It has been shown that the upper oesophageal sphincter has both radial and axial asymmetry. Stanciu & Bennett (1974) found that, in a small number of normal individuals, c atheter openings arranged to record at 120 degrees showed a significant difference in sphincter pressure between the three readings, indicating that spatial orien t a t i o n of catheter tips influences the pressure recorded. Berlin et al
(1977), using a six lumen catheter with openings at 60 degrees, found that the pressure in the upper oesophageal sphincter was non-uniform with a substantially greater pressure in the anterior-posterior direction with pressures of about 80mm Hg anteriorly and posteriorly. Greater resting pressures (average 100 mmHg) have been recorded from anterior and posterior catheter orifices than from the lateral orifices(33 mmHg) by Winans (1972) using an eight lumen catheter.
Welch et al (1979) compared the upper oesophageal sphincter manometry produced by three types of catheter. They were an eight lumen radially perfused probe similar to that used by Winans (1972), a conventional Honeywell three transducer probe and a circumferentially sensitive probe designed to measure upper oesophageal sphincter pressure without regard to probe orientation. The Honeywell probe produced significantly lower pressures than the other methods with wide intra subject variation. In contrast, upper oesophageal sphincter pressure measured by the circumferential probe was constant for each subject and was identical to the anteroposterior upper oesophageal sphincter pressure m easured by the eight lumen probe. Computer analysis of the upper oesophageal sphincter pressure recorded by the eight lumen probe showed that the normal three dimensional map of the upper oesophageal sphincter was anteroposterior accentuation of peak pressures and also axial asymmetry with anterior peak pressures occurring 0.8cm closer to the pharynx (Welch et al 1979).
Kahrilas et al (1987a) used a modified Dent sleeve (1976) to
measure upper oesophageal sphincter pressure. The principle of the sleeve device is that it records the highest pressure acting at any site along its sensing membrane (Linehan et al 1985). The sleeve was 6cm in length and flat in cross section (3.3x7.2mm) so that it conformed to the slit-like upper oesophageal sphincter anatomy and held an anterior or p osterior orientation. When compared to rapid p u l l - through measurement of upper oesophageal sphincter pressure using a standard manometry system, the sleeve sensor measured significantly lower upper oesophageal sphincter pressure with less variability between subjects suggesting that either the rapid pull-through technique or water perfusion into the upper oesophageal sphincter zone stimulates the upper oesophageal sphincter to contract. Simultaneous upper oesophageal sphincter recordings, using a sleeve sensor and a side-hole sensor during a station pull-through, produced almost equal pressure values at the peak of the high pressure zone but the side-hole sensor recorded s ignificantly lower pressures than the sleeve sensor at 0.5 cm or more from the peak of the high pressure zone. During prolonged recording at the peak of the high pressure zone the sleeve recorded greater pressures than the side-hole sensor. This suggested that the side-hole sensor had a tendency to move relative to the peak upper oesophageal sphincter pressure. When stationary for 1 to 2 minutes both sensors recorded significantly lower upper oesophageal sphincter pressure, again suggesting that m ovement of the catheter stimulates the sphincter to contract. One limitation of the sleeve system was its relative inability
to record abrupt pressure increases. This meant that the d urat i o n of upper oesophageal sphincter relax a t i o n was u n d e r e s t i m a t e d .
The anatomy of the cricopharyngeus explains the differential p ressures recorded. The cricopharyngeal muscle originates from the lateral borders of the cricoid cartilage and then forms a continuous posterior sling for the upper oesophagus. There is no insertion into a median raphe (Lund 1965). Contr a c t i o n causes flattening of the upper oesophagus against the cricoid arch in an anterior-posterior direction. This can be confirmed radiologically (Fyke & Code 1955). In 13 patients who underwent partial laryngectomy the upper oesophageal sphincter pressure fell after a cricopharyngeal myotomy. If no myotomy was performed, then the upper oesophageal sphincter pressure remained the same. There was no correlation between upper oesophageal sphincter pressure and ability to swallow as long as there was glottic competence (Berlin et al 1977). Studies of patients after total laryngectomy (Welch et al 1979) showed lower peak pressures (40% of control) with loss of radial pressure asymmetry. Three dimensional mapping showed that axial asymmetry had also vanished. It therefore appeared that the anatomical alterations produced by laryngectomy abolished upper oesophageal sphincter pressure asymmetry.
Peak upper oesophageal sphincter pressures are recor d e d in an antero-posterior direction. It is clear therefore that great care must be taken to ensure that the catheter o rifices are uniformly positioned. Meaningful comparisons cannot be made between studies unless this c o n v e n t i o n is
o b s e r v e d .