Mononeuropathy

Mononeuropathy refers to the damage or dysfunction of a single peripheral nerve, which includes any cranial nerve, spinal nerve, or nerve branch that connects the central nervous system (i.e., the brain and spinal cord) to the entire body. Most mononeuropathies cause both motor and sensory impairment, usually affecting the hands, arms, or feet. 

For most mononeuropathies that have an acute presentation, the underlying cause may be difficult to define since they often resolve rapidly and provide no opportunity for a complete examination. Nonetheless, mononeuropathies are usually the result of local nerve damage caused by compression-related events that are classified as either fixed or transient. 

Fixed mononeuropathies are mainly induced by nerve compression against a hard surface, such as a tumor, cast, or even a prolonged cramped posture (e.g., during hospitalization or surgery). As a result, there may be a change in sensation, and sometimes, in movement, of the affected area. Fixed mononeuropathies usually affect superficial nerves like the radial nerve, which extends along the forearm, or the peroneal nerve, which extends into the lower leg. Fixed mononeuropathies are more often noticed in very thin people, as the direct compression of nerves against bones can lead to the condition. 

The continuous pressure that causes fixed mononeuropathies can progressively thin the myelin sheath, the protective cover of the nerve, through a process called segmental demyelination. If the pressure is persistent, it may cut through the myelin sheath and injure the nerve axon. This is referred to as Wallerian degeneration, and it can also occur due to local injury, like a deep cut through a nerve. 

Additionally, the entrapment of nerves in narrow anatomical spaces can cause fixed mononeuropathies. For example, carpal tunnel syndrome describes when the median nerve is trapped, and cubital tunnel syndrome is when the ulnar nerve is trapped. Multiple risk factors can lead to the entrapment of a nerve in a restricted anatomical structure, including pregnancy and hypothyroidism (i.e., low levels of thyroid hormones), as well as rheumatoid arthritis, which causes swelling and stiffness of the affected joints.

Contrary to fixed mononeuropathies, transient mononeuropathies are triggered by repetitive actions that cause trauma, like aggressive muscular activity or sudden overextension of a muscle. 

The clinical presentation of mononeuropathy mainly depends on its underlying cause and the particular nerve that is impacted. In most cases, however, all sensory and motor symptoms are typically associated with the single nerve that is affected. Some clinical presentations of mononeuropathies can be abrupt and very painful, while others may present gradually and intensify over time. Transient mononeuropathies show symptoms that are evident only with the motion that originally damaged the nerve, whereas the symptoms of fixed mononeuropathies are more persistent. 

Oftentimes, individuals with mononeuropathy complain about changes in sensation, numbness, and paresthesia, a tingling sensation that feels like pins and needles in their feet or hands. Mononeuropathy can also cause pain, experienced as a stabbing or burning sensation. In rare instances, individuals with mononeuropathy may experience complete sensory loss.

To treat mononeuropathy, it is important to diagnose and treat its underlying cause. For most mononeuropathies that present with mild symptoms, symptoms can often resolve with adequate rest, application of heat, avoidance or removal of the causative activity, and moderate use of nonsteroidal anti-inflammatory drugs (NSAIDs). In certain circumstances, immobilizing the affected area with medical equipment, such as splints or braces, may improve symptoms. In addition, oral or injected corticosteroids and therapeutic ultrasounds may be suggested in order to reduce pain, increase circulation, and improve mobility. The newest treatment used to relieve carpal tunnel syndrome is hydrodissection under ultrasound guidance, a non-surgical procedure that relieves tension by using a solution to separate the affected nerve from the surrounding tissues.

In the case that conservative treatments are unable to resolve the associated condition, surgical decompression may be required. For instance, when external pressure is applied by a tumor to a nerve, surgery is the only way to relieve any symptoms. Notably, surgical procedures should also be considered in cases where progression of a mononeuropathy occurs despite conservative treatment.