Upper Extremity Nerve Injuries: The Broken “ARM”
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Upper Extremity Nerve Injuries
Upper extremity nerve injuries may occur secondary to humerus fractures.
Knowing the upper extremity anatomy, as well as the nerve distribution and dermatomes of the arms, will help you understand which nerves may be damaged based on the location of the fracture.
Furthermore, understanding the motor and sensory innervation of each nerve will allow you to perform an appropriate physical exam.
As with every EZmed lecture, you will learn a simple way to remember the content.
We will use the “ARM” mnemonic to help remember which upper extremity nerves may be damaged from a humerus fracture based on the location of the injury.
We will also discuss the innervation, motor, and sensory components of the axillary, radial, and median nerves.
The “ARM” Mnemonic
The “ARM” mnemonic is simple to use.
As mentioned above, the mnemonic will help you recall which nerve can be damaged from a humerus fracture based on the location of the injury.
The mnemonic is easy to remember, simply think broken “ARM”.
Here is how it works.
First, divide the humerus into 3 sections as demonstrated by the image below.
You will then label each section “A-R-M”, beginning superiorly and moving inferiorly.
The superior portion will be “A”, the middle section will be “R”, and inferior portion will be “M”.
This will help you remember which nerves may be injured based on the location of the humerus fracture.
Let’s see the mnemonic in action!
A = Axillary Nerve
“A” stands for “Axillary” nerve.
This will help you remember that a fracture to the head of the humerus can lead to an axillary nerve injury.
Let’s now get a better understanding of the axillary nerve.
The axillary nerve is one of the terminal branches of the brachial plexus.
Innervation: Deltoid, Teres Minor
Motor:
Arm abduction beyond 15 degrees - Deltoid
Remember the initial 15 degrees arm abduction is primarily from the supraspinatus.
Lateral/external rotation of the arm - Teres Minor
Sensory: Inferior lateral deltoid region
Therefore, when assessing for axillary nerve injury, you may consider asking the patient to abduct their arm beyond 15 degrees (sometimes limited due to pain), or you may assess the sensory innervation over the deltoid region.
R = Radial Nerve
“R” stands for “Radial” nerve.
This will help you remember that a fracture to the mid-shaft of the humerus may damage the radial nerve.
Remember the radial groove, a shallow depression where the radial nerve and brachial artery traverse, is located at the mid-shaft of the humerus.
The radial nerve is also one of the terminal branches of the brachial plexus.
Innervation: Triceps brachii, all 12 muscles in the posterior forearm compartment, extrinsic extensor muscles of wrist and fingers
Motor:
Elbow extension - Triceps brachii
Wrist extension - Extensor carpi radialis longus/brevis, Extensor carpi ulnaris
Forearm pronation/supination - Brachioradialis and Supinator
Finger extension - Extensor digitorum, Extensor digiti minimi, Extensor indices, Extensor pollicis longus/brevis
Thumb abduction - Abductor pollicis longus
Sensory: Posterior distal arm, posterior forearm, and posterior thumb, index, middle, and radial half of the ring fingers as well as the associated dorsal hand area.
Therefore, assessment of radial nerve function may include asking the patient to extend their elbow, wrist, and/or fingers.
You may also assess the sensory components by lightly touching the designated posterior portions of the hand outlined above.
M = Median Nerve
“M” stands for “Median” nerve.
This will help you remember a supracondylar fracture, or a fracture to the distal third of the humerus, may lead to a median nerve injury.
The median nerve is also one of the terminal branches of the brachial plexus.
Innervation: Flexor muscles of forearm, flexor muscles of wrist, and thumb
Motor: Flexor compartment of the forearm, thenar muscles, and intrinsic hand muscles
Sensory: Skin over thenar eminence, the palmar aspect of the thumb, index, middle finger, and the radial half of the ring finger, as well as the dorsal side of the distal phalanx of index and middle finger and half the ring finger.
Therefore, assessment of the median nerve may include wrist and finger flexion, as well as a sensory exam correlating with the anatomical locations discussed above.
Summary
Hopefully the “ARM” mnemonic gave you a simple method for remembering the nerve injuries that can occur secondary to humerus fractures based on the location of the injury.
Remember the following:
A - Axillary n. - Fracture to the head of the humerus
R - Radial n. - Fracture to the mid-shaft humerus
M - Median n. - Fracture to the distal third of the humerus
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https://www.ncbi.nlm.nih.gov/books/NBK493212/
https://www.ncbi.nlm.nih.gov/books/NBK534840/
https://www.ncbi.nlm.nih.gov/books/NBK448084/#:~:text=Structure%20and%20Function,-The%20median%20nerve&text=The%20median%20nerve%20also%20provides,medial%20aspect%20of%20the%20forearm.
https://radiopaedia.org/articles/median-nerve-2?lang=us