Welcome to another ultrasound-based case, part of the “Ultrasound For The Win!” (#US4TW) Case Series. In this case series, we focus on a real clinical case where point-of-care ultrasound changed the management of a patient’s care or aided in the diagnosis. In this case, a 53-year-old man with history of diabetes mellitus presents with right index finger swelling.

Learning Objectives

1. List the differential diagnosis for a patient presenting with a swollen and painful digit.
2. Describe the sonographic findings suggestive of flexor tenosynovitis.
3. Define the term ‘anisotropy’.
4. List Kanavel’s 4 cardinal signs.
5. Describe the management of flexor tenosynovitis.

Case Presentation

A 53-year-old man with a history of diabetes mellitus presents to the Emergency Department (ED) for evaluation of right index finger pain and swelling. His symptoms began after he inadvertently pricked his finger with a thorn of a shrub while gardening 2 days prior to his presentation. He has since noted progressively worsening erythema and pain to his digit. He visited his family doctor prior to his ED visit, and was prescribed a course of antibiotics. He denies numbness or tingling of the extremity. His review of systems is otherwise non-contributive.

Vitals

• BP 171/87 mmHg
• P 87 bpm
• RR 20 breaths/min
• O2 97% room air
• T 37.1 C

Differential Diagnosis

• Abscess
• Cellulitis
• Felon
• Flexor tenosynovitis
• Gout/Pseudogout
• Herpetic whitlow
• Paronychia
• Septic arthritis

On physical examination, you examine a well-appearing man in no acute distress. Examination of his right hand reveals a mildly symmetrically swollen second digit, which is slightly flexed at rest [Fig. 1]. There is a small healed puncture wound on the lateral aspect of the proximal index finger. There is tenderness to palpation of the digit, which is worse on the volar surface. Passive range of motion of the digit does not elicit pain. There is erythema and induration to the digit with no palpable fluctuance or crepitus. Neurovascular examination of the hand is unremarkable.

Figure 1. Patient’s right hand revealing a mildly swollen and erythematous index finger with a small healed puncture wound.

Laboratory studies are obtained:

• White blood cell count: 9.1 x10^9 cells/L
• Erythrocyte sedimentation rate: 8.2 mm/hr

The emergency physician performed a point-of-care ultrasound of the patient’s affected finger.

Point-of-Care Ultrasound:

Figure 2. Affected digit imaged in the transverse plane reveals circumferential anechoic fluid surrounding the flexor tendon, suggestive of flexor tenosynovitis.

Figure 3. Flexor tendon sheath (encircled) with circumferential anechoic fluid (arrow) surrounding the tendon, suggestive of flexor tenosynovitis.

Figure 4. Affected digit with flexor tenosynovitis (FTS) compared to the normal adjacent digit.

Figure 5. Surrounding hyperemia evident with color doppler.

Ultrasound Image Quality Assurance (QA)

The ultrasound images of the affected digit were obtained with a high-frequency linear transducer which is helpful when examining superficial structures such as extremities. Tendons are identified as band-like structures composed of organized linear fibrillar components that exhibit “anisotropy” [Fig. 6]. Anisotropy describes the sonographic artifact seen with reflective and highly organized structures including tendons and nerves, and less so with ligaments and muscles. This artifact is directionally dependent, relying on the principle that the transducer beam should be oriented perpendicular to a nerve or tendon to be optimally visualized. Thus, anisotropy is evident with fanning of the probe, which alters the angle of the beam to individual fibers, making the tendon more anechoic then hyperechoic as the transducer beam’s axis changes.

Figure 6. Tendon with a prominent example of anisotropy.

With musculoskeletal ultrasound, the use of a water bath or standoff pad are techniques that can be useful in improving image quality and patient compliance. By placing an extremity in a container of water, the transducer can be placed a few centimeters above the area of interest as the water acts as an excellent conductive medium [Fig. 7].

This technique has several benefits.

1. The area of interest can be better positioned within the transducer’s focal zone improving visualization
2. A water bath avoids direct contact of the transducer to the patient’s digit which will aid in a less painful examination.

Commercial standoff pads are available that provide a similar effect, or alternatively, the use of a bag of normal saline can be substituted.

Figure 7. A water bath can be helpful when using the ultrasound for superficial structures such as the extremities.

The use of color doppler can be useful in identifying hyperemia, suggestive of an inflammatory or infectious process [Fig. 5]. Normal tendons should not demonstrate vascularity, but hyperemia may be evident with flexor tenosynovitis.

Disposition and Case Conclusion

Given the concern for flexor tenosynovitis (FTS) on point-of-care ultrasound despite the non-specific physical examination findings, intravenous antibiotics was started, and plastic surgery was consulted. The patient was taken to the operating room for surgical decompression and irrigation of the sheath. Intraoperatively, purulent fluid was found within the tendon sheath. After a course of antibiotics, the patient was discharged from the hospital, and a followup visit revealed full mobility and range of motion of his digit with no complications.

FTS is a relatively rare surgical emergency that requires early diagnosis and treatment with intravenous antibiotics and surgical consultation to prevent significant morbidity.¹ The digital flexor tendon sheath is composed of both retinacular and synovial tissue. Infection causes elevated pressures within this sheath, which can result in spread of infection to surrounding bursae and fascial planes that track into the hand and forearm, as well as tendon necrosis due to impaired blood flow.¹ Patients who present with an advanced disease course or with a delay in diagnosis or treatment are at high risk for significant morbidity including permanent digital stiffness or amputation.¹ Amputation rates have been shown to be as high as 17% when FTS is misdiagnosed.²

The clinical course of FTS typically begins as a puncture wound that introduces bacteria (most commonly Staphylococcus aureus) into the flexor tendon sheath.¹² Patients typically present 2-5 days after an injury with clinical symptoms that most commonly include a swollen and painful digit. Kanavel’s cardinal signs, described in 1912 (!) have been historically used to distinguish between FTS and other conditions that present similarly.³

Table 1. Kanavel’s Cardinal Signs of Flexor Tenosynovitis³

However, Kanavel’s signs have not been validated and physical exam findings may be unreliable, especially early in the disease course.

Point-of-care ultrasound can aid in identifying early stages of FTS when physical examination findings may not be diagnostic. Although the sensitivity and specificity of ultrasound to identify FTS has not yet been established, sonographic findings that are suggestive of FTS in the right clinical setting include circumferential anechoic fluid surrounding the flexor tendon. Additionally, the use of color doppler can reveal peritendinous hyperemia.

Take Home Points

1. Flexor tenosynovitis (FTS) is a relatively rare infection with non-specific clinical symptoms and are at risk for significant morbidity including severe stiffness of the digit or amputation.
2. Kanavel’s cardinal signs may be helpful for distinguishing FTS from other conditions, but they have not been validated.
3. Prompt diagnosis and treatment including intravenous antibiotics and surgical consultation for decompression and irrigation is essential for preventing tendon and digit ischemia.
4. Point-of-care ultrasound findings of fluid and hyperemia surrounding the affected tendon can aid in identifying FTS in its earlier stages when physical exam findings may not be diagnostic.
5. Anisotropy is an artifact that occurs with reflective and highly organized structures and can be useful in identifying tendons and nerves.

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