ULTRASOUND OF TRUE KNOT

OF THE UMBILICAL CORD

True knots are difficult to diagnose because:

1. The gestational age at which they occur is still unclear. They have been reported to form in all three trimesters (1).
2. The entire cord length is not routinely scanned.
3. Parts of the cord may be obscured or concealed by the fetus in the third trimester.
4. Some workers believe that true knots originate intrapartum at the time of delivery (6).
5. There is no characteristic prenatal appearance.

ULTRASOUND

• Simple true knots may present with a "cloverleaf pattern" on gray scale images (2).

• Triple knots may present as a "four-leaf clover" pattern (3).
• Bizarre multicolored patterns may be seen on color doppler images (1).
• “Hanging noose sign” – transverse section of the umbilical cord surrounded by a loop of umbilical cord (7). This sign together with the image of the section of the cord under pressure from the knot are very suggestive of a true knot especially if these signs remain after fetal movement.

CLASSIFICATION

• Loose knots.
• Do not elevate perfusion pressure.
• Tight knots.
• Venous congestion (1).
• Arterial congestion may or may not be present.
• Partial / complete vascular thrombotic occlusion is rare as the majority of knots appear to be protected against occlusion by the thickness of Wharton's jelly and the large cord radius (1). Browne and co-workers (4) suggest that perfusion pressure is proportional to traction applied and inversely proportional to cord radius. Loose knots therefore do not elevate perfusion pressure.
• Flattening and dissipation of Wharton's jelly.
• Color Doppler - supplies information on:
• Direction of blood flow.
• Hemodynamic significance of a knot. The stenotic effect that a knot has on the venous (and occasional arterial) waveform can be assessed by measuring flow velocities in the pre and post stenotic portions of the cord. High velocities and disturbed flow is present in the post-stenotic segment.
• Systolic notching of the flow velocity waveform in the umbilical artery has been described in hemodynamically significant knots (5).
• False positive diagnosis.
• Multiple opposed loops of normal cord.

REFERENCES

1. Gembruch U, Baschat AA. True knot of the umbilical cord: transient constrictive effect to umbilical venous blood flow demonstrated by doppler sonography. Ultrasound Obstet Gynecol1996;8:53-56.
2. Collins JH. First report: prenatal diagnosis of a true knot (letter). Am J Obstet Gynecol 1991;165:1898.
3. Collins JC, Muller Rj, Collins CL. Prenatal observation of umbilical cord abnormalities: a triple knot and tortion of the umbilical cord. Am J Obstet Gynecol 1993;169:102-104.
4. Browne FJ. On the abnormalities of the umbilical cord which may cause antenatal death. J Obstet Gynaecol Br Common 1925;32:17-48.
5. Jacobi P, Weiner Z, Goren T, Thaler I. Systolic notch in umbilical artery flow velocity waveform associated with a tight true knot of the cord. J Matern Fetal Invest 1994;4:119-121.
6. Blickstein I, Shoam-Schwartz Z, Lancet M. Predisposing factors in the formation of true knots of the umbilical cord. Analysis of morphometric and perinatal data. Int J Gynecol Obstet 1987;25:395-398.
7. Lopez Ramon Y Cajal C, Ocampo Martinez R. Prenatal diagnosis of true knot of the umbilical cord. Ultrasound Obstet Gynecol 2004;23:99-100.