Age / Sex : 29 / M

Chief complaint : Right wrist drop, 1DA


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Courtesy : 문승규 (Sung Gyu Moon), 건국대병원 (Konkuk University Hospital)

Compressive neuropathy of the radial nerve (saturday night palsy)

Discussion

Findings:

Right upperarm MRI:

CI: right wrist drop after drunken sleep

Segmental swelling and T2-weighted signal increase of radial nerve at mid to distal humeral level across lateral intermuscular septum

   with segmental enhancement on postcontrast image

   -> c/w radial nerve mononeuropathy (saturday night palsy)

Discussion:

The radial nerve arises from the posterior cord of the brachial plexus. After exiting the axilla, the radial nerve travels around the posterolateral humeral shaft and descends dorsally along the spiral groove between the lateral and medial heads of the triceps muscle. It innervates the triceps, brachioradialis, extensor carpi radialis, and supinator muscles, as well as the skin along the posterior upper arm. The radial nerve enters the anterior compartment after piercing the lateral intermuscular septum approximately 10 cm proximal to the lateral epicondyle, and subsequently gives off superficial and deep branches

The term Saturday night palsy has been coined to describe a classic radial neuropathy secondary to intoxication, whereby sleeping with the arm draped over a chair causes direct pressure on the radial nerve. Nerve injury within the spiral groove may occur due to a fracture of the humeral shaft. Other causes of radial neuropathy include misuse of axillary crutches and deep intramuscular injections of the arm. Entrapment neuropathy can also be caused by a fibrous arch of the lateral head of the triceps muscle in weight lifters. Because the lateral head of the triceps muscle attaches to the lateral intermuscular septum and humerus, the point of entrapment occurs where the nerve passes through the septum located directly on the humeral cortex.

A proximal radial neuropathy in the spiral groove leads to functional loss of accessory forearm supination, elbow flexion, wrist and digital extension, and thumb abduction, as well as sensory loss in the dorsolateral hand. Elbow extension is spared because the branches to the triceps muscle come off proximal to the spiral groove, helping distinguish this neuropathy from a higher level of compression.

Acute or subacute radial neuropathy manifests with diffuse increased signal intensity on T2-weighted MR images in all or some of the muscles supplied by the radial nerve. Signal abnormalities in more proximal muscles such as the triceps, extensor carpi radialis longus, and anconeus muscles are consistent with high radial nerve entrapment. US, with its higher spatial resolution and capacity for allowing contralateral comparison, is more useful in this regard. With entrapment, the nerve is often focally swollen and uniformly hypoechoic, with loss of the normal fascicular pattern

The prognosis for Saturday night palsy depends on the extent of the injury, which is determined by the force and duration of compression. Mild damage results in neuropraxia, a transient conduction block without nerve degeneration. This type of injury will almost always result in complete recovery. Moderate damage results in axonotmesis, characterized by axonal damage and Wallerian degeneration that can have incomplete or late recovery. Severe damage results in neurotmesis, characterized by complete axon degradation and Schwann cell death with a low chance of full recovery. Patients with this degree of injury will often need surgical intervention. The degree of damage can be difficult to determine based on electromyography alone, and prediction of prognosis can be difficult early on. Recovery is not rapid, with even mild cases resolving at the earliest in 2-4 months and often much longer.

References:

Multimodality imaging of peripheral neuropathies of the upper limb and brachial plexus. Radiographics. 2010 Sep;30(5):1373-400. doi: 10.1148/rg.305095169.

Saturday Night Palsy. 2020 May 13. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–.