ASSIGNMENT ON: RADIAL NERVE PALSY

MedicoPlexus

ARJUN AJITHAN

Abstract 

Radial nerve is a nerve emerging from the posterior cord of the brachial plexus which in turn originating from the ventral roots of spinal nerves C5 to T1. To generalize the Radial nerve innervates the posterior portion of upper limb, forearm, and the overlaying skin area. This project serves to describe the anatomy and the palsy of the above-mentioned nerve. 

Fig. 1 represents the brachial plexus and radial nerve 

Origin and structure 

The radial nerve is a terminal branch of the posterior cord of the brachial plexus and arises posteriorly to the axillary artery in the axilla region. It leaves the axially inferiorly through the triangular interval to supply long and lateral heads of the triceps brachii muscle. The nerve then travels through the radial groove of the humerus along with the deep branch of the brachial artery and supplies a branch to innervate the medial head of triceps brachii. 

The radial nerve enters the forearm through the anterior to the lateral epicondyle and transverses through the cubital fossa and then terminates into two branches- the deep motor branch and the superficial sensory branch. 

Fig. 2 shows the path of radial nerve and some innervated structures 

Motor and Sensory Functions of the Radial nerve 

The posterior arm and forearm muscles are innervated by the radial nerve.  

It innervates the three heads of the triceps brachii which functions as to extension of arm at the elbow. It also give branches to supply the forearm muscles- the brachioradialis and the extensor carpi radialis longus.  

The cutaneous branches of the radial nerve- inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm originate in the posterior compartment of the arm. Both penetrate lateral head of triceps brachii and overlaying fascia to become subcutaneous. 

The superficial branch innervates the dorsum of hand, thumb, index finger, middle finger, and some portion of the ring finger. It is important to note that the nail beds of these fingers are innervated by the Median nerve rather than the radial nerve. The deep branch also called the posterior interosseous nerve is predominantly motor and passes between the supinator muscle and supply posterior muscles of the forearm. To oversimplify, the muscles that serve to extend the wrist and finger joints and to supinate the forearm are innervated by the deep terminal branch of the radial nerve.  

To summarize,  

1. The radial nerve innervates: 

• Triceps 
• Extensor carpi radialis longus 
• Extensor carpi radialis brevis 
• Brachioradialis 

2. And the posterior interosseous nerve supplies: 
   • Extensor digitorum 
   • Supinator muscle 
   • Extensor digiti minimi 
   • Extensor carpi ulnaris 
   • Abductor pollicis longus 
   • Extensor pollicis longus 
   • Extensor pollicis brevis 
   • Extensor indicis proprius 
3. The cutaneous sensory innervation provided by: 
   • Posterior cutaneous nerve of arm 
   • Posterior cutaneous nerve of forearm 
   • Superficial branch of radial nerve 
   • Inferior lateral cutaneous nerve of arm 

Fig. 3 visualizes radial nerve supply 

Radial nerve palsy 

An injury to the radial nerve at different levels can manifest different symptoms. The location of the injury can be classified into three categories. 

In the Axilla 

A radial nerve injury at the axilla is commonly due to a dislocation of the shoulder joint or a fracture of proximal humerus. It can also be the result of excess pressure on the nerve exerted by crutches or acute trauma as in ‘Saturday night palsy‘. 

Damage to the radial nerve at the axilla can lead to motor and sensory symptoms. 

• Motor: The triceps brachii and posterior compartment muscles are affected. The patient has inability to extend at the forearm, wrist, and fingers. Wrist-drop occurs as a result of unopposed flexion of wrist. 
• Sensory: All the cutaneous branch off radial nerve is affected and loss of sensation over lateral and posterior arm, posterior forearm, and dorsal surface of lateral three and a half digits. 

In the Radial Groove 

The radial nerve runs within the radial groove of the humerus. Therefore, a fracture to the humeral shaft usually results in nerve damage. 

• Motor: The triceps brachii is weakened but not paralyzed since branches supplying lateral and long heads arise proximally. Posterior forearm muscles are the most affected. Consequently, the patient is unable to extend at wrist and fingers. Wrist-drop is present. 
• Sensory: The superficial branch of the nerve is damaged, as a 

result loss of sensation in dorsum of hand and dorsal part of lateral three and a half digits occurs. Sensation in forearm and the arm is present since the cutaneous branches have already arisen. 

In the forearm 

Damage in the forearm may result in the injury of superficial branch and/or the deep branch. 

Typically, the superficial branch is damaged when lacerated. No loss of motor function occurs as a result. Sensory loss in the lateral three and a half digits and the associated area on the dorsum of the hand is present. 

A fracture to the radial head or posterior dislocation of the radius causes injury to the deep branch. Majority of the muscles in posterior forearm are affected. Wrist-drop is not seen since the extensor carpi radialis longus is unaffected and maintains extension. No sensory functions are lost. 

Diagnosis 

Radial nerve palsy can be diagnosed by physical examination, X-ray, MRI, or EMG. 

Conclusion 

Palsy of the radial nerve is a result of an injury to the radial nerve emerging from the brachial plexus. It is important to identify and differentiate this condition from others. Therefore, good anatomical knowledge is necessary for diagnosing and treating radial nerve palsy.