Wrist drop refers to an impairment in hand extension at the wrist. There is usually concurrent finger extension weakness (“finger drop”). The most common localizations are peripheral processes, such as a C7 radiculopathy or radial nerve compression at the spiral groove (“Saturday night palsy”).
The rapidity of onset and the duration of weakness should be assessed. Primary motor cortex has significant representation of hand movement, and abrupt onset wrist drop and hand weakness can be seen with cerebral infarction or hemorrhage affecting the contralateral frontal lobe (“hand knob”). When concurrent new-onset neck or radicular pain is present, a C7 radiculopathy is most likely.
On clinical examination, a C7 radiculopathy cannot be distinguished from a middle trunk plexopathy, as both have a combination of wrist extension, arm extension, and forearm pronation weakness and numbness of the middle finger. In this instance, nerve conduction studies and needle electromyography (NCS/EMG) are indicated. NCS should demonstrate normal sensory responses in the setting of radiculopathy. In a middle trunk plexopathy, the radial sensory response should be abnormal.
A radial neuropathy with arm extension weakness indicates that the lesion must be in the axilla, which is proximal to the innervation of the triceps muscle. A radial neuropathy that spares the triceps is most commonly seen as part of a Saturday night palsy, in which the patient experienced prolonged compression of the radial nerve at or near the spinal groove, distal to the innervation of the triceps. This results in numbness of the dorsal forearm and hand, and weakness of the wrist and finger extensors and the brachioradialis muscle; elbow extension is spared. The radial nerve divides into a superficial radial sensory nerve that conveys sensation on the dorsal lateral hand, and the posterior interosseous nerve, which innervates wrist and finger extensors. The brachioradialis muscle is innervated prior to this division, thus a patient with finger extension and wrist extension weakness without weakness of either arm extension (triceps) or elbow flexion when the forearm is half pronated (brachioradialis) likely has a posterior interosseous neuropathy.
A posterior cord plexopathy is a rare cause of wrist drop and can be easily confused with a radial neuropathy at the axilla as both cause weakness of arm extension, elbow flexion when the forearm is half pronated, wrist and finger extension and numbness of the dorsal forearm and lateral hand. A posterior cord plexopathy should be suspected when there is also arm abduction weakness, since the axillary nerve also arises from the posterior cord.