The Microanatomy of the Brachial Plexus and Peripheral Nerves
André P. Boezaart* DProfessor of Anesthesiology and Orthopaedic Surgery, Departments of Anesthesiology and Orthopaedic Surgery and Rehabilitation, Division of Acute and Perioperative Pain Medicine, University of Florida, College of Medicine, Gainesville, Florida, USA
Abstract
The aims of this chapter are to explain and present the older and new concepts and understanding around the microanatomy of nerve roots, trunks, and peripheral nerves. More recent work over the past 3 or 4 years looked at nerves with high-definition ultrasound and electron microscopy and illustrated that the paraneural or circumneural sheath is what neurosurgeons for years have been calling the “gliding apparatus” of the nerve. The space just deep to this layer is the subcircumneural (subparaneural) space, which should most probably be the target space for successful and safe single-injection block and catheter placement for continuous nerve block. The different microanatomical features of spinal roots, plexus trunks, and peripheral nerves are discussed and compared, as well as the microanatomical explanation of the different sonographical appearance of these three types of nerves.
Keywords: Anterior motor spinal root, Anterior scalene muscle, Arachnoid mater, Arachnoid villi, Arachnoid villus, Circumneural sheath, Circumneurium, Dura mater, Epimysium, Epineurium, Long thoracic nerve, Paraneural cyst, Paraneural sheath, Phrenic nerve, Pia mater, Posterior scalene muscle, SPosterior sensory spinal root, Prevertebral layer of deep cervical fascia, Scalene minimi muscle, Subcircumneural space, Subepimyseal space, Subparaneural space, Ventral middle scalene muscle..
* Corresponding author André P Boezaart Professor of Anesthesiology and Orthopaedic Surgery, Departments of Anesthesiology and Orthopaedic Surgery, Division of Acute and Peri-operative Pain Medicine, University of Florida College of Medicine, 1600 SW Archer Road, Gainesville, Florida 32610, USA; Tel: 352-846-0913 (Work); Fax: 352-392-7029; E-mail: [email protected]. Introduction
For effective regional anesthesia and nerve blocks in acute pain medicine, it is not only vital to understand the cross-anatomical positioning of plexuses and nerves (macroanatomy) and how to find the nerves (sonoanatomy and functional anatomy), but also what membranes and tissue layers surround them and how the nerves are connected to each other and to the central nervous system.
This becomes especially crucial in acute pain medicine, where not only the primary block, during which a high volume of a high concentration of local anesthetic agent is usually injected, but also the secondary block must succeed. The secondary block comes into play when the effect of the primary block wears off; the continuous (secondary) block must take effect thereafter. Local anesthetic agents have their effect by blocking the electrical activity of the nerve axons (by blocking the sodium channels) and can only reach the axons after being directly injected onto the axons in the same fascial compartment, which has certain inherent dangers, or by diffusing through the various tissue layers to the axons against a concentration gradient. This gradient is not constant or static, but changes regularly as the blood stream continuously and at a variable rate, depending on the regional blood flow, removes the drug on both sides of the tissue membrane.
Drug diffusion over membrane and fascia barriers follows Fick’s Law [1] (simplified as dQ/dt = P × ΔC, where dQ/dt is the rate of diffusion (flux), P is the permeability constant of the drug and membrane conductance, and ΔC is the concentration gradient over the membrane. This is simplified, but the membrane conductance (P) will obviously change as the thickness and permeability of the membrane changes and the permeability constant of the drug changes (see also http://en.wikipedia.org/wiki/Fick's_laws_of_diffusion).
There is an optimal place where a local anesthetic drug must be placed for optimal primary block as used in regional anesthesia, or where a catheter for continuous nerve block has to be placed for continuous nerve block for ongoing pain. Over the years, this has been referred to as the “sweet spot of the nerve,” a term with different meanings at different times. In the era when nerve blocks were mainly done for local and regional anesthesia for surgery, the quest was to have a safe and dense motor and sensory block with a fast onset and relatively long duration of action for use as the sole anesthetic, replacing general anesthesia. At that time, the practitioners of yesteryear used paresthesia to identify the “sweet spot” and the phrase “no paresthesia, no anesthesia” was coined [2].
In later years, when nerve stimulators become popular in identifying the “sweet spot” of the nerve, different currents were proposed to explain why certain nerve blocks were more successful than others with their quicker onset and longer duration than others using the same dose of local anesthetic agent. It was generally thought that a nerve stimulator output between 0.2 and 0.5 mA put the needle tip on the correct side of the relevant membranes around the nerves, but not deep enough so that the needle was inside the nerve fascicles where the raw unprotected axons are. This notion was never met with undisputable scientific verification, but was popularized by expert opinion. Experience over many years taught us that this was indeed most probably the case.
Enter the era of continuous nerve block, first in an effort to improve regional blood flow, and later for the management of ongoing acute perioperative pain [3]. Initially, it was thought that if a catheter were placed near enough to a nerve [4], the continuous secondary block would be successful. This was met with failure and disappointment; it was later realized that the catheter also had to be placed in the “sweet spot” of the nerve [3]. Practitioners applied ...
