Carpal Tunnel Injection

Median nerve compression under flexor retinaculum

Causes and findings:
• Overuse or trauma, post-Colles' fracture
• Pregnancy, hypothyroidism, acromegaly
• Rheumatoid arthritis, psoriatic arthropathy
• Idiopathic
• Pins and needles in the distribution of the median nerve, especially at night
• Paraesthesia can be reproduced by tapping the median nerve at the wrist (Tinnel's sign) or by holding the wrist in full flexion for 30 seconds and then releasing (Phalen's sign). Longstanding median nerve compression may cause flattening of the thenar eminence

Equipment:
Syringe - 1ml
Needle - 23G 1.25 inches (30mm) blue
Kenalog 40 - 20 mg
Lidocaine - Nil
Total volume - 0.5 ml

Anatomy:
The flexor retinaculum of the wrist attaches to four sites: the pisiform and the scaphoid, the hook of hamate and the trapezium. It is approximately as wide as the thumb from proximal to distal and the proximal edge lies at the distal wrist crease. The median nerve lies immediately under the palmaris longus tendon at the mid-point of the wrist, and medial to the flexor carpi radialis tendon. Not every patient will have a palmaris longus so ask the patient to press tip of thumb onto tip of little finger; the crease seen at mid-point of the palm points to where the median nerve should run.

Technique:
• Patient places hand palm up
• Identify point midway along proximal wrist crease, between flexor carpi radialis and median nerve
• Insert needle at this point then angle it 45°. Slide distally until needle end lies under mid-point of retinaculum
• Inject solution in bolus

Aftercare:
The patient rests for 1 week and then resumes normal activities. A night splint helps in the early stages after the infiltration and the patient is advised to avoid
sleeping with the wrists held in full flexion - the 'dormouse' position.

Comments:
No local anaesthetic is used here because the main symptom is paraesthesia, not pain, and it is not advisable to increase the pressure within the tunnel. Care should be taken to avoid inserting the needle too vertically, when it will go into bone, or too horizontally, when it will enter the retinaculum. If the patient experiences pins and needles, the needle is in the median nerve and must be withdrawn slightly and repositioned. Although one injection is often successful, recurrences do occur. Further injections can be given if some relief was obtained, but if the symptoms still recur surgery may be required.

Alternative approach:
The injection can be equally well performed by inserting the needle between the median nerve and the flexor tendons, using the same dose and volume.