Evaluation of Thenar Muscle Biomechanics and Thermal Sensory Thresholds in Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is the most common neuropathy resulting from the compression of the median nerve as it passes through the carpal tunnel. The carpal tunnel is a narrow passage bounded by the transverse carpal ligament superiorly and the carpal bones inferiorly. It houses nine flexor tendons and the median nerve. CTS develops due to mechanical trauma, high pressure, and ischemic damage affecting the median nerve. When intratunnel tissue pressure rises above physiological levels, it leads to microvascular ischemic injury, intraneural venous congestion, impaired axonal transport, and progressive epineurial-perineurial fibrosis.

In clinical practice, CTS typically begins with nocturnal paresthesia, numbness, and a burning pain in the area corresponding to the median nerve's sensory distribution (the volar aspect of the first three digits and the radial half of the fourth digit). Patients often find relief by shaking their hands. As the duration and severity of the compression increase, sensory dysfunction progresses to permanent hypoesthesia and motor involvement; this may manifest as a loss of thumb opposition and abduction strength, atrophy of the thenar muscles, and a significant decline in fine motor dexterity. However, clinical complaints-influenced by a patient's subjective symptom severity, pain tolerance, and mechanisms of central sensitization-may not accurately reflect the degree of actual structural damage to the nerve fibers.

Electroneuromyography (ENMG) is considered the gold-standard diagnostic method for carpal tunnel syndrome (CTS), offering high sensitivity (85-90%) and high specificity (>95%). Routine electrophysiological protocols involve the transcutaneous stimulation of the median nerve to measure the latency, amplitude, and conduction velocity of sensory and motor action potentials. Nevertheless, routine ENMG only assesses the conduction of large-diameter myelinated sensory (A-β) and motor (A-α) fibers. Small-diameter myelinated A-δ fibers and unmyelinated C fibers-which are responsible for the hyperesthesia, dysesthesia, and thermal abnormalities experienced by patients in the early stages-cannot be recorded via routine ENMG. This discrepancy serves as the primary pathophysiological explanation for cases where patients present with severe clinical symptoms yet exhibit normal or only mildly abnormal ENMG findings.

Among diagnostic tests, ultrasonography (USG) reveals the structural morphology of the median nerve. USG has been shown to be as effective as electrophysiological testing in the diagnosis of CTS. Due to compression, the median nerve becomes enlarged proximally at the entrance to the carpal tunnel. An increase in the cross-sectional area (CSA) of the median nerve-specifically at the level of the pisiform bone (≥10 mm²)-offers high sensitivity for the diagnosis of carpal tunnel syndrome (CTS). However, neither ultrasound (USG) nor electroneuromyography (ENMG) can directly measure the microstructural and viscoelastic alterations that nerve damage induces in the target peripheral effector muscle.

Chronic entrapment of the median nerve does not merely cause atrophy in the thenar muscles due to subclinical denervation; it also leads to changes in the resting tone of muscle fibers, an increase in non-contractile connective tissue, fibrotic remodeling, and viscoelastic impairment. Manual muscle testing and conventional dynamometry are insufficient for quantitatively differentiating these micro-biomechanical characteristics. Myotonometry, which has entered clinical practice in recent years, enables the quantitative and objective measurement of muscle resting tone, stiffness, and elasticity through non-invasive mechanical impulses applied to superficial muscle tissue. Similarly, computerized thermal sensory analyzers map the functional integrity of A-δ and C fibers by assessing thresholds for cold detection (CDT), warm detection (WDT), cold pain (CPT), and heat pain (HPT).

The primary hypothesis of this study is that CTS is not merely a focal conduction block, but rather a neuromechanical pathology characterized by damage to small cutaneous nerve fibers and alterations in the peripheral architecture of the thenar muscles. In this context, the objective of our study is to... The objective is to analyze the biomechanics of the abductor pollicis brevis muscle and thermal sensory thresholds in patients with mild to moderate carpal tunnel syndrome (CTS) in comparison to healthy individuals, and to determine the relationships between these parameters and disease severity, the cross-sectional area of the median nerve (as measured by ultrasonography), and symptom scores.

This research was conducted as a prospective, cross-sectional cohort study at the Department of Physical Medicine and Rehabilitation of Karaman Training and Research Hospital. The study protocol was reviewed and approved by the Local Scientific Medical Research Ethics Committee of Karamanoğlu Mehmetbey University Faculty of Medicine (Decision Date: 10/03/2026; Ethics Committee Decision No: 322884). All procedures were carried out in full compliance with the ethical and deontological standards set forth in the World Medical Association Declaration of Helsinki and the Guidelines for Good Clinical Practice.

Consecutive patients who presented to our Physical Medicine and Rehabilitation outpatient clinic, underwent a detailed neuromuscular physical examination and electrophysiological studies (ENMG), and fully met the inclusion criteria were prospectively enrolled in the study. The control group consisted of asymptomatic, healthy healthcare worker volunteers, matched to the patient cohort in terms of age and gender, who exhibited no neurological symptoms in the upper extremities, no positive provocative test results, and no electrophysiological or ultrasonographic abnormalities.

Inclusion criteria for the study were defined as follows: being between 18 and 65 years of age; reporting paresthesia, numbness, or pain in the cutaneous distribution area of the median nerve for at least 3 months; and having a confirmed diagnosis of mild to moderate carpal tunnel syndrome (CTS) in accordance with standard electrophysiological guidelines. Conversely, the following were considered exclusion criteria: metabolic and nutritional neuropathies that could affect peripheral nerve or muscle architecture (e.g., diabetic polyneuropathy, vitamin B12 deficiency, uremic neuropathy secondary to chronic renal failure); metabolic endocrinopathies such as uncontrolled diabetes mellitus or significant hypothyroidism; systemic inflammatory rheumatic diseases; other concurrent neuromuscular pathologies affecting the upper extremity-such as cervical radiculopathy, brachial plexopathy, thoracic outlet syndrome, pronator teres syndrome, or ulnar entrapment neuropathies; and a history of major trauma, fractures, or surgical interventions involving the hand or wrist. Having received a local corticosteroid injection into the carpal tunnel within the last 6 months, or having undergone comprehensive physical therapy for the wrist region; pregnancy/lactation; and cognitive or psychiatric disorders that could limit full adherence to the study protocols were defined as exclusion criteria.

Ultrasonographic assessment was performed using a G.E. Logiq® P5 ultrasound system (GE Healthcare, Milwaukee, WI, USA) equipped with a high-frequency 7-12 MHz linear probe. With the participant in a seated position, their arm supported, forearm in supination, and fingers in a semi-flexed position, the probe was placed at the entrance of the carpal tunnel-at the level of the pisiform bone-perpendicular to the skin, without applying additional compression. The cross-sectional area (CSA, mm²) was calculated by manually tracing the hyperechoic outer boundary of the median nerve. Measurement of Thenar Muscle Biomechanics (MyotonPRO): The viscoelastic and mechanical properties of the Abductor Pollicis Brevis (APB) muscle-the primary superficial abductor of the thenar muscle group-were evaluated using a digital myotonometer (MyotonPRO, Myoton AS, Tallinn, Estonia). Participants were seated in a resting position with their forearm fully supinated to ensure muscle relaxation. The probe was applied perpendicular ($90\^\circ$) to the skin at the most prominent point of the APB muscle belly. Following a standard mechanical preload (0.18 N), the device delivered a 0.4 N mechanical impulse for 15 milliseconds, applied three consecutive times at 0.8-second intervals (multi-scan mode). The following parameters were recorded automatically: Oscillation Frequency (Hz): Reflects muscle tone (intrinsic tension at rest). Dynamic Stiffness (N/m): Represents the muscle's capacity to resist deformation. Logarithmic Decrement: Indicates the mechanical elasticity of muscle tissue (the rate at which it returns to its original state following deformation and recovers impact energy); as the decrement increases, elasticity decreases. Quantitative Sensory Testing (QST): The quantitative assessment of thin-fiber function (A-δ and C fibers) was performed using the Medoc TSA-2 Air® (Medoc Advanced Medical Systems, Ramat Yishay, Israel) thermal sensory analyzer. A 30 × 30 mm thermode surface, based on the Peltier principle, was applied to the thenar eminence-an area within the cutaneous distribution of the median nerve. The initial baseline temperature was set at 32 °C. To prevent thermal tissue damage, the temperature range was limited to between 0 °C and 50 °C. A linear temperature change rate of 1 °C/s was used for the measurements. Participants were instructed to press a handheld button the moment they perceived a sensation: Cold Detection Threshold (CDT, °C): Reflects the function of A-δ fibers. Warm Detection Threshold (WDT, °C): Reflects the function of unmyelinated C fibers. Cold Pain Threshold (CPT, °C): The temperature at which a cold stimulus is perceived as painful (thermal hyperalgesia). Heat Pain Threshold (HPT, °C): The temperature at which a heat stimulus is perceived as painful.

For each parameter, three consecutive measurements were taken, and the mean value (°C) was calculated. Clinical, Functional, and Provocative Assessments-Provocative Tests: The Tinel sign, Phalen test (onset of paresthesia during 1 minute of full flexion), reverse Phalen test (triggering of symptoms during 1 minute of full extension), and Durkan carpal compression test (direct compression for 30 seconds) were recorded as binary variables (positive = 1, negative = 0). Grip Strength: Maximum hand grip strength and two-point (tip-to-tip) pinch strength were measured using a Baseline® hydraulic hand dynamometer and pinch gauge, in accordance with American Society of Hand Therapists (ASHT) standards, and recorded in kilograms (kg). Cutaneous Sensory Tests: Light touch sensation was assessed using the Semmes-Weinstein monofilament kit (2.83 = normal; ≥ 3.61 = diminished touch), and cortical tactile discrimination was evaluated using a Baseline® Two-Point Discriminator (measured in cm). Symptom and Functional Scales: Numeric Rating Scale (NRS): Assessed on a scale from 0 (no pain) to 10 (unbearable pain). Boston Carpal Tunnel Questionnaire (BCTSQ): The validated Turkish versions of the 11-item Symptom Severity Scale (SSS) and the 8-item Functional Status Scale (FSS) were administered (score range: 1-5). CTS-6 Scale: A brief symptom severity scale (0-26 points). LANSS Scale: Used to assess the neuropathic pain component (a score of ≥ 12 is suggestive of neuropathic pain).