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Peripheral nerves may be injured by medications or vaccine injections. The injury may be from mechanical trauma caused by the needle itself, or also by the substance injected. If the injection is within the nerve, then the pressure created by the volume of fluid injected can cause damage. Depending on how irritating the medication is to the nerve, it may also cause chemical nerve damage, both acutely, or in a delayed fashion. As with all traumatic nerve injuries, subsequent scarring may also lead to additional nerve dysfunction. Rarely, an adjacent artery may be lacerated by the injection, causing a hematoma that secondary compresses the nerve; this usually occurs in anticoagulated patients.
Medication and vaccine injections frequently use an intramuscular route of administration, being either injected into the deltoid or gluteal muscles. Gluteal injections are more common in children, while deltoid injections are more frequently used in adults. If a deltoid injection is placed too posteriorly, it may damage the axillary nerve, or selectively affected its anterior or posterior divisions, as they pass around the neck of the humerus underneath the deltoid muscle. Occasionally, the radial nerve itself may be damaged by the injection also. This can occur if it is placed to too caudal (near the spiral groove), or posterior where the radial nerve passes into the posterior compartment of the upper arm. Gluteal injections are traditionally placed in the upper lateral quadrant of the buttock region specifically to avoid the sciatic nerve. When injections are not strictly in this quadrant, the sciatic nerve may be injured. Because the peroneal division is more superficial and lateral than the tibial division, it is more commonly injured, causing a foot drop. In cacechtic or atrophic patients (e.g., old age, cancer, prolonged bedrest, denervation), properly placed injections can still cause nerve damage because the muscle mass is much reduced, causing the needle to be deeper than usual. Another risk factor is when injections are administered when patients twist over in bed. This may cause their anatomical landmarks to be distorted. Therefore, for gluteal injections the patient should be completely lateral or prone when receiving the injection, preferably even standing. For deltoid injections the patient should be in a neutral position with their arms at their sides, not twisted towards the injection.
Patients usually present with acute pain and/or paresthesias radiating down the sensory distribution of the damaged nerve at the time of the injection. Immediate weakness and numbness may occur, while in some patients a neurological deficit manifests more subacutely over hours. This delayed response is presumably from chemical irritation to the nerve. In patients with injections that the irritating substance is only in the vicinity of the nerve (i.e., no mechanical damage to the nerve), the neurological deficit or pain can occur in a delayed fashion a few days later. Depending on the degree of injury, the nerve palsy may be complete or partial.
Documenting the relationship of the symptoms to the injection is key to making the diagnosis. For delayed symptoms, a recent history of an injection near the affected nerve is confirmatory also. If the palsy is complete or partial, then a detailed physical examination confirms which nerve or neural element (of a plexus) is damaged. Although supplementary tests, like electrodiagnostics or MR/CT imaging, may provide additional information, the diagnosis per se is often confirmed with the history and physical examination alone. For patients who are at least 3-4 weeks from their injury, electrodiagnostic testing may confirm the nerve injured, the extent of the injury, and serial examinations can be used to document early recovery, thus precluding surgery. CT scans are obtained when a hematoma causing nerve damage is suspected, this is rare however. High resolution MRI is becoming more popular in documenting the focality of nerve injury, as well as the degree of nerve edema present.
Considering injection nerve injuries are relatively rare, evidence based guidelines are not available to direct treatment. In the literature there is some disagreement regarding the timing and indication for nerve exploration in these patients. Some experts advocate an immediate exploration to wash away any irritating substance from in or around the nerve, and to perform both and internal and external neurolysis, determined by the location of the injection. Other authors suggest close follow-up with serial examinations and electrodiagnostic testing, with a consideration of surgical exploration at approximately three months if no recovery has occurred. For delayed explorations, intraoperative nerve action potentials are performed and the damaged nerve is resected and grafted if there is no action potential ellicited. Alternatively, a nerve transfer can be performed instead of nerve grafting (e.g., a triceps branch to the axillary nerve distal to the injury). For chronic (>1 year) palsies, tendon transfers may be helpful, including a posterior tibialis tendon transfer for foot drop. As with most nerve injuries, continued physical and occupational therapy with bracing, as required, remains the mainstay of treatment.
Anecdotally, injection injuries that do not spontaneously resolve in 1-2 months (i.e., neuropraxias) have a poor prognosis if untreated. If the peroneal nerve is involved, the prognosis often remains poor regardless of surgical repair, considering how proximal the injury is. Axillary nerve injuries have a better prognosis, not only because other uninvolved muscles may help partially replace the function of the deltoid, but more so because an axillary nerve injury and/or nerve repair occurs close to the denervated muscle, which often leads to robust and early reinnervation.