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  Treatment       Peripheral Nerve Blocks      
 
 
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Introduction

Peripheral nerve blocks are used in the pain relief clinic and operating theatre to block pain in the following ways:-

  • Help avoid general anaesthesia during surgery.
  • Acute pain control for painful conditions like fractured ribs, fractured hips etc.
  • Treat Neuralgic pain caused by an injury / disease process affecting the peripheral nervous system.
  • Nerve pain caused by cancer.

You can learn more about the causes of peripheral neuralgic pain by looking at the following pages:-

  • Peripheral Neuropathy - neuralgic pain caused by a number of disease processes / toxins which affect the function of different components of peripheral nerves.
  • Entrapment Neuropathy - neuralgic pain caused by peripheral nerves being trapped by different anatomical structures in the body.
  • Post Traumatic Neuropathy - neuralgic pain which comes on after trauma or surgery.
  • Post Herpetic Neuralgia - neuralgia which arises due to damage to the peripheral and central nervous system by the herpes virus.
  • Phantom Limb Pain - neuralgia which comes on following traumatic or surgical amputation of a limb.
  • The term nerve block can mean a number of different things. In general it involves doing something to a nerve that alters the way in which it able to transmit painful signals - this is called a conduction block and is a form of neuromodulation. Nerve blocks can be short or long term depending on which technique is used.

The following techniques are commonly used:-

  • Local anaesthetic peripheral nerve injection
    • This gives a relatively short length of action, but can be useful in helping to diagnose the cause of the pain.
    • The aim is to get the local anaesthetic as close to the nerve as possible to ensure that as much of the nerve as possible is exposed to it.
    • The block is usually done somewhere between the source of the nerve injury and it's connection to the spinal cord.
    • If repeated several times, some patients notice a significant reduction in their neuralgic pain which outlasts the theoretical length of action of the local anaesthetic. This may be due to some form of neuromodulation in the dorsal horn on the spinal cord.
  • Local anaesthetic peripheral neuroma injection
    • When a nerve is severed, it tries to reconnect with the other severed end but is not able to. The result is a benign nerve growth (neuroma ) which looks like the shape of a piece of broccoli.
    • Neuroma's are more common after trauma, surgery, and amputation and are sensitive to direct pressure. They fire off electrically in a totally random way causing peripheral neuralgic pain.
    • Their activity can sometimes be dampened down by directly applying local anaesthetic around them an several occasions.
  • Radiofrequency techniques (RF)
    • Application of a high frequency electrical signal to a nerve causes it to heat up.
    • Classical RF lesioning involves the nerve being "cooked" at 80 deg C for 60 - 90 seconds, causing nerve coagulation and disruption (rhizotomy) with permanent interruption of pain signal conduction.
    • A newer technique called pulsed radiofrequency (PRF) gently warms the nerve to 42 deg C for 10 -15 minutes. This technique does not seem to cause nerve destruction, but still achieves neuromodulation of the pain.
  • Neurolytic peripheral nerve injection
    • This technique is usually reserved for those patients with terminal conditions like cancer, and who are close to the end of their life.
    • Nerve destructive substances like phenol or absolute alcohol are applied directly to the nerves which supply the part of the body responsible for causing the pain.
    • The neurolytic solution acts by causing coagulation of the vasa nervorum (small vessels supplying the nerves). The nerve then ceases functioning and dies off secondary to oxygen starvation.
    • The effect can last 6 - 8 weeks and therefore needs to be timed carefully, as a chemical neuritis can develop afterwards which can be difficult to treat.
    • If the nerve is easily accessible, the block can usually be repeated.
Anatomy and Indications

Please look at the table below for information about the anatomy and indications for different nerve blocks in the body.

Region
Nerve
Diagram
Indications
Head
Greater Occipital
Occipital Neuralgia, Shingles
Supratrochlear, Supraorbital, Infraorbital
Facial Neuralgia, Shingles
Gasserian Ganglion
Trigeminal Neuralgia, Shingles, Facial Cancers
Maxillary, Mandibular
Trigeminal Neuralgia, Shingles, Facial Cancers
Neck
Superficial Cervical Plexus
After Neck Surgery, Trauma
Deep Cervical Plexus
After Neck Surgery, Trauma
Shoulder
Suprascapular
Chronic Shoulder Joint Arthritis
Arm
Brachial Plexus
Interscalene Approach
Brachial Plexus Injuries, Upper Limb Injuries, Pancoast Lung Cancer
Supraclavicular Approach
Infraclavicular Approach
Axillary Approach
Elbow
Median, Radial
Forearm and Hand Trauma, Neuralgia
Ulnar
Forearm and Hand Trauma, Neuralgia
Wrist
Median, Ulnar
Hand Trauma, Neuralgia
Dorsal Radial
Dorsal Radial Neuralgia, Trauma
Hand
Digital
Digital Neuralgia, Trauma
Chest
Intercostal
Intercostal Neuralgia, After Thoracotomy
Abdomen
Intercostal
Intercostal Neuralgia, After Nephrectomy
Intra-operative Pain Relief For Abdominal Surgery
Pelvis
Pudendal
Pudendal Neuralgia, Pain relief for forceps delivery in labour
Groin
Iliohypogastric, Ilioinguinal
After Hernia Surgery
Thigh
Sciatic Posterior Approach
Sciatic Neuralgia, Buttock, Thigh and Leg Trauma
Anterior Approach
Lateral Approach
Femoral
Femoral Neuralgia, Thigh Trauma
Obturator
Obturator Neuralgia, Pelvic Trauma
Lateral Femoral Cutaneous
Meralgia Paraesthetica
Knee
Common Peroneal, Posterior Tibial
Tibial, Ankle and Foot Trauma
Saphenous
Saphenous Neuralgia, After Varicose Vein Surgery
Ankle
Saphenous, Sural, Posterior Tibial, Superficial Peroneal, Deep Peroneal
Foot Neuralgias and Trauma
Foot
Digital
Mortons Neuroma, Trauma

Block Characteristics

The end result of a nerve block is predictable once you understand where the nerve goes, what it connects to, what type of nerve it is, and what has been injected around it.

The factors that determine the effect of a nerve block are:-

  • Type of local anaesthetic - the duration of action of these differ considerably:-
    • Lignocaine (Lidocaine, xylocaine) - 2 hours
    • Prilocaine (Citanest) - 4 hours
    • Bupivacaine (Marcaine) - 6 hours
    • Levobupivacaine (Chirocaine) - 6 hours
  • Dose of Local Anaesthetic - the duration of action is related to the total dose (mg) injected. The larger the volume and the higher the % concentration, the higher the dose and the longer the length of action.
    • Dose (mg) = Drug % x Volume (ml) x 10
    • Therefore 10 ml of bupivacaine 0.5% = 0.5 x 10 x 10 = 50 mg
  • Block Extenders - a nerve block tends to wear off because the local anaesthetic is absorbed into the local tissues / circulation, reducing the amount available to block the nerve. Adding a drug which reduces blood flow (vasoconstrictor) to the area has the effect of reducing tissue / circulation uptake, and therefore makes the block last longer. Weak doses vasoconstricting drugs like adrenaline and clonidine are often added to nerve blocks to prolong them.
  • Acidity of the solution - how easily a local anaesthetic penetrates a nerve depends on the acidity of the solution injected. In general making a solution less acidic by adding a small amount of alkali increases the ease of penetration into the nerve, and therefore improves the onset time. Sodium bicarbonate (alkali) can be added to an epidural injection during a caesarian section to speed up the onset time of the block and the delivery of the baby. Local anaesthetics do not work as effectively in the presence of infection, because the tissues in the infected area are more acidic, making it more difficult for the drug to penetrate the nerve.
  • Type of Nerve - blocking a sensory nerve will only result in loss of sensation, whereas blocking a mixed sensory / motor nerve can result in loss of sensation and loss of motor power (weakness).
  • Size of Nerve - smaller diameter nerves are more susceptible to being blocked when compared to larger diameter ones. Smaller nerves therefore have a faster onset time and vice versa for larger ones.
  • Degree of Nerve Myelination - within each main nerve there are individual nerves fibres which have differing characteristics (see table below). In general fibres with the smallest diameter and least amount of myelin , are the most susceptible to being blocked with local anaesthetic. These characteristics also explain why the order of onset of the block is Pain block 1st, Sensory block 2nd, and Motor block 3rd. The recovery of the blockade is in the reverse order ie. movement recovers first, followed by sensation, followed by pain.
Fibre Type
Function
Diameter
Myelin
Speed
Block Susceptibility
C
Slow Pain
Smallest
-
Slowest
++++
A-delta
Fast Pain
Small
+
Medium
+++
A-beta
Light Touch
Medium
++
Fast
++
A-alpha
Motor
Large
+++
Fastest
+

Complications
  • Failure to produce a block - missing the nerve can happen if the block is done "blind". Using a nerve stimulator improves the chances of success, and reduces the risk of damaging the nerve with the needle. Obesity is a definite cause of failure as sometimes we don't have needles long enough to reach the necessary location. Sometimes unusual anatomy can defeat even the most experienced operator.
  • Infection - can be prevented by taking normal sterile precautions. Nerve blocks shouldn't be done in the presence of local skin / tissue infection at the site of the injection. Although an infection may be far away from the site of the proposed nerve block, bacteria can still spread via the blood stream to the site of the block and cause an infection. My personal practice therefore is not to perform nerve blocks at all in the presence of infection anywhere in the body. There may however be some medical professionals who disagree with this opinion.
  • Bleeding - the block needle may have to pass through vascular tissues like muscles, or close to major blood vessels, to get to the necessary nerve. If the coagulation status is abnormal, this may produce local bruising. Severe bleeding around a nerve after the procedure can cause nerve compression with permanent nerve injury. If you are taking blood thinners (aspirin, warfarin, heparin etc), or have been found to have a bleeding tendency, then your doctor should be informed prior to performing the block.
  • Intravascular injection can occur when the nerve is anatomically close to a vein or an artery. Large doses of local anaesthetic injected quickly into these vessels causes grand mal seizures, brain stem depression affecting the respiratory and cardiovascular centres, and cardiac arrest. It is important therefore that all injections are performed slowly, and that the upper dose limit is adhered to, to help reduce this risk. The safety profile of the local anaesthetics is (safest first) Lignocaine > Prilocaine> Levobupivacaine > Bupivacaine. Levobupivacaine is fast replacing bupivacaine in hospital practice due to it's improved safety profile. Bupivacaine intravenously can cause DC shock resistant ventricular fibrillation requiring the use of intravenous mexiletine in doses up to 500 mg. Injecting intravenous boluses of Intralipid 10 - 20% has been show to improve survival in this situation. The fat gloules in the intralipid act as a local anaesthetic sump, helping to lower levels in the blood stream.
  • Unexpected spread to other nerves can occur following a nerve block. Mostly this is an irritating side effect that wears off with time without any further action required. Occasionally it may have more serious consequences, and therefore the operator has to be prepared for such an occurrence and take appropriate action. The reasons why local anaesthetic spreads to other nerves are outlined below:-
    • Needle slightly out of position during injection - e.g. a femoral nerve block may develop after performing an iliohypogastric block in the groin, causing unexpected leg weakness and a longer stay in hospital whilst it wears off. This happens because the local anaesthetic is injected too deeply under the transversus abdominis muscle.
    • High volumes of local anaesthetic may track along tissue planes to reach other nerves close by - e.g. phrenic and recurrent laryngeal block may develop after an interscalene brachial plexus block in the neck causing paralysis of one side of the diaphragm and hoarseness respectively. Diaphragmatic paralysis can be a problem in those with critical chest disease, but not in normal adults. Recurrent laryngeal block can cause food and liquids to be inhaled into the trachea (windpipe), and therefore the patient must fast afterwards until it wears off.
    • Close anatomical relationship - if one or more nerves is anatomically close to the one being blocked, whatever you do both nerves end up being blocked - e.g. the recurrent laryngeal nerve is always blocked at the same time as the cervical sympathetic chain during a stellate ganglion block.
  • Prolonged block - sometimes a block lasts longer than expected. This can be due to:-
    • Too high a dose - sitting it out for a few hours usually sees everything returning to normal again.
    • Pre-existing nerve injury - in this situation the effects of the block may last days or weeks with persisting numbness and weakness. I would suggest that blocks should be avoided in the presence of pre-existing nerve injury.
    • Excessive vasoconstrictor - this causes severe vasoconstriction with oxygen starvation to the nerve and subsequent permanent damage. Vasoconstrictors should not be used near end arteries (e.g. fingers, penis) due to the risk of producing gangrene.
    • Faulty positioning after the block can cause excessive pressure on a nerve whilst the area is numb. Prolonged pressure causes oxygen starvation and permanent nerve injury. The part to be blocked should always be protected / padded / positioned properly afterwards to prevent this happening.
    • Intra-neuronal injection - injecting the local anaesthetic directly into the nerve, rather than around it, causes injury to the nerve directly proportional to the volume of local injected. This complication can be avoided by the proper use of a nerve stimulator which finds the nerve electrically using a small current passed directly through the needle tip.
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