Intersection syndrome is a condition that affects the first and second compartments of the dorsal wrist extensors. The condition is thought to occur as a result of repetitive friction at the junction in which the tendons of the first dorsal compartment cross over the second, creating a tenosynovitis. This is typically noted as a pain just proximal and dorsal to the radial styloid, or also noted anatomically by 4 cm - 6 cm proximal to Lister's tubercle.
The first dorsal compartment of the wrist is comprised of the abductor pollicis longus (APL) and extensor pollicis brevis (EPB). These tendons have a unique anatomical pathway proximally in which they cross over the second dorsal compartment tendons just proximal to the extensor retinaculum and radial styloid. The second dorsal compartment of the wrist is comprised of the extensor carpi radialis brevis (ECRB) and extensor carpi radialis longus (ECRL).
The syndrome is typically the result of repetitive extension and flexion exercises or activities. It is commonly seen in sporting activities such as rowing or canoeing, skiing, racquet sports, and horseback riding. There is no significant difference in injury pattern found in men versus women.
The repetitive extension-flexion results in a friction injury at the crossover junction of the first dorsal compartment (abductor pollicis longus and extensor pollicis brevis) and the second dorsal compartment (extensor carpi radialis brevis/extensor carpi radialis longus) tendons leading to an inflammatory response and subsequently a tenosynovitis. The presentation is typically one that the patient complains of pain or tenderness over the dorsal aspect of the wrist proximal to the radial styloid. There may also be swelling and crepitus that is palpable on the exam with wrist and/or thumb extension.
Intersection syndrome was first described in the literature by Alfred-Armand-Louis-Marie Velpeau a French anatomist and surgeon in 1841. He is also credited for the first accurate description of leukemia. The term of intersection syndrome was first coined by James H. Dobyns in 1978 at the Mayo Clinic. Although the accepted vernacular is intersection syndrome, it has been described in the medical literature by many other names: Oarsmen's wrist, crossover syndrome, squeaker's wrist, abductor pollicis longus bursitis, abductor pollicis longus syndrome, subcutaneous polymyositis, and peritendinitis crepitans. 
Intersection syndrome is a clinical diagnosis, although a musculoskeletal ultrasound can easily confirm it. The initial steps for diagnosis include a focused physical exam of the elbow, wrist, and hand.
As with all musculoskeletal exam, you must have a structured approach that includes inspection, a range of motion, palpation, muscle testing, and other special tests. Each joint above and below the injury should be tested in all motions. Look for swelling over the distal forearm as there can be some cases that present with a palpable finding on exam 4 cm - 6 cm proximal to Lister's tubercle. Crepitus is a very common finding on the exam over the site of irritation. This is a finding that is specific to intersection syndrome. As the two dorsal compartments cross the movements of pronation and supination, create friction resulting in the exam finding of crepitus. Pronation is typically found more uncomfortable than supination.
When developing or working through your differential diagnosis, resisted pronation that leads to the recreation of the patient's pain, along with the palpable finding of crepitus about 2 cm - 3 cm proximal to the radial styloid, can help differentiate from tenosynovitis of De Quarvein Syndrome. De Quarvein Syndrome is a condition that also involves the first dorsal compartment of the wrist extensors. This condition is noted below the radial styloid and can be classically tested via the Finkelstein maneuver.
Plain film imaging and CT will not be helpful in the diagnosis of Intersection syndrome. MRI would give excellent soft tissue picture and diagnosis, although MRI would not be a cost- or time-effective choice.
Ultrasound technology has pushed musculoskeletal medicine forward in both diagnosis and treatment provided by physicians. There have been some that say it can be as specific as MRI in the hands of the skilled user. Remember that as in most musculoskeletal conditions, the anatomy is mostly superficial. Therefore a linear ultrasound probe is utilized. When observing Intersection syndrome under ultrasound, the ideal image is in the transverse plane in short axis. The findings that would correlate to the diagnosis would be a hypoechoic area in between the two dorsal compartments as they are on top of each other. This represents swelling/edema as caused by friction. There also may be a thickening of the tendon sheaths.
Treatment is conservative management with rest and activity modification. Corticosteroid injection has shown significant improvement and is a known next best step if little or no improvement has been made with other conservative treatment. 
Anti-inflammatory medications are maybe useful for acute injury and pain relief. Common medications are ibuprofen, naproxen, meloxicam or diclofenac. Acetaminophen also may be utilized for pain relief as well. Typically rest and activity modification will be more effective. Ice is also an effective treatment. A temporary splint for protection and comfort at night may also be beneficial. There is no compelling evidenced based rehab protocol for intersection syndrome at this time. One may consider extrapolating the use of eccentric strengthening and stretching for rehab protocols.
When conservative measures are not effective corticosteroid injection under ultrasound guidance can be utilized. Using the ultrasound visualization technique noted in the evaluation process; confirm the diagnosis. The typical injection is a one to one mixture of a corticosteroid and anesthetic (0.5ml to 1ml of steroid, commonly used is triamcinolone 40mg/1ml along with a local anesthetic of choice at 0.5ml to 1ml). A 23 to 25 gauge needle at a length of 1 to 1.5 inch is preferred. 
Using the in-plane or out of plane needle injection technique, guide the needle to where the first dorsal compartment (abductor pollicis longus and extensor pollicis brevis) is crossed over the second dorsal compartment (extensor carpi radialis brevis/extensor carpi radialis longus). After the injection is completed, have the patient pronate and supinate the wrist, observe for crepitus and tenderness with palpation. Resolution of the pain can help solidify the diagnosis. The steroid will take time to reach full potential. We recommend that rehab exercises be used in tandem with the injection after day 3 to 5. Additionaly prolotherapy is another injectable option, this is also performed under ultrasound guidance with the same technque as steroid. The basis of prolotherapy is to minimize the side efect profile of steroid and produce similar outcomes for improvements in pain and function. Prolotherapy utilizes an anesthetic usually lidocaine and a mixture of dextrose in sterile water(5-20%). Prolotherapy is thought to create a proinflammatory state which then triggers the release of growth factors and ultimately collagen deposition, leading to strengthening of tissue. 
In rare, recalcitrant cases surgical debridement and release is indicated.
A team of the clinician and nursing splinting the injury and providing close follow up will result in the best outcome. [Level V]
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