Ultrasound-Guided Perineural Injections
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In-plane injections should be used for peripheral nerves because of the relative vulnerability of the target.
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The image should be optimized for maximum conspicuity of the nerve and to reliably identify the border of the outer epineurium.
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The nerve should be scanned proximally and distally to the injection field in both the short- and long-axis views prior to the procedure.
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Applying a small amount of hypoechoic injectate adjacent to the nerve will increase the nerve’s conspicuity.
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The flow of the injectate should be observed carefully to ensure that it is not intraneural.
Peripheral Nerve Anatomy and Image Optimization
Anatomy
Peripheral nerves have a characteristic fascicular pattern that is readily distinguished from the fibrillar pattern of tendons ( Fig. 31.1 ). The hypoechoic fascicles are seen among the intervening hyperechoic inner epineurium. In the short-axis view, the contrast of this tissue creates the appearance of a honeycomb. The fascicles reflect the bundle of nerve fibers surrounding by perineurium. The inner epineurium consists of connective tissue and intraneural vascular structures. The highly reflective outer border is the outer epineurium.
Scanning and Image Optimization
Peripheral nerves should initially be scanned in the short-axis view for identification and recognition of the surrounding anatomy. The long-axis view also should be included for a complete perspective ( Fig. 31.2 ). When the nerve is surrounded by relatively isoechoic tissue, such as tendons, and is difficult to identify, scanning should be done to follow the nerve to a more conspicuous location such as surrounding hypoechoic muscle. Once reliably identified, the nerve can be followed back to the preferred location. Increasing the speed of scanning of the nerve in the short-axis view can also improve its conspicuity.
The peripheral nerve image can be optimized by toggling the transducer to create an orthogonal position of the nerve and minimize anisotropic artifact. Typically, the highest-frequency transducer that can adequately penetrate to the desired depth should be used. The depth should be set so that the nerve takes up a large portion of the image rather than a small area at the surface. The focal zone should be placed at the level of the nerve, and the gray-scale gain should be set to create the greatest contrast between the outer epineurium and surrounding tissue.
Common Pathology
The most consistent abnormal finding with focal neuropathies is enlargement of the nerve. The mechanism of this is complex but can include: blocking of axoplasmic flow, endoneurial edema, distal axonal degeneration, perineural and endoneurial inflammation, perineural fibrosis, axons sprouting, remyelination, and thickening of the perineurium and epineurium. Assessment of the cross-sectional area at the area of maximal nerve swelling is the most commonly used parameter to distinguish nerve abnormality. This is performed by tracing the inner border of the outer epineurium and the calculation is performed by most ultrasound machines. Techniques that help distinguish pathologic enlargement include comparison to unaffected areas of the same nerve and, when needed, comparison to the opposite limb. Sufficient inspection should be performed to determine whether the enlargement is focal, which is typical of entrapment neuropathies, or more diffuse, as seen with stretch injuries. The internal echotexture of the nerve also should be assessed. Enlargement or distortion of the internal fascicles often suggests more severe injury. The effect of surrounding tissue, such as scarring, should also be identified. In the context of trauma, careful inspection should be performed to establish that the nerve is in continuity. Identification of a complete neurotmesis leads to the consideration of surgical treatment for repair.
Indications for Peripheral Nerve Injections
Ultrasound-guided peripheral nerve injections are performed for anesthetic blocks for pain management during surgery or other procedures, diagnostic blocks to help determine the source of pain, and therapeutic treatment of a known peripheral nerve injury. The nature of the injectate and relative volume is based on the indication and type of procedure. Ultrasound guidance has changed the nature of anesthetic and therapeutic injections by allowing direct visualization of the nerve, enabling the clinician to change the needle position and target multiple nerves from the same external landmark. , Diagnostic blocks are performed by injecting a small amount of local anesthetic around the nerve, instead of wide infiltration into the surrounding tissue, in an effort to help determine the pain generator. Ultrasound guidance provides greater precision of diagnostic injections over nonguided injections. Procedures performed for therapeutic effect can consist of any combination of local anesthetic, steroid, saline, and dextrose. The purpose is to provide direct therapeutic effect from the injectate and, when appropriate, use the injected volume to alleviate compression and encroaching scar by hydrodissecting the nerve. ,
Peripheral Nerve Injection Technique
Equipment
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Needles:
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27-gauge, 1.0-inch needle to anesthetize the skin
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25-gauge, 1.5-inch needle for most superficial injections
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25-gauge, 3.5-inch needle for deeper injections
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Injectate options:
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Local anesthetic (e.g., 1% lidocaine, 0.25% bupivacaine)
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Normal saline solution
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5% dextrose
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Steroid (e.g., dexamethasone 4 mg/mL, triamcinolone acetonide (Kenalog) 40 mg/mL)
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Platelet products: platelet-rich plasma (PRP) or platelet lysate
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Ultrasound machine with a high-frequency (10 to 15 MHz) linear array transducer
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Sterile conduction gel
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Sterile gloves
Peripheral Nerve Injection Principles
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There should be complete knowledge of the function of the peripheral nerve prior to the procedure.
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The nerve and surrounding tissue should be thoroughly pre-scanned in both short- and long-axis views. This includes both cranially and caudally to the injection field.
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The field should be scanned with varying transducer pressure to assess for neighboring vessels that could be potentially collapsed.
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Doppler imaging should also be used, with pre-scanning to further assess for vascular structures.
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The image should be optimized so the margins of outer epineurium are adequately visualized.
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The injections should be performed with an in-plane view of the needle and a short-axis view of the nerve.
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Aseptic technique should be used with all injections.
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Anoblique stand-off of sterile conduction gel can help with needle visualization prior to insertion. This is particularly beneficial with superficial targets.
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It is helpful to slowly introduce a small amount of the hypoechoic injectate to increase the conspicuity of the outer epineurium ( Fig. 31.3 ), then proceed with the remainder of the injection.
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