Cutaneous Innervation

Clinical definition of the distribution of sensory changes affecting a specific area within one extremity implies a single nerve root, portion of a plexus, or peripheral nerve damage. Focal complaints involving a few digits or a distinct area within an extremity (such as the lateral outer thigh), almost always define a peripheral nervous system (PNS) lesion, either the cervical or lumbosacral nerve root or a specific mononeuropathy.

A careful neurologic examination allows differentiation of specific anatomic sites of involvement. Finger numbness is a very familiar clinical neurologic complaint. The carpal tunnel syndrome results from median nerve entrapment at the wrist; this is the most common mononeuropathy. Here, sensory symptoms are predominant early in this disorder. The clinical history of nocturnal awakening or symptoms present on awakening in the morning or precipitated by various activities, particularly driving or working with the hand, helps the clinician differentiate this disorder from other somatosensory system lesions. Sometimes the patient provides the most accurate assessment by roughly defining the area in question using a finger to outline the area of diminished sensation. Often, it then becomes clear that the pattern of sensory loss fits the distribution of a particular peripheral nerve or nerve root dermatome.

Unfortunately, sometimes the symptom distribution is not clearly defined by patients, particularly those who are not good self-observers. Often, they are unable to decide whether their symptoms involve the classic lateral 3.5 digits of the hand, namely the thumb, index, and middle fingers, and the lateral aspect of the ring finger, typical of a median nerve lesion, versus all fingers—raising the possibility of a cerebral cortical lesion. However, when there is concomitant focal weakness with numbness, it is much easier to differentiate a peripheral nerve lesion from a nerve root lesion.

Median nerve lesions primarily affect the palmar aspects of fingers 1 to 3.5; the dorsal tips of these fingers are also compromised to the distal interphalangeal joints (DIPs). In contrast, ulnar neuropathies also manifest with finger numbness but with a different anatomic distribution involving the medial 1.5 fingers, specifically the entire little (fifth) finger, and the medial aspect of the ring (fourth) finger. The ulnar nerve is the only nerve in the hand to equally affect the palmar and posterior portions of the fingers and hand. Parenthetically, an early medial cord or lower trunk brachial plexus lesion may also present with numbness mimicking the ulnar nerve, that is, the medial fingers of the hand. Although a C-8 radiculopathy has a similar sensory distribution, it is usually accompanied by significant neck pain. The radial nerve primarily innervates the dorsum of the proximal thumb, index, middle, and lateral half of the ring finger to the DIPs, as well as the dorsum of the hand in continuity with these fingers, thus sparing the fingertips because of their full median innervation. Thus when the finger tips are involved, there is either a median or ulnar nerve lesion or a nerve root lesion present.

A cervical radiculopathy, C6 to C8, is the other common disorder that leads to numbness developing in the fingers. The C6 and C7 nerve roots are the two most commonly compromised sites at the nerve root level. Often, the history of nerve root impingement is relatively abrupt in onset, commonly preceded by intrascapular or neck pain. Sometimes the patient may not present to their physician until after the painful lesion has resolved and its history is then no longer important to the individual and thus forgotten. Annoying finger numbness may be the only clinical residua of a recent C6, C7, and/or C8 nerve root irritation. In contrast to a peripheral nerve lesion affecting primarily the palmar surface, such as the median nerve in the carpal tunnel syndrome, a C6 or C7 nerve root lesion will compromise sensation of both palmar and posterior finger surfaces. Thus three separate and distinct peripheral nerves, namely the median, ulnar, and radial, as well as C6 to C8 radiculopathies, and most uncommonly a medial plexus lesion, when affected, can manifest with a numb finger.

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