Capacitación y entrenamiento

4.1.3.1 Manometry

Several methods have been developed for measuring anal canal pressures, the values recorded depending upon the method employed. Each technique can influence the pressure that it is recording, and each has its own advantages and disadvantages. The most commonly used systems in current practice are microballoons and continuous perfusion.

Microballoons ; an air or water filled balloon inserted into the anal canal and connected to a pressure transducer in a closed system will transmit the net circumferential pressure exerted on the balloon over the length of anal canal along which there is contact between the balloon and anal lining. The recorded pressure will therefore vary according to the size, shape and compliance of the balloon, problems which are overcome by using a standard small balloon. The compliance of the anal canal wall will also affect the recorded pressures; rigid scar tissue may exert high pressures on the balloon although functionally the scarred tissue may well not be as effective at closing the anal canal as normal tissue from which lower pressures may be obtained. On the other hand, this may be seen to reflect resistance to distension sensitively. The balloon should be of sufficiently small diameter so as not to induce anal contraction. Although a balloon system can be used to measure pressures at different levels along the anal canal, this cannot be done simultaneously. Thus the tube connecting the balloon to the transducer must be marked such that the position of the balloon within the anal canal is known. Further, since the functional length of the canal may change after fistula surgery, a method must be used such that the observer is sure that he or she is recording pressures at the same site in the anal canal pre- and postoperatively. Fistulotomy may only shorten the canal length by a reduction in its distal part, and thus the reference point must be at the craniad end of the canal. A suitable reference point would

be 5mm caudal to the point of first rise in pressure on withdrawing the balloon from the rectum and other pressures measured at set distances from that level, that is every one centimetre. Microballoons cannot register radial differences in pressure, which may be of significance in the context of fistula surgery in which part of the sphincter may be radially divided as in standard fistulotomy. However it may be argued that continence is a function of the overall circumferential pressure and that intraluminal pressure at a given level is the composite of vector forces exerted from around the whole circumference, in keeping with Pascal's Law that pressure in a closed space is uniform. From this viewpoint, radial pressures may be considered artefactual resultant upon leverage of the probe within the canal.

Miller at ai (1989b) devised an air filled microballoon system which they claimed had certain advantages over the water filled system; no potential problems could be engendered by air bubbles in the balloon or connecting catheter, the device did not need to be zeroed on each occasion against the level of the anal verge, and there would be no artefactual changes in pressures recorded due to catheter movement. Although the measured pressures were lower than the equivalent pressure recorded with the water filled microballoon (and therefore the two systems are not interchangeable) there was a high degree of correlation between the two methods, which, in the absence of a "gold standard" implies that neither is more or less accurate than the other.

Perfusion systems; the pressures measured by a system involving the continuous perfusion of fluid through a port in a tube inserted into the anal canal will give an index of the resistance to flow out of the catheter and will therefore depend upon the diameter of the catheter, its compliance, and the rate of perfusion. Modern systems use small diameter tubes of low compliance so that changes in pressure are registered rapidly, and they have a constant low perfusion rate to avoid

accumulation of fluid in the anorectum which might lead to involuntary contractions. Perfused side holes will detect radial as well as longitudinal pressure differences, and with multiple side ports these can be measured simultaneously. As with any measuring system, the position at which the pressure is recorded must be known, and for purposes of analysis of data, a catheter with perfused side holes arranged longitudinally at set distances or arranged radially at the same level would be the most practical.

The Anorectal Physiology Unit at St. Mark's Hospital employs a water filled microballoon system, and this has been used in the study. Manometry was performed by the station pull-through technique (Rogers at al 1989) using a standard 4mm diameter water filled microballoon attached to a catheter (marked at one cm intervals below the microballoon midpoint) which was connected, via a pressure transducer (Medex MX 860, Medex Inc., Haslington, UK) to an amplifier (Lectromed MT & P, Lectromed, St. Peter, Jersey) and recording device (Multitrace 6, Lectromed, St. Peter, Jersey). The level of the anorectal junction was judged as the level at which the recorded pressure rose steeply on withdrawal of the microballoon from the low pressure rectum into the higher pressure anal canal. Resting anal pressures were measured at one centimetre stations along the length of the anal canal, starting 0.5cm distal to the anorectal junction. Squeeze pressures (the highest incremental rise in pressure on 3 separate maximal voluntary contractions of the sphincters) and total pressures (the sum of resting and squeeze pressures) were then measured at the same levels.

Functional anal canal length at rest was measured as the distance between anorectal junction and anal verge using the same microballoon.

4.1.3.2. Anal epithelial electrosensltlvlty

Anal epithelial electrosensitivity was measured using a bipolar ring urethral electrode (Dantec 21L10, Bristol, UK) mounted on a urinary catheter and lubricated with antiseptic (electrically conductive) mucilage. The electrode was placed in the mid anal canal with the lower electrode positioned 1cm above the anal verge. The electrode was connected to a constant current stimulator (Neuromatic 2000M, Bristol, UK) and square wave stimuli at 5Hz and of lOOpis duration were applied. The stimulating current was gradually increased from zero until the stimulus was perceived by the patient, usually as a tapping, pricking or burning sensation. The test was repeated 3 times and the lowest threshold stimulating current was recorded (Roe et al 1986; Rogers et al 1988). The mounted electrode was then withdrawn from the anal canal, and a strip of insulating tape placed around three-quarters of the circumference of the electrode. The electrode was then repositioned in the anal canal at the same level, and electrosensitivity recorded in each of the four quadrants

(anterior, posterior, left lateral and right lateral).

4.1.3.3 Pudendal latencies

The left and right pudendal nerve terminal motor latencies (PNTML) were measured using the St.Mark's pudendal electrode (Dantec UK Ltd, Bristol) connected to a Neuromatic 2000M neuromyograph (Dantec UK, Ltd, Bristol). The surface tip electrode adheres to the gloved index finger which is passed into the rectum to stimulate the pudendal nerve transrectally in the region of the ischial spine on each side (Kiff and Swash 1984). External sphincter contraction is recorded using a separate electrode positioned at the base of the finger. A square wave stimulus of 0.1ms duration, at IHz was applied and the minimum current used to obtain an easily reproducible latency.

4.1.3.4 Perineal position

The position of the anal verge relative to the ischial tuberosities was measured at rest and on straining, using the St.Mark's perineometer (Henry et al 1982).

4.1.3.5 Rectal sensitivity

4.1.3.5.1 Rectal sensation to balloon distension

Although rectal compliance and sensitivity to distension would not be expected to change as a result of surgery for anal fistula, it has a bearing on continence; accordingly, threshold, urge and maximum tolerated volumes to balloon distension of the rectum were recorded using a balloon attached to a catheter and air filled syringe, the base of the balloon positioned 10 cm from the anal verge. The balloon was initially inflated and then deflated to allow correct configuration within the rectum. The balloon was progressively distended with air and the volumes recorded when the patient was asked to report the initial sensation (threshold volume), the sensation of urgency (urge volume) and the limit of tolerance (maximum tolerated volume) (Henry et al 1985).

4.1.3.5.2 Rectal mucosal electrosensitivity

Rectal mucosal electrosensitivty was assessed using the same bipolar ring electrode as was used to assess anal epithelial electrosensitivity, but with the electrode placed 10cm from the anal verge and with a stimulus of lOHz frequency and of 500 ^sec duration (Kamm and Lennard-Jones 1990), with a gradually increasing current until the patient perceived the stimulus. The test was repeated 3 times and the lowest threshold stimulus was recorded.