What are the types of mononeuropathy?

Types of Mononeuropathy

Mononeuropathies are classified into three main categories: single mononeuropathy (affecting one nerve), mononeuritis multiplex (affecting multiple individual nerves in an asymmetric pattern), and compressive/entrapment neuropathies (affecting nerves at specific anatomical sites). 1, 2

Single Mononeuropathy

Single mononeuropathy involves isolated damage to one peripheral nerve and can affect both cranial and peripheral nerves.

Upper Extremity Mononeuropathies

• Median neuropathy (carpal tunnel syndrome) is the most common mononeuropathy overall, with a prevalence of 50 per 1,000 in the United States and 5% in the general population 2, 3
• Ulnar neuropathy at the elbow is the second most common entrapment neuropathy, with an incidence of 20.9% 2
• Radial neuropathy commonly occurs from compression or trauma 2, 4

Lower Extremity Mononeuropathies

• Peroneal (fibular) neuropathy is the most common compressive neuropathy of the lower extremity 2
• Tibial neuropathy affects the posterior tibial nerve, often at the tarsal tunnel 2
• Femoral neuropathy involves the femoral nerve, typically from compression or stretch injury 2

Cranial Nerve Mononeuropathies

• Oculomotor nerve (CN III) palsy presents with ptosis, mydriasis, and impaired eye movement 5
• Trochlear nerve (CN IV) palsy causes vertical diplopia 5
• Abducens nerve (CN VI) palsy results in horizontal diplopia and impaired lateral eye movement 5
• Cranial mononeuropathies are relatively uncommon compared to peripheral nerve involvement 6

Mononeuritis Multiplex

Mononeuritis multiplex represents asymmetric involvement of multiple individual nerves or nerve roots, distinguished from polyneuropathy by its non-length-dependent, patchy distribution.

Clinical Characteristics

• Asymmetric sensory and/or motor deficits affecting multiple individual nerves in a non-contiguous pattern 1, 7
• Pain is a prominent feature, particularly when caused by inflammatory or vasculitic processes 1, 7
• Can involve cranial nerves, causing focal deficits like facial weakness or visual disturbances 1

Pathophysiological Distinction

• The asymmetric presentation suggests different underlying pathology than symmetric polyneuropathy, such as vasculitic neuropathy or inflammatory processes 1
• May require immunosuppressive therapy if caused by inflammatory processes, unlike typical polyneuropathy management 1, 7
• MRI may show nerve root enhancement or thickening in some cases 1

Compressive and Entrapment Mononeuropathies

These result from mechanical injury to nerves at specific anatomical sites where they are vulnerable to external pressure or internal anatomical constraints.

Mechanisms of Injury

• Compressive neuropathy results from acute or chronic repetitive external pressure 4
• Entrapment neuropathy occurs from chronic distortion or angulation of the nerve from an internal anatomical source 4
• Other mechanisms include crush injury, stretch injury, and transection 4

Clinical Patterns

• Median and ulnar mononeuropathies typically have gradual onset and affect the dominant limb 6
• Other mononeuropathies (peroneal, radial, femoral) tend to have acute onset with no predilection for either side 6
• When bilateral involvement of the same nerve is excluded, multiple mononeuropathies are relatively uncommon (found in only 5 of 51 diabetic patients in one study) 6

Important Clinical Distinctions

Relationship to Systemic Neuropathy

• Diabetic mononeuropathy can occur independently of peripheral and autonomic neuropathy, with almost half of patients showing no significant background neuropathy on objective testing 6
• However, minimal background nerve damage present in diabetic patients may render them more susceptible to factors causing mononeuropathy 6
• The distinction between mononeuropathy and polyneuropathy is crucial as they often have different underlying causes requiring different treatment approaches 1

Diagnostic Approach

• Nerve conduction studies are essential to document the pattern of involvement and distinguish focal from diffuse processes 1, 7
• Measurement of distal latency, amplitude, conduction velocity, and identification of focal conduction block help localize focal mononeuropathies 4
• Needle electrode examination provides information about distribution of nerve damage, activity and chronicity of axon loss, and can help date the onset of injury 4