It can also evaluate for additional disease processes, neurologic disorders, and soft-tissue masses. Magnetic resonance imaging (MRI) can provide fine detail that may not be visible on ultrasound and can also identify which muscles have been affected. It can also be highly beneficial in the early identification of obvious nerve disruption and hastening early surgical intervention for these cases. Ultrasound can be a low-cost, low-risk modality that can assist with visualizing the nerve and identifying areas of damage or disruption.
Electromyography and nerve conduction studies are able to localize lesions anatomically, which can help differentiate between cervical radiculopathies, brachial plexopathies, and peripheral neuropathies. However, additional diagnostic tools can be helpful in evaluating alternative causes and complications as well as for predicting prognosis.
The evaluation and diagnosis of Saturday night palsy are primarily clinical, and many patients that have a clear history and physical exam may not require further diagnostic measures. Additionally, patients may exhibit loss of the triceps reflex, which is controlled by radial nerve innervation. Sensory deficits will often involve the posterior or lateral upper arm, with symptoms extending distally to affect the posterior forearm, posterior hand, and posterolateral aspect of the lateral three-and-a-half digits. Providers should be aware that patients can still extend their fingers at the level of the proximal and distal interphalangeal joints, as these are controlled by the ulnar nerve. Patients also lose the ability to extend the thumb, resulting in difficulty with opening the hand and grasping objects. This leads to an inability to extend the wrist and fingers at the level of the metacarpophalangeal joints. On physical exam, patients may demonstrate a characteristic wrist drop, which results from the loss of extensor muscle function controlled by the radial nerve branches and preservation of the flexor muscles controlled by other nerves in the hand and arm. Patients may report numbness, weakness, tingling, pain, or any combination of these. Symptoms can also begin several days after the initial insult, leading to a delayed presentation. It is possible that patients may not provide this info until prompted, as it can go unrecognized as the inciting event. Otherwise, patients may report some other mechanism by which compression would have been unnaturally placed on the upper medial arm or axilla. Patients will often report symptom onset after consuming a large amount of alcohol and then sleeping in an unnatural position. This includes but is not limited to compressive clothing or accessories, improper use of crutches, prolonged blood pressure cuff usage, and more. While these are the classically described presentations, one must be aware that Saturday night palsy can be caused by any unnatural positioning or use of the limbs that can cause compression by a similar mechanism. Honeymoon palsy, on the other hand, refers to an individual falling asleep on the arm of another and consequently compressing that person's nerve. Saturday night palsy classically involves an individual falling asleep with the arm hanging over a chair or other hard surface, leading to compression within the axilla. Intoxicated individuals may not retain the reflexive ability to correct their position while asleep. Both names suggest a scenario where immobilization in an unnatural position can result in prolonged compressive damage onto the radial nerve. Saturday night palsy has in other instances been referred to as "honeymoon palsy".
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