Efficacy of the Mannheim Peritonitis Index (MPI)

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Introduction

Peritonitis is inflammation of the peritoneum. It may be classified as either primary or secondary, localised or generalised, or based on the presence of infectious agents (septic or non-septic). Secondary peritonitis is mostly septic and often results from gastrointestinal tract pollution. It can be secondary to trauma, ischemic damage, neoplasia, ulceration, foreign body blockage, or dehiscence of previous surgical wounds. Gastrointestinal contents, including bacteria and digestive enzymes, seep into the peritoneal cavity following perforation of a hollow viscus. This sets off an inflammatory reaction resulting in peritonitis, sepsis, and circulatory collapse.

Depending on where the perforation occurred, perforation peritonitis might appear differently clinically. For instance, odynophagia, vomiting, and abrupt chest pain are common symptoms of oesophageal perforations, but sudden, severe stomach pain is usually the first sign of gastroduodenal perforations. Conversely, colonic perforations could proceed more slowly and result in localised abscesses or secondary bacterial peritonitis. Abdominal examination findings include tenderness, guarding, and systemic inflammatory symptoms such as hypotension and tachycardia. Perforation peritonitis is a surgical emergency, and the objectives of surgery are to identify and treat the infection's cause, lower the abdomen's infectious burden, and facilitate healing.

One of the commonest causes is perforation of the gastrointestinal tract, which causes extensive inflammation and infection of the peritoneal cavity (perforation peritonitis) that can be rapidly fatal. With its quick development, it poses a serious issue, with mortality ranging from 10% to 40%, even with recent advances in medical and surgical therapy [1].

Accurate risk classification systems help better predict patient outcomes, prioritise resources, and customise treatment plans. Numerous scores, such as the Sequential Organ Failure Assessment (SOFA) score, the Mannheim Peritonitis Index (MPI), and the Acute Physiology and Chronic Health Evaluation (APACHE) score, have been created for this purpose. Although the APACHE score is considered one of the most effective scoring systems for assessment of disease severity, it is not specific to peritonitis. Factors such as the aetiology of the perforation or type of peritonitis are not included, potentially making MPI a more effective tool in this clinical setting.

The MPI was first described in Germany in the 1980s to forecast outcomes for individuals suffering from peritonitis, using a straightforward grading method that considered clinical, laboratory and operative findings [2]. Age, gender, organ failure, malignancy, preoperative length of peritonitis, source of sepsis, extent of peritonitis, and the type of exudate are the eight prognostic criteria included. The overall score may vary from 0 to 47, with a particular value allocated to each element. According to a pilot study by Billing et al., higher MPI scores are associated with more severe illness and a greater probability of death [3]. While MPI has shown prognostic value in prior studies, its utility in Indian populations, characterised by unique demographic and clinical features, remains underexplored. Furthermore, in resource-limited settings like India, bedside prognostic tools play an important role in prognosticating patients.

Materials and methods

This study was a prospective observational study conducted over two years (June 2022 to December 2024) in the Department of General Surgery at a tertiary care hospital in Western Maharashtra, India. The sample size was calculated using the following formula:

\[ n = \frac{\text{DEFF} \times N \times p(1 - p)}{\left( \frac{a^2}{Z^2_{1 - \alpha/2}} \times (N - 1) \right) + p(1 - p)} \]

Where hypothesised percentage frequency of the outcome factor in the population (p): 14% ±10; confidence limits as a percentage of 100 (absolute ±%): 10%; Z is the value from the standard normal distribution corresponding to the desired confidence level (Z = 2.68 for 95% CI); p is the expected true proportion = 3.3% (p = 0.045), hence q = 1-p = 0.955; e is the desired precision = 5% (e = 0.05).

Adding these values to the above formula results in a sample size (n) of 47 [4]. If we add 10% missing data or non-response, the minimum required sample size will be 52. Our study had a final sample size of 72 patients who fit the inclusion criteria.

The study population comprised patients aged over 18 years who presented to the emergency department with clinical and radiological suspicion of perforation peritonitis, later confirmed intraoperatively. Pregnant women and patients below 18 years of age were excluded. The study protocol was approved by the institutional ethics committee, and written informed consent was obtained from all participants, with adherence to the standards for research involving human participants.

On presentation, using history, clinical examination, laboratory investigations, and intraoperative findings, variables found in the MPI were assigned the values indicated (Table 1), and the total MPI score for each patient was calculated by adding these. The minimum possible score was zero if no adverse factors were present, and the maximum was 47 if the presence of all was confirmed. Organ failure included renal, cardiovascular, pulmonary and gastrointestinal insufficiency. Oliguria (<20 mL/hour), elevated creatinine (>1.6 mg/dL), or elevated urea (>60 mg/dL) was counted as acute kidney injury. Low pO₂ (<50 mmHg) or high pCO₂ (>50 mmHg) represented respiratory insufficiency, while the presence of shock represented cardiovascular failure. Intestinal obstruction was defined as either bowel paralysis lasting more than 24 hours or a complete mechanical obstruction.

Table 1

Mannheim Peritonitis Index

Source: Reference [2]

Risk Factor	Adverse	Points Assigned	Favourable	Points Assigned
Age	>50 years	5	<50 years	0
Gender	Female	5	Male	0
Malignancy	Present	4	Absent	0
Organ dysfunction	Present	7	Absent	0
Evolution time	>24 hours	4	<24 hours	0
Origin	Non colonic	4	Colonic	0
Extent of peritonitis	Generalised	6	Localised	0
Peritoneal exudate	Fecal	12	Clear	0
	Purulent	6

After diagnosis, every patient underwent the required operative procedure at the earliest. Choice of incision and most appropriate procedure (e.g., resection and anastomosis, primary closure, stoma creation, etc.) and other operative decisions were left to the lead surgeon on a case-by-case basis. Adequate wash of at least three to four litres of warm saline was ensured in every case. Postoperative care was provided as necessary.

Demographic personal data such as age and gender, dates of admission and discharge from the hospital, days hospitalised, date of surgery and information related to the surgical procedure performed and operative findings were recorded. Patients were followed up for 30 days postoperatively, and outcomes were recorded in terms of discharge, morbidity, or mortality. Morbidity was further specifically measured in terms of postoperative surgical site infections, wound dehiscence, intra-abdominal abscess formation, acute renal failure and pulmonary complications. A surgical site infection was defined as an infection occurring up to 30 days after surgery near the surgical site. Acute renal failure was defined by the RIFLE criteria (Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease) [5]. Pulmonary complications were identified by pulmonary insufficiency, which was defined as the presence of any of the following on clinical examination, arterial blood gas analysis or chest X-ray. This included an oxygen saturation of less than 85% on room air, or pO₂ < 50 mmHg or pCO₂ > 50 mmHg on arterial blood gas analysis. Chest X-ray findings included the presence of radiological signs like bat-wing opacities, Kerley B lines, upper lobe diversion, peribronchial cuffing and airspace opacification.

Cases were divided into three groups based on their total MPI score, similar to the papers by Billing et al. [3], Notash et al. [6], and Ntirenganya et al. [7]. These included those with a total score below 21 points, those between 21 and 29 points, and those above 29 points, for further analysis and prognostication. The correlation between total MPI score and patient morbidity and mortality was analysed to determine if the MPI is an effective prognostic index in patients with perforation peritonitis. Statistical analysis was performed using IBM SPSS Statistics for Windows, Version 26 (Released 2020; IBM Corp., Armonk, New York, United States).

Results

This prospective observational study was carried out at a tertiary care teaching institute in Pune, India, over two years, with a study population of 72 patients. Our relevant findings in terms of age and sex distribution, mortality rates, and postoperative morbidity in terms of surgical site infections, wound dehiscence, intra-abdominal abscess formation, and renal and pulmonary complications are shown below.

Age and gender distribution

The majority of our patients lie in the 20 to 50 year age bracket, with a strong male predominance. The age and gender distribution of our population was given in Table 2 and Figure 1.

Table 2

Age Distribution of Study Population

Age Distribution	Number of Subjects	Percentage of Total
≤20 yrs	6	8.3
21-30 yrs	16	22.2
31-40 yrs	15	20.8
41-50 yrs	15	20.8
51-60 yrs	6	8.3
>60 yrs	14	19.4
Total	72	100

Figure 1

Gender Distribution of the Study Population

Source of perforation

An analysis of the source of perforation found intra-operatively showed the leading cause to be pre-pyloric in the study. This likely correlates with the lower socioeconomic strata that our hospital primarily serves, as well as the unique demographic characteristics of the area (Table 3 and Figure 2).

Table 3

Source of Perforation

Source of Perforation	Number of Patients	Percentage
Appendicular	14	19.4
Duodenal	4	5.6
Gallbladder	6	8.3
IIeal	9	12.5
Jejunal	9	12.5
Pre-pyloric	27	37.5
Rectum	3	4.2
Total	72	100

Figure 2

Source of Perforation

GB: Gallbladder

Mortality

Of the study population of 72 patients, 60 survived, while 12 succumbed. Of the 12 patients that succumbed, 10 had an MPI score greater than 29, while almost half the patients that survived (26 out of 60) had an MPI score less than 20 on admission. A chi-square test was performed to interpret the results, and the p-value was found to be <0.05. This indicates a statistically significant association, highlighting the strong prognostic value of the score in terms of postoperative mortality. The relationship between MPI and mortality rates is present in Table 4 and Figure 3.

Table 4