Ulnar Neuropathy

As has been implied above, UNP may refer to a variety of conditions that share common features: affected individuals often claim pain, motor deficits, and sensory alterations. Of note, intense pain may limit mobility and thus mimic motor dysfunctions such as reduced grip strength. Sensory deficits may comprise, but are not limited to, numbness, paresthesias, and dysesthesias. Determined arm positions and movements such as extension, flexion, or rotation, may exacerbate complaints. The palpation of compromised segments may elicit pain. Extensive knowledge regarding the anatomical structures innervated by the UN, their position and function, are required to associate a clinical picture of upper limb neuropathy with lesions of the UN. To this end, the following disorders should be considered:

• Compressive entrapment of the UN: cubital tunnel syndrome, Guyon's canal syndrome/ulnar tunnel syndrome [3] [4]
• Metabolic disorders manifesting in form of peripheral neuropathy or polyneuropathy, e.g., diabetic neuropathy [5]
• Infectious and inflammatory conditions
• Traumatic lesions of the UN
• Mass lesions as caused by cysts or tumors [6]
• Deformities associated with advanced-stage rheumatoid arthritis and osteoarthritis, among others [7]
• Developmental anomalies, e.g, Martin-Gruber anastomosis, Riche-Cannieu anastomosis [8]

These categories are not mutually exclusive. For instance, space-occupying lesions may exert mass effects and trigger compressive entrapment of the UN [9].

The diagnosis of UNP is based on anamnestic and clinical data. Results obtained in a general examination may also indicate an underlying pathology, but additional diagnostic measures are usually required to identify the primary disorder. In this context, diagnostic imaging plays a primordial role: plain radiography is mainly carried out to assess osseous structures, but the UN cannot be visualized using this technique. Therefore, sonography is to be preferred [1]. For high-resolution images of soft tissues, magnetic resonance imaging may be conducted [10]. UN function may be assessed by means of nerve conduction tests, which are generally combined with electromyographic studies. Moreover, laboratory analyses of blood samples (blood counts, blood chemistry, immunohistochemical studies) are recommended and may reveal enhanced inflammatory parameters and the presence of autoantibodies, among other pathological alterations.

The ulnar nerve (UN) originates from the brachial plexus and descends along the medial side of the upper limb. Its proximal segment is embedded in the medial bicipital groove of the upper humerus, and more distally, the UN pierces through the medial intermuscular septum to reach the epicondylar groove located posterior to the medial epicondyle of the humerus. Here, the UN reaches its most superficial position, which is best illustrated by this well-known scenario: an unfortunate bump to the elbow may provoke a tingling sensation along the respective forearm and hand if the UN is struck. Subsequently, the UN enters the cubital tunnel, a fibro-osseous channel delimited by the medial epicondyle of the humerus (medially), the olecranon (laterally), the elbow joint capsule and medial collateral ligament (anteriorly), and Osborne's ligament (posteriorly) [1]. Distal to the elbow, the UN travels between the humeral and ulnar heads of the flexor carpi ulnaris muscle and above the flexor digitorum profundus muscle. In close proximity to the wrist, the UN divides into a superficial sensory and deep motor branch. On its way towards the fourth and fifth digit, the latter has to pass through the Guyon's canal (also referred to as ulnar canal), which extends over approximately 4 cm, from the pisiform to the hook of the hamate [2]. It is further confined by the flexor retinaculum of the carpal tunnel, pisohamate and palmar carpal ligament, palmaris brevis and abductor digiti minimi muscle.

The UN contains both motor and sensory nerve fibers and gives off branches to innervate the following structures:

• Flexor carpi ulnaris and flexor digitorum profundus muscle (motor control)
• Third and fourth lumbrical muscles, hypothenar muscles, palmar and dorsal interossei muscles, and adductor pollicis muscle (motor control)
• Medial half of the palm, medial half of the back of the hand (sensation)
• Fifth digit and ulnar side of the fourth digit (sensation)

Ulnar neuropathy (UNP) is a general term referring to lesions of the UN without specifying the site, degree, and cause of injury. Consequently, any of the aforementioned anatomical structures may be compromised. The precise clinical picture may allow to narrow down the possible sites of UN damage.

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