What Is an Interscalene Block for Shoulder Surgery?

An interscalene block is a regional anesthesia technique that numbs the nerves of the brachial plexus as they pass between the scalene muscles on the side of the neck, providing powerful pain relief to the shoulder, upper arm, and sometimes the elbow. It is widely considered the gold standard nerve block for shoulder surgery, and it works by depositing local anesthetic around the upper roots and trunks of the brachial plexus before they branch out toward the arm. The block can serve as the sole anesthetic for a procedure or, more commonly, as a complement to general anesthesia that dramatically reduces pain and opioid needs after surgery.

Why It Is the Go-To Block for Shoulder Surgery

The brachial plexus supplies nearly all sensation and motor control to the shoulder and arm. At the interscalene level, the nerve roots that matter most for shoulder pain (C5 and C6, and to a lesser extent C7) are bundled closely together, which makes a single injection at this spot remarkably effective for procedures on the shoulder joint. Interscalene brachial plexus block provides optimal analgesia for shoulder surgery and has become a standard part of perioperative care for procedures ranging from arthroscopic rotator cuff repair to total shoulder replacement.¹

A meta-analysis pooling data from multiple randomized trials found that a single-shot interscalene block reduced opioid consumption for up to 12 hours after surgery, cut the rate of postoperative nausea and vomiting at 24 hours, and sped up discharge from both the recovery room and the hospital.² Beyond the recovery room, a retrospective study showed that a preoperative interscalene block cut intraoperative opioid use by roughly half and reduced the concentration of inhaled anesthetic gases needed during surgery by about 15%.³ For patients, this translates to less grogginess, less nausea, and a faster transition to going home.

How Ultrasound Changed the Technique

Before ultrasound became standard in regional anesthesia, interscalene blocks were placed using anatomical landmarks on the neck and a nerve stimulator, a small device that sends electrical impulses through the needle tip to confirm proximity to the nerve by causing a muscle twitch. That approach worked, but it was imprecise. The anesthesiologist could feel and hear the twitch, yet could not see where the needle tip sat or how the local anesthetic was spreading.

Ultrasound guidance changed the equation substantially. A comparative study found that blocks placed under direct ultrasound visualization achieved surgical-quality anesthesia in 99% of patients, compared with 91% when guided by nerve stimulation alone. The ultrasound group also had better sensory blockade, better motor blockade, and more complete coverage of the shoulder region.⁴ Equally important, seeing the anatomy in real time lets the anesthesiologist use less local anesthetic, place the needle more precisely, and avoid blood vessels and other structures. Today, ultrasound-guided interscalene block is the dominant technique worldwide.

The Breathing Side Effect Everyone Should Know About

The single most predictable side effect of an interscalene block is temporary paralysis of the phrenic nerve on the same side. The phrenic nerve runs very close to the brachial plexus at the interscalene level, and local anesthetic routinely spreads to it. This paralyzes half the diaphragm, a condition called hemidiaphragmatic paresis. In a healthy person with two functioning lungs, losing half the diaphragm’s movement is usually just mildly noticeable, perhaps a feeling of slight breathlessness, but it is measurable.

One classic study found that an interscalene block produced diaphragmatic paralysis in every single patient examined, confirmed by both lung function tests and chest X-ray.⁵ A more recent review described this as an inevitable consequence of the standard technique.⁶ The paralysis is temporary, resolving as the block wears off, but it creates a real problem for patients who already have compromised lung function. Interscalene block is generally considered relatively contraindicated for people with significant lung disease, including severe COPD, prior pneumonectomy (removal of a lung), or contralateral phrenic nerve palsy.⁷

Reducing the Impact on Breathing

Because phrenic nerve paralysis is the block’s Achilles heel, researchers have explored two broad strategies to minimize it: using less local anesthetic volume at the traditional interscalene site, and targeting the nerves at a slightly different anatomical location altogether.

Low-Volume Injections

Ultrasound allows precise placement of small volumes of local anesthetic right where they are needed, and studies show that dropping the volume from a standard 20 mL down to 5 mL significantly reduces respiratory impact. In one trial, all patients who received 20 mL had paradoxical diaphragm movement (meaning that half of the diaphragm moved upward instead of downward during breathing), while only 45% of those who received 5 mL showed the same finding. Lung volume measurements confirmed that the low-volume group lost far less breathing capacity.⁸ An MRI-based study confirmed that larger volumes spread more frequently toward the intervertebral foramen and the phrenic nerve, while lower volumes produced less aberrant spread and a more favorable safety profile overall.⁹

The trade-off is that lower volumes may produce a shorter or slightly less dense block, so the anesthesiologist balances respiratory safety against the duration and completeness of pain relief. For patients with healthy lungs undergoing outpatient surgery, a standard volume is often acceptable. For someone with borderline lung function, a low-volume approach may allow them to receive a block they would otherwise have been denied.

Superior Trunk Block

A more recent strategy targets the superior trunk of the brachial plexus, which sits slightly farther from the phrenic nerve than the traditional interscalene injection point. The idea is to still capture the C5 and C6 nerve fibers that supply the shoulder while physically distancing the injection from the phrenic nerve’s path.

Results from multiple randomized trials have been encouraging. One trial reported that hemidiaphragmatic paralysis occurred in about 5% of patients receiving a superior trunk block versus about 71% in the standard interscalene group, with no difference in pain scores in the recovery room.¹⁰ Another trial found that complete diaphragmatic paralysis occurred in 38% of the interscalene group and in none of the superior trunk group, again with equivalent pain control.¹¹ The evidence is not perfectly consistent across all studies. A third trial still found hemidiaphragmatic paresis in about 76% of superior trunk block patients (versus 97.5% in the interscalene group), though complete paralysis dropped dramatically, from about 73% to just 5%.¹² The variation across studies likely reflects differences in injection technique, volume used, and how paresis was measured. Still, the general trend is clear: targeting the superior trunk substantially reduces full diaphragmatic paralysis while maintaining equivalent shoulder analgesia.

Rebound Pain When the Block Wears Off

One of the most common complaints patients have after an interscalene block is rebound pain, a sudden surge of intense pain that arrives as the block resolves, typically somewhere between 10 and 16 hours after injection. The block provides such complete numbness that patients sometimes assume the surgery itself was not that painful. When sensation returns, the contrast between “no pain” and “full surgical pain” hits hard. One trial measured the mean time to worst pain at about 14.6 hours and the mean time to first need for rescue painkillers at about 11.3 hours after the block.¹³

Managing rebound pain has proven surprisingly tricky. That same trial tested whether giving a dose of oral hydromorphone six hours after the block, before it wore off, could pre-empt the pain surge, and found no significant benefit compared to placebo. Simply taking an opioid in advance did not blunt the rebound. However, intravenous dexamethasone (a steroid) given alongside the block did help: a randomized trial found that the incidence of rebound pain dropped from about 73% in the control group to about 30% in the group that received dexamethasone, with lower pain scores after the block resolved and less opioid use overall.¹⁴ Multimodal shoulder injections given by the surgeon at the time of surgery have also shown promise, reducing pain scores through the first 48 hours and lowering opioid consumption.¹⁵

The practical upshot: if you are going home after shoulder surgery with a single-shot block, plan ahead for pain management before the block wears off. Having your oral medications on hand and starting them on schedule matters more than waiting to see if you really need them.

Single-Shot Versus Continuous Catheter

A single-shot interscalene block provides strong pain relief for roughly 8 to 16 hours, depending on the local anesthetic used and whether adjuvants are added. For some procedures, that window covers the worst of the postoperative pain. For more invasive operations like rotator cuff repair or shoulder replacement, pain can remain severe for days, and a single injection may not be enough.

A continuous interscalene catheter extends coverage by leaving a thin tube near the brachial plexus through which local anesthetic is infused over two to three days. Randomized trials consistently show that the continuous approach lowers pain scores and reduces opioid use on the first postoperative day compared to a single injection.¹⁶ For minor arthroscopic procedures, a continuous infusion added to a single-shot block reduced pain especially with movement during the first 24 hours, though the advantage narrowed by the second day.¹⁷

A more recent trial following patients through three months found that those with a continuous catheter had markedly lower pain scores at every time point through the first week and even at one month, along with less opioid consumption and better shoulder function scores at three months.¹⁸ A separate trial found that a three-day continuous block improved sleep quality and cut opioid use compared with a single shot after rotator cuff repair.¹⁹

Catheters are not without downsides. About 10% of catheters pull out prematurely before their intended removal, and adverse events are somewhat more common in catheter groups.²⁰ Patients need clear instructions about caring for the catheter site, what to watch for (signs of infection, worsening numbness, increasing shortness of breath), and when to seek help. And because the arm remains numb while the infusion runs, protecting the limb from accidental injury in those first days is essential.

Adjuvant Medications That Extend the Block

One of the most active areas of research involves additives mixed into the local anesthetic to prolong the block’s duration without needing a catheter. Two drugs in particular, dexamethasone (a corticosteroid) and dexmedetomidine (a sedative-analgesic), have shown striking results when combined and added to the local anesthetic solution. A retrospective study found that this “dex-dex” combination extended the median block duration from half a day to about three and a half days. Two days after surgery, 84% of patients in the dex-dex group still had active analgesia from the block, compared with none in the control group. Opioid consumption in the dex-dex group was less than half that of controls.²¹ A case series reported that some patients receiving the combination required no opioids at all after shoulder surgery.²²

Liposomal bupivacaine, a slow-release formulation of a common local anesthetic, is another attempt at extending a single injection. The evidence here is more mixed. A double-blinded randomized trial comparing liposomal bupivacaine with plain bupivacaine for shoulder arthroplasty found no clinically meaningful difference in pain scores, time to first opioid use, total opioid consumption, or patient satisfaction.²³ However, a large retrospective study of nearly 500 total shoulder arthroplasty patients reported that those who received liposomal bupivacaine had a higher rate of tolerable pain in the first 24 hours and lower opioid consumption compared with standard bupivacaine or no block.²⁴ A systematic review and meta-analysis concluded that single-injection liposomal bupivacaine blocks resulted in lower pain and lower opioid use across the first three postoperative days compared with other single-injection blocks and even compared with continuous catheter infusions in some analyses.²⁵ The picture is still evolving, and individual study quality varies, but the trend suggests liposomal bupivacaine may offer meaningful extended relief, particularly when a catheter is impractical.

Nerve Injury Risk

Any time a needle is placed near a nerve, there is a small risk of nerve injury. A systematic review focusing specifically on interscalene blocks found that the overall incidence of neurologic injury after a single-injection block was about 3%, and about 5% after continuous catheter infusions. Those numbers sound alarming, but the vast majority of these injuries are mild and transient: less than 0.5% of symptoms persisted beyond one year in either group.²⁶ Symptoms usually involve numbness, tingling, or mild weakness in the hand or fingers that gradually resolves over weeks to months. Permanent damage is rare but has been documented, which is why consent discussions before the block are important.

It is also worth noting that shoulder surgery itself can cause nerve injuries through surgical traction, positioning, or swelling, so distinguishing between a block-related injury and a surgery-related one is not always straightforward. Ultrasound guidance has likely helped reduce the incidence compared with older techniques, since the anesthesiologist can see the needle tip and avoid intraneural injection, though definitive large-scale data comparing injury rates before and after the ultrasound era remain limited.

Other Side Effects Beyond Breathing

Phrenic nerve paralysis gets the most attention, but the interscalene block can cause a few other side effects related to the dense nerve traffic in the neck. Horner’s syndrome, a combination of a drooping eyelid, a constricted pupil, and reduced sweating on one side of the face, occurs when local anesthetic spreads to the sympathetic chain near the cervical spine. It looks alarming but is harmless and resolves when the block wears off. Hoarseness from recurrent laryngeal nerve involvement is another possibility, as is a sensation of a lump in the throat. These effects are usually mild and temporary, though they can be unsettling if nobody warns you in advance.

How Better Pain Control Affects Rehabilitation

Pain after shoulder surgery is not just an experience to endure; it directly affects recovery. Patients in too much pain tend to guard the shoulder, limiting the early gentle motion that physical therapists prescribe to prevent stiffness. A trial comparing continuous catheter infusion with a single shot after rotator cuff repair found that the catheter group had better shoulder extension at six months and significantly stronger external rotators, suggesting that better early pain control translated into measurable functional gains months later.²⁷ Another trial tracking patients with a continuous catheter through three months found superior shoulder function scores in the catheter group compared to the single-shot group.²⁸

Continuous interscalene blocks have also been applied beyond routine postoperative care. In a series of patients with complex regional pain syndrome of the shoulder, a condition where pain becomes disproportionately severe and disabling, continuous infusion of local anesthetic allowed patients to participate in intensive rehabilitation. External rotation improved from an average of about 5 degrees before treatment to about 30 degrees at one month, and abduction roughly doubled, gains that were maintained at one year.²⁹ This is a niche application, but it illustrates how effective nerve blockade can unlock rehabilitation that would otherwise be impossible because of pain.

Protecting a Numb Arm at Home

One practical concern that often gets overlooked in preoperative discussions is what it feels like to have a completely numb arm for hours or even days. The interscalene block does not just eliminate pain; it typically eliminates most sensation and motor function in the shoulder and upper arm, and sometimes the hand. During that window, the arm has no protective pain reflexes and reduced awareness of its position in space. Patients have been known to lean on a numb hand without realizing it, roll onto the arm during sleep, or accidentally bump it against a door frame. Keeping the arm in a sling and being mindful of its position, especially during the first 24 hours, helps prevent inadvertent injury.

For patients sent home with a continuous catheter and pump, the responsibility goes further. They need to keep the insertion site clean and dry, recognize signs of catheter migration or disconnection, and understand that new or worsening numbness in unexpected areas, increasing shortness of breath, or signs of infection at the catheter site all warrant a call to the surgical or anesthesia team. Most outpatient catheter programs provide a 24-hour contact number for exactly these situations.