Axillary nerve is a terminal branch of the posterior cord of the brachial plexus formed by C5 and C6 spinal roots. It descends backward with the posterior humeral circumflex artery and exits axilla posteriorly through a quadrilateral space. Then it forms anterior and posterior branches beneath the deltoid muscle. It innervates the deltoid and teres minor muscles, and gives cutaneous supply over the deltoid region and superior posterior arm [6,7].

The axillary nerve could be damaged by intramuscular injection into the deltoid. Tak et al. [8] investigated 310 patients with nerve injuries after intramuscular injections. Among them, 3 (1%) patients had axillary nerve injury; it is the third most common lesion following sciatic and radial nerve injuries. Davidson et al. [6] reported a case of axillary nerve injury after a deltoid intramuscular injection of antiemetics. Choi et al. [9] also reported a case of axillary nerve injury after deltoid intramuscular injection of atropine and hydroxyzine as a preoperative medication.

If the injury is caused directly by the needle tip, the patient typically complains of immediate, sharp, and electric–like pain over the shoulder at the time of injection. However, if the pure motor branch of the axillary nerve is injured, the patient could not suffer pain and could only have deltoid muscle wasting and shoulder weakness, especially abduction [6,9].

The suitable intramuscular injection site in upper limb is the deltoid muscle. The deltoid is a triangular-shaped muscle that originates from the lateral one third of the clavicle, the acromion, and the spine of the scapula. It converges downward into the deltoid tuberosity near the middle of the humerus. As the axillary nerve courses beneath the deltoid muscle and supplies the muscle, it is clearly vulnerable to needle injury. Subdeltoid bursa and circumflex humeral arteries are other at-risk structures from a deltoid injection (Fig. 1) [10,11].

Although deltoid intramuscular injection is considered a routine clinical procedure, occasionally it is performed incorrectly. One of the most extensive investigation on this topic was scrutinized by McGarvey and Hooper [11]. They reviewed deltoid intramuscular injection technique by 50 general practitioners and 50 nurses, and found that there was a lack of awareness of anatomy and proper injection techniques.

The recommended injection site is the midpoint of the deltoid muscle (the densest part of the muscle) or approximately 3–5 cm below the lower edge of the acromion on midway between acromion and deltoid tuberosity (Fig. 2). Excessive proximal site could lead to bursa injection, while excessive distal site could lead to nerve damage or injection into vessels [11,12]. However, cadaver studies indicated that the distance from the tip of the acromion to the axillary nerve had as wide a range of 4–8 cm, with an average distance of 5–6 cm. So, a deltoid injection near 5 cm below the acromion may injure the axillary nerve. If the patient’s arm is abducted, the axillary nerve moves closer to the acromion and the risk of the nerve injury increases. Furthermore, the subdeltoid bursa may extend 5 cm below the acromion. Therefore, unfortunately, no site offers an absolute protection against at-risk structures [7,11,13].

The deltoid intramuscular injection is not recommended in young children due to the inadequate muscle mass. Even in adults, it is not recommended in patients who are thin with debilitating diseases as the same reason. Additionally, the number and volume of injections should be limited due to its small muscle mass. The suggested maximum injection volume at a deltoid site is 1–2 ml [10,12].

The selection of needle length is also crucial. The needle should be long enough to reach the muscle mass and to prevent subcutaneous injection, but not so long to involve underlying nerves, blood vessels, or bones. Appropriate needle length in adults depends on patient’s sex and weight (Table 1) [14,15].

It is important to recognize the possibility of nerve injury following an intramuscular injection. If a patient experiences any abnormal pain or paresthesia during the injection, the needle should be immediately withdrawn. Careful monitoring and observation of the injection site is also important for an early detection of nerve damage. Two to four hourly post- injection observation of the injection site is recommended [12].

The radial nerve is the major outflow from the posterior cord of the brachial plexus formed by C5–T1 spinal roots. It descends down the posterior aspect of the humerus along the radial groove and wraps around the humerus in a lateral direction. Then it moves anteriorly over the lateral epicondyle of the humerus and passes the antecubital fossa (Fig. 3) [16].

It provides motor innervation to the posterior compartment of the arm and forearm responsible for extension of the elbow, wrist and fingers. It also provides sensory innervation to the posterior arm, posterior forearm, and lateral two-thirds of the dorsum in the hand and fingers [5,16].

The radial nerve is the most commonly injured nerve in the upper extremity after intramuscular injection. Tak et al. [8] investigated 310 patients with nerve injuries after intramuscular injections. Among them, 29 (9.4%) patients had a radial nerve injury. Pandian et al. [17] identified 29 (43.9%) patients having suffered a radial nerve injury among the 66 intramuscular injection related nerve injury patients. Above two reports, radial nerve injury is the second most common lesion after sciatic nerve injury. There are also several case reports of radial nerve palsy following intramuscular injection into the arm [18].

Venipuncture near or at the elbow level can also involve the radial nerve. Edwards and Fleming [19] described a patient with a complete radial nerve palsy after the cephalic vein cannulation in the lateral aspect of his arm near the elbow. This lesion probably occurred as a result of a direct needle puncture of the radial nerve through the extensor carpi radialis longus muscle.

Pain is the most frequent symptom of a nerve injury. It is usually described as a severe shooting or burning sensation at the injection site with radiation to the dorsum of the forearm and hand. Patients could also present numbness, paresthesia, hypoesthesia and other sensory disturbance. Simple sensory deficits without pain may be functionally less significant because they involve the anatomical snuffbox and the radial dorsum of the hand. Motor function injury induces weakness when extending the forearm, wrist, and fingers. This condition is called “wrist drop” or “finger drop” [16,18,19].

The radial nerve locates very superficially in the middle third of the lateral aspect of the arm. Therefore, an intramuscular injection into this site could result in damaging the nerve. The intramuscular injection site in upper limb should be selected midway between the acromion and deltoid tuberosity of humerus, i.e. the deltoid muscle [16,18].

For venipuncture near the elbow level, selection of an injection site and a proper needle procedure are important. Detailed technique is described in “Venipuncture technique in the antecubital fossa.”

Ulnar nerve is a continuation of the medial cord of brachial plexus formed by C8 and T1 spinal roots. The nerve maintains a position medial to the axillary and brachial arteries in the upper arm and descends on the posteromedial aspect of the humerus. It innervates most of the intrinsic muscles of the hand, and muscles of the flexor carpi ulnaris and medial half flexor digitorum profundus in the arm and forearm. It also provides sensory innervation to medial surface of dorsum and palm of the hand [5,20].

Injection-related ulnar nerve injury seems to be relatively rare. Kim et al. [20] analyzed 645 patients with ulnar nerve injury for 30 years and discovered only 2 injuries (0.3%) were caused by injection. Geiringer and Leonard [21] reported 2 cases of ulnar nerve injury at the arm level who received intramuscular injection for immunization. The patients were asked to fixate their hands on the hips and rotate their arms. Then the injection was administered to patients’ sidearm while the nurse was standing at the patients’ side. Combining this position of arm rotation with a sidearm delivery facilitated administration on the medial-lying neurovascular bundle instead of the target deltoid muscle. Salanga and Hahn [22] also reported an arm level ulnar nerve injury caused by a jet injection for vaccination. Relatively small size of the patient and small muscle mass were the deduced causes of the nerve injury. Excessive arm rotation by the operator during the injection exposed the nerve making it more vulnerable to an injury by the liquid jet.

As other nerve injuries, pain is the most frequent symptom that patients suffer. The symptoms could include numbness, paresthesia, hypoesthesia and other sensory disturbance in the innervated area. Motor function injury could result in weakness of finger abduction and adduction, fourth and fifth finger flexion, wrist adduction and wrist flexion. Atrophy of hypothenar eminence and interosseous muscles is also a frequently reported symptom [21,22].

Intramuscular injection frequently targets the deltoid muscle as the injection site. Although the ulnar nerve in the upper arm is located in the medical aspect and far from the deltoid, inappropriate position such as excessive arm rotation could facilitate missing the muscle completely and damaging the ulnar nerve. Superficial location of the ulnar nerve under the skin makes it vulnerable to such injury. Patients that lack subcutaneous tissue and muscle could face more vulnerability to this traumatic injury. Injection complications can be prevented by administering the injection by properly trained staff that holds sufficient anatomical knowledge [21,22].

The median nerve arises from the lateral and medial cords of brachial plexus formed by C5-T1 spinal roots. It then runs down with the brachial artery on the medial side of the arm and enters the anterior compartment of the forearm via the antecubital fossa. It provides motor innervation to anterior compartment of the forearm responsible for flexion of the wrist and fingers and sensory innervation to the lateral two-thirds of the palm in the hand and fingers [5].

At 5–8 cm distal to the medial epicondyle of the humerus, the anterior interosseous nerve arises on the radial aspect of the median nerve. It is a pure motor nerve to the flexor pollicis longus, flexor digitorum profundus to the index and sometimes middle fingers, and to the pronator quadratus [23].

Kohn et al. [24] reported a patient with permanent median nerve injury. The patient intermittently received intravenous heparin. A large hematoma was developed after an antecubital venipuncture and the median nerve was found to be compressed by a blood clot. The anterior interosseous nerve injuries were also discovered after a routine blood sampling or peripherally inserted central catheter line insertion into the antecubital fossa [23,25,26].

Proximal median nerve lesion induces functional loss of forearm pronation and flexion of the wrist and fingers with median nerve distribution sensory disturbance. In case of the isolated anterior interosseous nerve injury, although no sensory loss occurs, pain may be present in the forearm along the course of the nerve. Most patients have weakness to flex the distal phalanges of the index finger and thumb. Patients typically are unable to make an “OK” sign or to pinch a sheet of paper using their thumb and the index fingers [23,25,26].

For venipuncture near the elbow level, cautious selection of an injection site and proper needle procedures are important. Detailed technique is described in “Venipuncture technique in the antecubital fossa.”