anesthetic reaches it in close to every block. The diaphragm on that side is stilled for the length of the block, and the lung loses about a quarter of its capacity. The block is avoided in a patient with poor lungs, who cannot spare a diaphragm for the hours it lasts.
How is the breathing effect reduced?
By using less local anesthetic, around five to ten milliliters where twenty or thirty were once given, and placing it to the outer side of the groove away from the phrenic nerve. Ultrasound makes the smaller volume work by showing where the drug spreads. The effect is reduced by these steps, so the lungs are still weighed before every block.
Is an interscalene block safe with ultrasound guidance?
Ultrasound shows the roots, the needle, and the spread, so the tip is set beside the plexus and away from the vertebral artery and the lung. The drug is given in small steps with the syringe drawn back to check for blood. The main effects to know are the near-certain diaphragm paralysis and the harmless Horner syndrome and hoarseness that can come with it.
Does an interscalene block reach the hand?
Not well. The block catches the upper roots of the plexus and often spares the lowest ones. The hand and the inner forearm can be left with feeling. For surgery below the elbow, a supraclavicular or more distal block is the better choice.
About the Author
Julien Mercier
Senior R&D Engineer · Medical Ultrasound Transducer Development
Senior R&D Engineer with an M.S. in Applied Physics and over 15 years of experience in medical ultrasound transducer development, specializing in the design verification and performance testing of high-frequency imaging transducers. Currently leading the development and verification of the company’s next-generation high-frequency linear-array transducer, responsible for imaging performance evaluation and reliability analysis in preclinical testing. Brings extensive hands-on experience in piezoelectric element tuning, beamforming parameter optimization, and system-level performance testing.
