What it is
On the back of the forearm, on the thumb side, two muscles that move the thumb (abductor pollicis longus and extensor pollicis brevis) run diagonally across two tendons that bend the wrist back (extensor carpi radialis longus and brevis). Intersection syndrome is pain, swelling and often a creak or squeak where they cross, after repetitive gripping and wrist movement. It has also been called oarsman's wrist and crossover syndrome, and 'bugaboo forearm' in skiers.
Why it happens is debated. The usual explanation is friction between the two groups, inflaming the tendon linings. At operation, one surgeon found the main problem was a tight sheath around the wrist tendons, another found bulky thumb muscles pressing on the tendons beneath them, and an elite rowing team's surgeons describe it as true inflammation of the tendons from that pressure.
Sources 1, 2, 3, 4, 5, 6, 7
Where it hurts
The crossing is a few finger-widths above the wrist. Measured from the small bony bump in the middle of the back of the wrist (Lister's tubercle), it sat 3.5 to 4.8 cm higher in 10 forearms studied after death, about 4 cm higher on MRI (magnetic resonance imaging), and the thumb muscles spanned 4 to 7.7 cm above it by touch in 5 volunteers. Reviews put the pain about 4 to 6 cm above the wrist.
A rarer form sits at the wrist itself, where the thumb's long tendon crosses the same wrist tendons: 3 of 21 cases in one ultrasound series.
Sources 8, 9, 10, 1, 3, 11
Who gets it
People who grip and move the wrist over and over. In the Australian national rowing team, 2 or 3 rowers a year had it over 3 years (4% to 6% of the team), each losing an average of 16 to 34 days of training. In one week of deep-powder skiing, 5 of 42 heli-skiers developed it, from pulling planted poles out of deep snow. Canoeists, kayakers, racket players, weightlifters, horse riders, mountain bikers and ice-axe climbers get it too.
Work can bring it on: a supermarket cashier in France, where it is recognised as an occupational disease, and farmers threshing, planting and hammering in Thailand, where it made up 0.37% of 8,080 hospital patients with arm or hand pain, 70% of them men. It was found in 1.9% of patients having a hand and wrist ultrasound scan in an Italian hospital.
Sources 7, 4, 1, 3, 12, 13, 14, 15, 11
How it feels, and how it differs from de Quervain's
Pain on the back of the forearm on the thumb side, a few finger-widths above the wrist, with swelling (73% of 30 patients in one series) and crepitus, a creak or squeak you can feel or hear as the wrist or thumb moves (40%). The NHS lists a grating or crackling feeling as a sign of tendonitis. The area may look red. Gripping, twisting and bending the wrist back and towards the thumb bring it on.
De Quervain's involves the same thumb tendons, but it hurts lower down, at the bony bump on the thumb side of the wrist and the base of the thumb; our de Quervain's guide covers it. In a Japanese clinic, intersection syndrome was more often in men and the dominant hand than de Quervain's, and came on over a shorter time. The Finkelstein test (thumb tucked into a fist, wrist bent towards the little finger) can hurt with both, so it doesn't tell them apart. A 26-year-old rower was treated for de Quervain's for 9 months, including an injection, before an ultrasound scan showed intersection syndrome.
Sources 15, 16, 17, 3, 18, 2, 19
How it's diagnosed, and what scans do and don't meanEmerging evidence
Usually from the story and an examination: tenderness, swelling and crepitus at the crossing. Ultrasound shows fluid and swelling around the tendons there, as it did in all 4 patients in one series. MRI shows swelling around the tendons spreading up from the crossing, but a standard wrist scan may stop below it, so the scan needs to include the forearm, and long-standing cases may show little. In one case the wrist MRI was normal, and a bedside ultrasound found the fluid.
Scans need care: in 19 elite rowers with no symptoms, ultrasound found fluid around one of these wrist tendons in 53% and around the other in 32%. A finding fits the diagnosis only when the examination does too.
Sources 2, 17, 20, 11, 21, 8, 9, 3, 22
How it usually goesEmerging evidence
Most cases settle within weeks once the activity that set them off is cut back, according to reviews of small reports and case series; there are no trials. In a Thai series, all 30 patients were treated without surgery, and none of the 14 still followed 12 to 18 months later had it come back. Reviews describe a few weeks of rest and a splint, then several weeks of gradual return. In elite rowers it can cost weeks of training.
Sources 15, 3, 2, 7
