What is the anatomy, physiology, and management of carpal tunnel syndrome?

Anatomy and Pathophysiology of Carpal Tunnel Syndrome

Carpal tunnel syndrome results from compression of the median nerve as it passes through the rigid carpal tunnel at the wrist, leading to a combination of mechanical trauma, increased pressure, and ischemic nerve damage. 1, 2

Anatomical Structure

The carpal tunnel is a confined anatomical space at the wrist where the median nerve travels alongside flexor tendons. 3, 4 Key anatomical features visible on imaging include:

• The median nerve normally maintains a rounded contour, but in CTS becomes enlarged proximally and flattened distally as it passes through the tunnel 5
• The flexor retinaculum (transverse carpal ligament) forms the roof of the tunnel and characteristically bows volarly when pressure increases within the carpal tunnel 5
• Anatomic variants that predispose to CTS include bifid median nerve and persistent median artery, both identifiable on ultrasound 5

Pathophysiological Mechanism

The disease process involves multiple interconnected mechanisms:

• Mechanical compression is the primary injury mechanism, with the median nerve subjected to direct pressure within the rigid boundaries of the carpal tunnel 1, 2
• Elevated carpal tunnel pressure creates both mechanical trauma and ischemic damage to nerve fibers 2
• Demyelination occurs as a consequence of chronic compression, affecting nerve conduction 2
• Both small and large nerve fibers are affected, though the pattern and timing of involvement varies between patients 2
• Ectopic impulse generation develops in compressed nerve segments, contributing to paresthesias and pain 2

Clinical Manifestations Based on Nerve Distribution

The median nerve provides specific sensory and motor innervation that explains the symptom pattern:

• Sensory symptoms affect the palmar aspect of the thumb, index and middle fingers, and radial half of the ring finger - the classic median nerve distribution 6
• Motor involvement causes decreased grip strength and hand weakness as thenar muscle innervation becomes compromised 3
• Symptoms typically include burning, tingling, or numbness in the affected distribution 3

Epidemiology and Risk Factors

• CTS affects 3-6% of the adult population, making it the most common entrapment neuropathy 3
• Prevalence has increased dramatically in recent decades, paralleling increased technology usage and repetitive hand activities 3
• Risk factors include obesity, monotonous wrist activity, pregnancy, genetic predisposition, and rheumatoid inflammation 1
• Bilateral CTS may indicate systemic conditions or amyloidosis, particularly cardiac amyloidosis when occurring without rheumatoid arthritis or known trauma 5, 7

Diagnostic Imaging Findings

Ultrasound Features

• Median nerve cross-sectional area ≥10 mm² at the carpal tunnel inlet is the primary diagnostic criterion, with high sensitivity and specificity 5
• Nerve enlargement and flattening are additional supportive findings 5
• Space-occupying lesions and tendon pathology (such as tenosynovitis) can be identified as contributing factors 5

MRI Characteristics

• MRI demonstrates nerve enlargement, flattening, and flexor retinaculum bowing with high accuracy 5
• MRI can stage disease severity based on structural nerve alterations with moderate accuracy 5
• However, MRI is typically not indicated for routine evaluation due to cost and availability considerations 5

Common Pitfalls

• Idiopathic CTS is the most common presentation, but failing to recognize bilateral CTS as a potential marker of systemic disease (especially amyloidosis) can delay critical diagnoses 5, 7, 1
• Overlapping symptoms with other conditions (such as cervical radiculopathy or proximal median nerve compression) require careful clinical differentiation 3
• The disease typically progresses rather than spontaneously regressing, so early recognition is important 4