The goal of this prospective, randomized, double-blinded, head-to-head comparative study is to compare the efficacy of ultrasound-guided 4 ml 5% dextrose perineural injection versus 3 ml 5% dextrose with 1 ml 40 mg/ml methylprednisolone perineural injection in patients with carpal tunnel syndrome (CTS).

The main questions it aims to answer are:

Is there a difference in efficacy between interventions regarding 1. pain alleviation, 2. improvement in symptoms severity and functional status, 3. improvement in grip strength, 4. size of median nerve cross-sectional area, and 5. improvement in quality of life during the six-month post-intervention follow-up period?

The research will be conducted in the General and Veteran Hospital "Croatian Pride" Knin.

The research will be a prospective, randomized, double-blinded, head-to-head comparative study.

The goal of the research is to compare the efficacy of ultrasound-guided 5% dextrose injection versus 5% dextrose with 40 mg methylprednisolone injection in patients with carpal tunnel syndrome (CTS).

Randomization of patients will be performed before the intervention using computer software.

All participants will sign written informed consent and the study will be performed under the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. The study is approved by the Ethical Committee at the General and Veteran Hospital "Croatian Pride" Knin (approval number: 510.01/23-01/79).

Adult participants aged between 18 and 80 years with a clinical and electrophysiological diagnosis of idiopathic CTS referred to the Physical Medicine and Rehabilitation clinic will be randomly assigned into two groups using a random number generating software in a 1:1 manner.

Since patients can have CTS in one or both hands, randomization will be conducted according to the patient, thus the same patient can receive the intervention only on one side which will be considered worst.

The minimum sample size of a total of 70 participants (35 in each group) was estimated by analyzing the power of the t-test for independent samples according to the following characteristics: type one error (α=0.05), power (1-ß=0.80), and effect size (d=0.68).

The power analysis of the test was performed using the computer program G*Power for Windows, version 3.1.3.

IBM SPSS Statistics version 29.0 software will be used for the data analysis.

Group one will receive an ultrasound-guided 4 ml single perineural injection of 5% dextrose in the carpal tunnel and group two ultrasound-guided single perineural injection of 3 ml 5% dextrose plus 1 ml 40 mg/ml methylprednisolone acetate in the carpal tunnel.

Participants, the physician who will perform the injection procedure and the assessor will be blinded for the treatment allocation. The only person who will know which intervention drug it is about will be the nurse who will prepare the medicine according to the randomization.

Clinical diagnosis of CTS will be made in the presence of at least one symptom (1. pain, 2. pricking, 3. tingling, 4. burning, or 5. numbness in the innervation area of the median nerve on the hand) and at least one sign (1. objective disturbance of the sense of touch, i.e. hypoesthesia or dysesthesia in the innervation area of the median nerve on the hand, 2. positive Tinel's sign, 3. positive Phalen's sign, or 4. positive compression test).

Inclusion criteria are age between 18 and 80 years and the diagnosis of idiopathic CTS based on clinical and electrophysiological criteria.

Exclusion criteria are contraindications for local application of methylprednisolone or 5% dextrose (hypersensitivity to the drug, infection, and skin damage at the application site), previous wrist surgery, traumatic wrist injury within 2 years, previous wrist injection within 6 months, previous physical therapy within 6 months, history of traumatic peripheral nerve injury in the upper extremity, brachial plexopathy, severe cervical radiculopathy and thoracic outlet syndrome, history of diabetes, history of thyroid disease, history of inflammatory rheumatic disease, pregnancy and inability to cooperate with the study protocol.

Basic demographic data will be obtained: gender, age, Body Mass Index (BMI), level of education, occupation, rating of hand activity and force level applied on the hand during the day, cigarette smoking, hand dominance, affected hand, symptom duration, and history of comorbid conditions.

Pain intensity (using a 10 cm Visual Analogue Scale; VAS pain, 0-10; 0=no pain, 10=the most severe pain), symptoms severity and functional impairment (using the Boston Carpal Tunnel Questionnaire; BCTQ), grip strength (using the Jamar hydraulic hand dynamometer in kilograms), median nerve cross-sectional area (CSA) (using diagnostic ultrasound at the carpal tunnel inlet), and quality of life (QoL) (using 36-Item Short Form Survey; SF-36) will be measured immediately before injection, at 4, 12, and 24 weeks post-injection.

The primary outcome measure will be the change in pain intensity at 4 weeks post-injection.

Secondary outcome measures will be the change in pain intensity at 12 and 24 weeks post-injection, and the changes in symptoms severity and functional impairment, grip strength, median nerve CSA, and QoL at 4, 12, and 24 weeks post-injection.

During the study, acetaminophen and tramadol will be allowed for occasional pain relief except for two days prior to the evaluation sessions due to possible interference with the results.

Physical therapy, use of wrist splints, acupuncture, and invasive interventions for CTS other than those included in the study protocol will not be allowed during the study.

Inclusion Criteria:

• age between 18 and 80 years,
• the diagnosis of idiopathic CTS.

Exclusion Criteria:

• contraindications for local application of methylprednisolone or 5% dextrose (hypersensitivity to the drug, infection and skin damage at the application site),
• previous wrist surgery,
• traumatic wrist injury within 2 years,
• previous wrist injection within 6 months,
• previous physical therapy within 6 months,
• history of peripheral traumatic nerve injury in the upper extremity, brachial plexopathy, severe cervical radiculopathy, and thoracic outlet syndrome,
• history of diabetes
• history of thyroid disease,
• history of inflammatory rheumatic disease,
• pregnancy,
• inability to cooperate with the study protocol.