Continuous ambulatory peritoneal dialysis—a guide to imaging appearances and complications
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The aim of this article is to review and illustrate the typical imaging findings for a patient on continuous ambulatory peritoneal dialysis (CAPD) and its complications, examining the uses and limitations of multimodality imaging.
CAPD is a commonly and increasingly used method of renal replacement therapy in end-stage renal failure (ESRF). From the set-up and insertion of the peritoneal catheter through to the actual treatment, there are pitfalls and complications that may adversely affect the patient and compromise the success of the dialysis. Complications can be either immediate or delayed, and can also be categorised into infectious and non-infectious aetiologies, including catheter failure, dialysate leaks, hernias and encapsulating sclerosing peritonitis.
Early recognition of complications, both clinically and on the different imaging modalities, is essential in the management of CAPD in order to reduce treatment failure and limit patient morbidity and mortality.
Complications of peritoneal dialysis cause patient morbidity and treatment failure.
Early recognition of complications from normal appearances is essential to limit dialysis failure.
Multimodality imaging plays an important role in the diagnosis of these complications.
KeywordsContinuous ambulatory peritoneal dialysis (CAPD) X-ray computed tomography Ultrasonography Catheters Peritonitis
Renal replacement therapy is a rapidly evolving specialty. For patients with end-stage renal failure (ESRF) there are a limited number of options. They include peritoneal dialysis (PD), haemodialysis (HD) and, with increasing frequency, renal transplant, where available. By the end of 2005, the number of patients undergoing treatment for ESRD was estimated to have reached 1.9 million worldwide, of which 1.455 million were being treated by dialysis and the remaining 445,000 were living with a functional renal graft. Of the global dialysis population in 2005, 89 % underwent HD treatment and only 11 % were treated by PD. This equates to a global total approaching 160,000 at this time .
With the aim to optimise patient outcomes and quality of life, renal transplant if successful is in most cases the desired option. This is, however, not widely available due to limitations in the availability of organ donors and also appropriately skilled and trained surgeons. In these cases the patient has the option of HD and CAPD with the pros and cons different for both with CAPD utilised in approximately one-third of new ESRF cases in the US and Europe .
The main advantages of CAPD are speed and ease of use, relative inexpense, absence of need for a highly skilled operator and lack of need for anticoagulation. It is preferred where vascular access is challenging or in patients with cardiovascular disease as it induces less cardiovascular stress [5-7].
Disadvantages include many potential complications, some of which occur more frequently with CAPD than other renal replacement techniques. These may be medical, such as electrolyte/acid–base imbalance or infection, surgical or mechanical catheter related. The most common cause for failure of treatment is infection due to the in-dwelling catheter; however, there are many causes for treatment failure such as dialysate leaks, hernias, intestinal obstruction and most seriously encapsulating sclerosing peritonitis (ESP) .
CAPD was at one stage the treatment of choice for both acute (ARF) and chronic renal failure (CRF) before advances in HD and continuous renal replacement therapies (CRRT) demonstrated improved outcomes, making it no longer the treatment of choice in ARF patients. There are several relative contraindications to consider with CAPD, including recent abdominal or cardiothoracic surgery, diaphragmatic pleuro-peritoneal connections, peritonitis, severe respiratory failure and abdominal wall cellulitis . Alternative uses for PD other than renal replacement therapy have been seen for the treatment of acute pancreatitis  and hyperthermia/hypothermia [9-11], which have had limited success and are not routinely in clinical use.
Imaging techniques and normal appearances
Plain film radiography is cheap and readily available but of limited value. It can be used to identify catheter position, which should normally follow a path once inside the abdominal cavity caudally into the pelvis, and some complications such as ESP when advanced, perforation and hydrothorax if a pleura-peritoneal communication exists. It is, however, less sensitive and specific than computed tomography (CT) for diagnosing dialysate leaks and other complications, but can still be used as a first line in the investigation of complications .
Ultrasound with its lack of ionising radiation is a highly useful test that can identify many complications such as abdominal wall or intra-abdominal collections, peritoneal thickening, peritoneal calcification or thickened and dilated small bowel loops but has limitations due to operator dependence and lack of sensitivity to rule out pathologies such as focal peritoneal thickening seen in early SP compared with CT .
Indications for CT peritoneography
Difficulty with fluid exchange
Recurrent peritonitis leading to adhesions and loculated peritoneal fluid collection or abscess
Abdominal wall swelling or soft tissue oedema suggesting dialysate leak
Hernias—umbilical, abdominal wall, inguinal
Dialysate distribution in the supine position within the peritoneal cavity has been estimated to be approximately 30–55 % within the pelvis surrounding mesentery and bowel, 10–20 % in the upper abdomen in perihepatic and perisplenic location and only 1–3 % within the lesser sac . CT peritoneography has been shown to be superior to standard CT for the diagnosis of complications such as dialysate leaks, the path and site of which can be better delineated, and intra-abdominal abscesses, which appear as unopacified fluid collections within the peritoneal cavity [6, 13].
Use of IV and oral contrast is not routinely required and can be used selectively for indications such as suspected intestinal obstruction, or inflammatory aetiologies such as pancreatitis or bowel ischaemia. However, IV contrast has the disadvantage of potential nephrotoxicity in patients with residual renal function and its use therefore needs to be evaluated in the context of risk-benefit to the patient by the attending physician. Also, altered enhancement of structures such as small or large bowel surrounded by the opacified dialysate may be obscured if CT peritoneography is performed.
Magnetic resonance imaging (MRI) can be of value with its greater soft tissue contrast, lack of ionising radiation exposure and multiplanar capability. It can be performed with or without a mixture of a gadolinium-based contrast agent into the dialysate and then similar to CT peritoneography, allowing a 30-min delay and the patient to mobilise to achieve adequate distribution of the fluid. T1-weighted imaging can then be utilised with no IV or oral contrast required. Alternatively, T2-weighted imaging can be utilised to delineate fluid collections without the use of gadolinium. However, in addition to cost and lack of availability, the spatial resolution is inferior to CT and given the uncertain potential to induce nephrogenic systemic fibrosis from gadolinium exposure within the peritoneal cavity this technique is not routinely recommended .
Scintigraphy with intra-peritoneal injection of a radioisotope has been utilised in the imaging of hernial complications related to CAPD prior to the widespread use of CT peritoneography for this purpose  or to detect the presence of a pleuro-peritoneal fistula , but is not routinely used in clinical practice.
Bacterial infections causing peritonitis are the commonest occurring complication, with a reported frequency of one episode every 20–30 months per patient  and are the commonest cause of catheter replacement . The likeliest route of infection is via the dialysis catheter but can occur secondary to other intra-abdominal infective or inflammatory pathologies such as cholecystitis or diverticulitis .
Dialysate leakages as a complication occur in >5 % of patients and are often not of clinical significance . They can be classified as early at ≤30 days, where the likely aetiology is catheter related, or late at >30 days, likely due to a mechanical or surgical tear in the peritoneal membrane. With increased intra-abdominal pressure from the infusion of dialysate there is increased likelihood of a leak from the peritoneal cavity as well as respiratory compromise from splinting of the diaphragm. Other mechanisms that raise intra-abdominal pressure, such as coughing, straining or obesity, can also predispose to leaks [19, 20].
Encapsulating sclerosing peritonitis
Encapsulating sclerosing peritonitis (ESP) also referred to as sclerosing peritonitis is an inflammatory process leading to the deposition of a thick fibrous membrane on the peritoneum, occurring in approximately 1 % of patients on CAPD overall, with prevalence increasing with length of CAPD treatment, up to nearly 20 % at 8 years . The aetiology of ESP is unclear, but risk factors for its development include increasing length of CAPD—typically for several years, type of dialysate and peritonitis episodes—both bacterial and chemical [23, 24]. Its early diagnosis is important, as cessation of CAPD can prevent progression and further complications of this condition.
Early changes include peritoneal thickening and calcification with non-specific clinical presentations, such as colicky abdominal pain or ultrafiltration failure. It is extremely important to detect early visceral and parietal involvement, as this would indicate the potential for the development of bowel complications. Though overall less sensitive than CT, ultrasound performed with the presence of dialysate fluid has been shown to be sensitive for the diagnosis of ESP, with dilated peristalsing small bowel loops seen earliest and other findings of peritoneal thickening both visceral and parietal, and fibrinous strands also possible to diagnose .
Rarely, ESP presents at a time after the cessation of CAPD. When advanced, treatment is difficult and therefore early diagnosis can lead to a switch in renal replacement therapy and possible prevention of progression, which can ultimately lead to significant morbidity and mortality. The condition is not reversible, though immunosuppression has been reported to have some benefit, and is only amenable to surgical therapy to treat for acute complications such as small bowel obstruction .
Subcapsular hepatic steatosis
CAPD plays an integral role in renal replacement therapy with its many advantages to patients; however, there are significant adverse risks associated with it, leading to serious risk of morbidity and mortality. Early recognition of complications, both clinically and on the different imaging modalities, is essential in the management of CAPD in order to reduce treatment failure and limit patient morbidity and mortality.
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