Differences between mechanical and paralytic ileus: causes, symptoms and treatment

By Cristiano Antonino On Sep 16, 2022

In medicine, ileus or intestinal obstruction refers to a pathological condition characterised by the partial or total cessation of the progression of intestinal contents, whether liquid, solid or gaseous

What is the difference between a mechanical ileus and a paralytic ileus?

What mainly differentiates a mechanical ileus from a paralytic ileus is the aetiology; depending on the cause, a distinction can be made:

an ileus is said to be mechanical when the occlusion is due to an actual obstruction that physically blocks the passage of material in transit in the direction of the anus;
an ileus is said to be paralytic (or adynamic) when there is no physical blockage in the intestine but transit is nevertheless impeded by a paralysis of the musculature proper of the intestine with consequent blockage of peristalsis, that is, of that coordinated contraction of the smooth muscles present in the digestive tract which allows food to proceed in the direction of the anus.

Intestinal subocclusion, on the other hand, is defined as a particular condition of mechanical ileus, in which the obstruction is only partial, manifested by subacute and/or recurrent episodes

In any case, the occlusion may be incomplete, characterised by chronic sub-occlusive episodes culminating in the critical occlusion phase, or complete, which begins with an acute and dangerous clinical picture.

It is also important to remember that a mechanical ileus can recognise three types of obstruction:

intraluminal: the obstruction is physically present within the canal and closes the lumen (e.g. clusters of parasites and faecalomas);
intramural: the obstruction is represented by the enlargement of the canal wall (e.g. a ring tumour);
extra-intestinal: the obstruction is a mass outside the canal, which has expanded so much that it compresses the lumen (e.g. tumour of a neighbouring organ).

An important difference between mechanical and paralytic ileus is that

mechanical ileus recognises a ‘local’ cause (with suffering generally affecting a circumscribed segment of the bowel while the rest of the viscera is only involved later): the segments upstream of the obstruction will gradually appear more and more dilated, while those downstream will be normal, allowing the expulsion of the material they contain and initially giving an illusion of normal canalisation.
A severe and untreated occlusion can have tragic results, leading in some cases to perforation and copious haemorrhaging; in paralytic ileus the suffering affects the entire intestine, i.e. the paralysis affects not a particular segment, but the entire bowel.

The anatomo-pathological picture initially shows a modest dilatation of the first intestinal tracts (which can be adequately decompressed with the introduction of a naso-gastric tube) and sometimes of the distal ones (for which a rectal probe is used for the same purpose).

Rarely does the paralytic occlusion reach worrying levels, as the paralysis usually regresses within a few hours; thus, this form presents a less severe clinical course than the mechanical form (a typical example is post-operative paralytic ileus).

The double meaning of ileus

In medicine, the term ‘ileus’ is used not only to indicate the halting of the progression of intestinal contents, but also – in human anatomy – to indicate the final part of the small intestine that comes after the duodenum and jejunum (the initial portions of the small intestine) and precedes the cecum (the initial portion of the large intestine).

The ileum is highlighted in green in the figure below.

Causes of mechanical ileus

The main causes of mechanical ileus are:

Obstruction. Very frequent is related to the presence of an obstruction of various kinds in the lumen of the small intestine. These can be:
ingested foreign bodies;
particularly voluminous foodstuffs;
clusters of hair or plant substances such as tricho- or phyto-bezoars;
gallstones (biliary ileum);
parasite clusters (intestinal worms);
adhesions.

Stenosis of the bowel

When tumour or inflammatory or malformative diseases narrow the lumen of the viscera.

A narrowing of the lumen may also be the result of an intestinal anastomosis, a haematoma in the intestinal wall or the scarring result of one or more duodenal ulcers.

Usually these forms are ingravescent and therefore the actual occlusion is preceded by sub-occlusive episodes that are often misrecognised.

Compression

This is the situation in which an extrinsic mass presses on the intestine, occluding it.

It is most often due to neoplastic pathology.

Angulation

It is a mechanism usually due to previous surgery (especially open surgery) or intra-abdominal pathologies that have led to the formation of single or multiple adherential bridges.

These adhesions attach the intestinal loops to each other or to other organs or to the wall, angling them and consequently occluding them.

Strangulation

The term indicates different situations distinguished by a common element: severe suffering of the loop due to compression of its vascular pedicle.

Strangulation occurs in:

Volvulus: when the entire intestinal loop and therefore the vascular pedicle contained in its mesentery rotates around its own axis, twisting.
Invagination: when an intestinal loop enters a contiguous loop (like a telescope segment) dragging the vascular pedicle along with it and compressing it.
Girdle strangulation: when an intestinal loop enters a loop or anatomical foramen and remains incarcerated there along with its pedicle.

Mechanical ileus caused by damage to the colon

A possible cause of mechanical ileus is an obstruction that impedes the progression of faeces in the colon, which in turn leads to stasis upstream in the small intestine.

A colonic obstruction may be caused and/or promoted by one or more factors, including:

tumours;
intestinal polyps;
inflammatory stenosis (adhesions);
accumulation of parasites (intestinal worms);
foreign bodies introduced via the anal route;
faecalomas.

Causes of paralytic ileus

The main causes of paralytic ileus are:

opening of the peritoneum or/and manipulation of the endo-abdominal viscera (from surgery): this is the most common situation;
presence of foreign bodies or biological substances (blood, bile, urine);
peritoneal irritation (peritoneal abscesses, perforation of viscera, vascular distress of the intestine, open or closed abdominal trauma) and abdominal viscera diseases (appendicitis, cholecystitis);
acute pancreatitis, retroperitoneal pathology (aneurysms), spinal trauma, severe pain pictures (renal colic), torsion of ovarian cysts;
organ pathologies (heart attack, pneumonia, stroke);
general (hydro-electrolyte alterations, dysmetabolism, drugs that block nerve transmission (ganglioplegics), or acetylcholine antagonists (anticholinergics), or antihistamines, or general anaesthetics (narcotics).

Symptoms and signs of paralytic ileus

In the case of paralytic ileus, the subject presents an often unspecific and nuanced picture, with:

nausea;
vomiting;
abdominal distension;
pain that is generally not very intense and poorly localised.

In general, the unremarkable extent of the symptoms serves the physician to differentiate a paralytic ileus from a mechanical ileus, which generally causes more severe signs and symptoms.

Symptoms and signs of paralytic ileus

In the case of mechanical obstruction, the symptoms and signs are generally more severe and specific than in paralytic ileus.

Symptoms and signs of mechanical ileus are:

Closure of the alvus to faeces and gas (absence of canalisation). This is a pathognomonic symptom although it occurs at varying times and in varying ways. In the case of a high occlusion, either at oesophageal or gastric level, the alvus will still remain open for some time as the intestine downstream of the obstruction is intact and therefore functioning. In the case of a low occlusion, at the level of the rectum, the closure of the alvo will vice versa be immediate. In low and incomplete occlusions there may be diarrhoea, pseudo diarrhoea, characteristically alternating with periods of constipation.
Vomiting (faecal or alimentary or biliary). The extent and time of onset of vomiting also depend on the level and type of occlusion. It will be early in high forms and the presence of bile (which is secreted at the level of the second duodenal portion) will help distinguish intestinal forms from oesophageal and gastric forms where it will be absent. In low-level obstructions vomiting will occur later, often taking on faecaloid connotations and becoming rarer or absent in rectal obstructions.
Pain. Pain is an important symptom related in particular to increased peristalsis. In the high forms it may be intense and intermittent. In the ileal forms it is cramp-like, paroxysmal, interspersed with periods of respite. In the low forms it takes on a dull, gravative character.

The sudden onset of a constant stabbing pain when accompanied by other peritoneal symptoms indicates serious complications such as strangulation, perforation and haemorrhage.

The sudden disappearance of the pain may signify the spontaneous resolution of the occlusive state, but more often than not it indicates an aggravation of it with the transformation of a mechanical ileus into a paralytic ileus.

Abdominal distension

Absent in the high forms, it is very evident in obstructions at the level of the descending colon or even lower.

The distension of the abdominal wall is the direct and visible consequence of that of the intestine, which in turn is linked to the accumulation of liquids and air.

Hypovolaemia

Is the subtraction of liquids to the total water mass, from the intestinal lumen of the segments upstream of the obstruction.

This hypovolaemia is aggravated by the losses that occur with vomiting and leads to blood concentration and hypovolaemic shock.

Loss of electrolytes and ions

Vomiting, depending on the level of obstruction, leads to significant losses of different ions and electrolytes.

This results in electrolyte imbalances, particularly in low occlusions, and acid-base imbalances in higher occlusions with metabolic alkalosis or acidosis.

Other signs: fever, tachycardia, drop in blood pressure, peristalsis evident through the abdominal wall, metallic noises on auscultation may be present to varying degrees.

Different treatment between mechanical and paralytic ileus

The treatment is also different: while in paralytic ileus the upstream pathology that caused it is treated (e.g. peritonitis or mesenteric vascular insufficiency, medical treatment), in the case of mechanical ileus the treatment is mainly surgical (with open surgery or with the use of colonoscopy), aimed at physically eliminating the cause of the blockage.

Finally, it is important to remember that a paralytic ileus may often represent the advanced phase of an initially mechanical ileus, while a mechanical ileus may more rarely represent the advanced phase of an initially paralytic ileus.

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