What are the causes of median nerve (MN) entrapment at the wrist, also known as carpal tunnel syndrome (CTS)?

From the Guidelines

Median nerve entrapment at the wrist is primarily caused by compression of the median nerve as it passes through the carpal tunnel, with common causes including repetitive hand movements, medical conditions like pregnancy and diabetes, and anatomical factors such as wrist fractures or bone spurs. The causes of median nerve entrapment can be broadly categorized into:

• Repetitive hand movements and prolonged wrist flexion or extension, particularly in occupations requiring repetitive hand motions like typing, assembly line work, or using vibrating tools 1
• Medical conditions that contribute to median nerve entrapment, including pregnancy (due to fluid retention), diabetes, rheumatoid arthritis, hypothyroidism, and obesity
• Anatomical factors such as wrist fractures, dislocations, or bone spurs that can reduce the space within the carpal tunnel, as well as wrist tenosynovitis or ganglion cysts
• Demographic factors, with women being more susceptible due to naturally smaller carpal tunnels, and genetic predisposition playing a role in some cases
• Aging, which increases risk as the protective tissues around the median nerve deteriorate over time Understanding these causes is essential for both prevention and treatment, which may include ergonomic modifications, wrist splinting, anti-inflammatory medications, corticosteroid injections, or in severe cases, surgical decompression through carpal tunnel release, with surgery being more effective than non-surgical treatment in most outcome measures, according to the best available evidence 1.

From the Research

Causes of Median Nerve Entrapment at Wrist

• The median nerve can be entrapped at the wrist due to various factors, including:
  • Chronic compression of the median nerve at the wrist within the space-limited carpal tunnel 2
  • Anatomical variations, such as a bifid median nerve, which can increase the cross-sectional area of the nerve and facilitate compression 3
  • Risk factors, including female gender, obesity, work-related factors, and underlying medical conditions, such as hypothyroidism, pregnancy, and amyloidosis 2
  • Trauma, repetitive maneuvers, and certain diseases, such as rheumatoid inflammation 4, 5
• The pathophysiology of median nerve entrapment at the wrist involves a combination of mechanical trauma, increased pressure, and ischemic damage to the median nerve within the carpal tunnel 5
• Other conditions, such as pronator syndrome and anterior interosseous syndrome, can also cause median nerve entrapment and should be considered in the diagnosis 6

Anatomical Variations

• Anatomical variations of the median nerve, such as a bifid median nerve, can occur frequently and may lead to diagnostic confusion if not recognized 3, 6
• These variations can be classified into different groups, including:
  • Group 0: extraligamentous thenar branch (standard anatomy)
  • Group 1: variations of the course of the thenar branch
  • Group 2: accessory branches at the distal portion of the carpal tunnel
  • Group 3: divided or duplicated median nerve inside the carpal tunnel
  • Group 4: accessory branches proximal to the carpal tunnel 3

Risk Factors

• Risk factors for median nerve entrapment at the wrist include:
  • Obesity 2, 5
  • Monotonous wrist activity 5
  • Pregnancy 2, 5
  • Genetic heredity 5
  • Rheumatoid inflammation 5
  • Female gender 2
  • Work-related factors 2
  • Underlying medical conditions, such as hypothyroidism and amyloidosis 2