Chapter 11 (page 46)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 11 (page 46)

Sonographic Anatomy

In transverse sonographic sections the normal pylorus presents as a hypoechoic ring with a central echogenic core (Chap 10, Fig 10.1). By comparing the sonographic image with histological sections Blumhagen and Coombs (l98l) were able to show that the hypoechoic ring corresponds to the muscularis externa layer of the wall, while the more echogenic central core is formed by the mucosal and submucosal layers (including the muscularis mucosae). By means of sonography the relative extent of the muscular and mucosal/submucosal elements of the ring can be determined accurately in normal, living subjects under physiological conditions.

In infants, Stunden et al (l986) found the maximum overall diameter of the normal pyloric ring (presumably with the aperture patent) to be 13.0mm. The hypoechoic muscular layer was 3.0mm in thickness. (The term "thickness" in sonography is synonymous with "height" in microscopic anatomy). The minimum overall diameter (presumably with the aperture closed) was 7.0mm, giving a radius of 3.5mm. In these cases the muscle thickness was 1.0mm, and the mucosal/submucosal layer consequently 2.5mm.

Stringer et al (l986) found the thickness of the mucosal/submucosal layer to vary from 2.5mm to 3.5mm in normal infants. According to Swischuk (l989) the muscularis externa component of the ring usually measures 1.0mm in thickness and the mucosal/submucosal component between 2.5mm and 3.5mm in normal infants. Minor variations have been found by other authors (Chap 10).

Present Investigations

Transverse sonographic sections of the contracted pylorus were obtained in 10 subjects without gastrointestinal symptoms, ranging in age from one month to 73 years; in all a typical "doughnut" appearance was seen (Fig 10.1). In all subjects it was clear that the ring-like, hypoechoic outer muscular layer was equal to, or thinner than the inner, echogenic mucosal/submucosal component of the ring. For instance, in a 5 year old child the radius of the ring was 6.0mm, the thickness of the muscular layer 3.0mm, and that of the mucosal/submucosal layer 3.0mm. In a 73 year old male the radius was 11.0mm, the thickness of the muscle layer 5.0mm, and that of mucosa 6.0mm. It is concluded that, during life, the muscular and mucosal/submucosal components of the ring are more or less equal in height.

Discussion

Many authorities look upon the pyloric ring as a sphincter (Chapter 2), implying that it is a purely muscular structure. At the other extreme Cole (l928) considered it to be a muco- membranous fold containing no muscular fibres at all. The present microscopic and sonographic images show that the ring has both muscular and mucosal/submucosal divisions. While the outer or peripheral part is formed by muscularis externa, the inner part of this muscular rim is capped by a fold of mucous membrane. With microscopy the height of the inner mucosal/submucosal division appears to be approximately one third that of the muscular part. Sonography of the normal, living ring (which is more accurate than microscopy in this instance, for reasons stated) shows that the muscular and mucosal/submucosal divisions are more or less of equal height in children and adults; in newborn infants the muscular division accounts for one-third, and the mucosal/submucosal division for two-thirds of the height of the ring.

In view of these findings it appears improbable that the pyloric ring as such constitutes a sphincter in the usually accepted sense.

References

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Cole LG. The living stomach and its motor phenomenon. Acta Rad l928, 9, 533- 545.
Edwards D. Some radiological aspects of pyloric disease. Proc Roy Soc Med l96l, 54, 933-937.
Edwards DAW, Rowlands EN. Physiology of the gastroduodenal junction. In: Handbook of Physiology, Sect 6, Vol 4, Motility. Edit Code CF. American Physiological Society, Washington DC, l968, l985-2000.
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