A 40 year-old male patient presents to an emergency department after suffering multiple dog bites, including one to the left thumb.  His wounds are irrigated, and he is suspected of having  a thumb extensor tendon injury on exam.  The lacerations are approximated with sutures and he is discharged on oral antibiotics.  He presents to another emergency department  less than 12 hours later with worsening left thumb pain, swelling, redness, and decreased movement.  An ultrasound was obtained in a water bath:

 

The volar aspect of the left thumb is closest to the probe.  The suture from the outside hospital can be seen in the near field overlying his laceration, with skin and soft tissues seen beneath.  The hyperechoic distal and proximal phalanx of the thumb are visible, along with the interphalangeal (IP) joint.  The flexor tendon is visualized superficial and parallel to the bone, with it’s insertion seen near the joint (green).  The flexor tendons of the hand lie within a tendon sheath that normally contains a small amount of fluid that varies among individuals.  In this clip a small amount of anechoic fluid is seen above and below the tendon (thick blue arrows).  

left thumb still with labels

 

Moving the probe proximally towards the palm, the metacarpophalangeal (MCP) joint is seen:

 

Again, there is anechoic fluid superficial to the flexor tendon.  The amount of fluid along the tendon is subtle, but abnormal compared to the patient’s unaffected right thumb:

 

The ultrasound images were concerning for inflammation or early infection of the tendon sheath.  The patient was started on broad spectrum antibiotics.   Orthopedics took the patient to the operating room for wound exploration, finding the tendon sheath penetrated by the dog bite.  An impaction fracture of the thumb and an extensor tendon laceration were also noted.  The patient underwent debridement and IV antibiotics were continued.  A few days later he returned to the OR for repeated debridement, tendon repair, and wound closure.

The red, swollen hand is a challenging presentation to the emergency provider.  Missed hand injuries and infections can quickly become worse and permanently affect function.  Differentiating simple surface infections (like cellulitis) from deeper space infections (abscess, septic joint, tendon synovitis) can be difficult on physical exam.  Ultrasound allows direct visualization of superficial and deep tissues, and can aid in proper diagnosis of hand infections.

The exam is best performed with a linear, high-frequency transducer.  Water baths are advantageous for several reasons:  They avoid direct contact with the patient’s skin (which can be painful) and they also eliminate contact artifact as the linear probe doesn’t conform well to smaller structures like the hand.  Imaging through several centimeters of water also optimizes the focal zone which is usually located in the middle field of the screen.  This provides better resolution of fine structures compared to images visualized immediately beneath the probe.

The epidermis and dermis usually appear as a thin slightly hyperechoic layer, with hypoechoic subcutaneous tissue beneath.  In long axis tendons will appear as hyperechoic linear structures that are usually parallel to the course of the underlying bone.  This appearance is due to the echoes created by the tightly packed collagen fibrils.  In short axis tendons can appear flat or round depending on their location in the body.  Just like the tightly bound fibers in a nerve, tendons exhibit a property called anisotropy.  This means that the image created is highly dependent on the angle of the ultrasound beam.  When the beam is perpendicular to the tendon (as in our above clips), the tendon will appear hyperechoic.  At all other angles less echoes are reflected back to the transducer, making the tendon appear more hypoechoic.  Understanding anisotropy is essential to avoid misinterpreting normal tendons as abnormal.

Most tendon pathologies will show a more hypoechoic tendon.  Tendons associated with a tendon sheath (like our above example) will normally have a small anechoic rim of fluid surrounding them.   A large amount of circumferential fluid collecting around the tendon, or any asymmetry compared to the unaffected side, are abnormal findings concerning for tenosynovitis.  However, it is difficult to determine based on ultrasound alone whether the cause is infectious versus other inflammatory (such as DeQuervain’s tenosynovitis).  For this reason ultrasound findings should be interpreted in the proper clinical context when assessing for deep space hand infections.

Pearls:

Point-of-care ultrasound can be used to improve the diagnosis of hand infections.

The superficial and deep structures of the hand are best imaged using a linear transducer and water bath.

Abnormal amounts of fluid surrounding tendon sheaths are concerning for injury or infection.

Tendons display anisotropy on ultrasound, and their appearance will change as the angle of the ultrasound beam changes.

-Mark Robidoux, Emergency Ultrasound Fellow

References

Dewitz, Andreas. Musculoskeletal, soft tissue, and miscellaneous applications. In: Ma JO, Mateer JR, Reardon RF, Joing SA, eds. Ma and Mateer’s Emergency Ultrasound. 3rd edition. McGraw-Hill Education; 2014.

Marvel B, Budhram, G. Bedside ultrasound in the diagnosis of complex hand infections: a case series. The Journal of Emergency Medicine. 2015; 48(1):63-68.

Padrez K, Bress J, Johnson B, Nagdev A. Bedside Ultrasound Identification of Infectious Flexor Tenosynovitis in the Emergency Department. Western Journal of Emergency Medicine. 2015;16(2):260-262.

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