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Intestinal obstruction can be the sequel to either mechanical or functional causes. Mechanical obstruction occurs due to physical blockage of the intestinal lumen whereas functional obstruction results from a decrease or inhibition of intestinal motility due to loss of smooth muscle contraction (Brown et. al, 2007).
===Mechanical Obstruction===

* Acute of chronic mechanical obstruction of the intestine can occur in all species.
** Acute obstruction usually involves the upper or middle [[Small Intestine - Anatomy & Physiology|small intestine]]
** Chronic obstruction typically occurs in the distal [[Small Intestine - Anatomy & Physiology|small intestine]] or [[Large Intestine - Anatomy & Physiology|large intestine]].
* Three main categories of causes of obstruciton:
** Intraluminal
*** E.g. foreign bodies, food impaction.
** Intramural
*** E.g. neoplasia
** Extrinsic
*** E.g. adhesions, neoplasia and prostate enlargement.

====Intraluminal Obstruction====

=====Foreign Bodies=====

* Foreign bodies of all types can be found in the intestines.
* While some may pass through posing no problems, others can cause acute obstruction,
* Quite common in dogs
** Rare in other species - tend to lodge in the oesophagus or in one of the ruminant [[Stomach and Abomasum - Anatomy & Physiology|stomachs]].)
** Enteroliths can be seen in horses greater than 4 years of age.
*** Are stones consisting of magnesium ammonium phosphate around a central nidus (often a metallic foreign body)
*** Typically lodge at the pelvic flexure or the transverse [[Colon - Anatomy & Physiology|colon]].
* '''Clinical'''
** Obstruction at pylorus produces repeated [[Control of Feeding - Anatomy & Physiology#The Vomit Reflex|vomiting]].
** Obstrustion lower down gives less dramatic effect.
*** Is still a problem if in the middle of the [[Small Intestine - Anatomy & Physiology|small intestine]]s.
*** May be vague signs; some [[Control of Feeding - Anatomy & Physiology#The Vomit Reflex|vomiting]] and off food.
* '''Diagnosis'''
** May not show up well radiographically (unless radio-opaque) for several days.
*** May also be objects that are semi solid or soft, e.g.
**** String
**** Plastic bags
**** Stringy things, like pieces of material- particularly in puppies.
** Make all of intestines have knotted appearance.
** May be seen in horses with baler twine.
* '''Pathogenesis'''
** Smooth, round objects, such as golf balls, lodge especially near the pylorus or lower down.
** Occasionally in cattle (piece of rope or piece of tarpaulin) produces a tangled mass in [[The Rumen - Anatomy & Physiology|rumen]].
** Cause pressure necrosis and eventually perforation.
** Foreign bodies can also be chronic, remaining for long periods of time without causing disturbance.

=====Impaction=====

* Impaction of the [[Colon - Anatomy & Physiology|colon]] can occur in all species.
** Dog and cat - main cause is dehydrated faecal material.
** Horse - faeces, digesta, sand, or fibrous material can all contribute.
*** There are certain predisposing factors:
**** Poor dentition
**** Water deprivation
**** A high roughage diet
**** General debility.
*Antihelminthic administration or large parasite burdens can also lead to impaction.

====Extrinsic Obstruction====

* Obstruction of the intestine due to external factors such as tumours, abscesses, and fibrous adhesions is a common occurrence.

=====Inflammatory Adhesions=====
[[Image:colon_adhesions_horse.jpg|thumb|right|150px|colonic adhesions in the horse (Courtesy of Elspeth Milne)]]

* Arise following gut perforation, peritonitis or surgery.
* Consist of fibrous tissue bands that may:
** Restrict intestinal motility
** Cause kinks in the mesentery.

=====Prostatic Enlargement=====

* In the dog
* Can lead to compression of the [[Rectum - Anatomy & Physiology|rectum]]

=====Neoplasia=====

* Neoplasi in structures adjacent to the intestines can spread and cause external compression.
* Pancreatic tumours in particular can extend and impinge on the [[Duodenum - Anatomy & Physiology|duodenum]].
* Pedicles of tumours such as lipomas in horses can become wound in loops of intestine leading to obstruction and possible strangulation.
[[Image:Pedunc_lipoma_closeup.jpg|thumb|right|150px|Closeup of a pedunculated lipoma (Courtesy of Elspeth Milne)]]
* '''Clinical'''
** Occasional [[Control of Feeding - Anatomy & Physiology#The Vomit Reflex|vomiting]] and ntermittent [[Intestine Diarrhoea - Pathology|diarrhoea]] over several weeks.
* '''Pathogenesis'''
** Seen occasionally in cat (rarer in dog)
** Usually towards end of intestines
*** E.g. at the ileocaecocolic valve.
** Gut proximal to tumour becomes thickened due to hypertrophy of smooth muscle as a result of trying to force ingesta past progessively narrowing lumen.
** Produces "hose pipe intestine".
** Seen with carcinoma, lymphoma, leiomyoma and other tumours.

===Functional Obstruction===

====Paralytic Ileus====

* A common condition.
* Occurs following trauma or abdominal surgery.
* Stasis of gut flow due to failure of [http://en.wikipedia.org/wiki/Peristalsis| peristalsis].
** Leads to distension with gas and fluid, as well as a flaccid intestinal wall.

=====Causes=====

* Anything which stops peristalsis, e.g.
** Damage to nerve supply to intestine (autonomic nervous system)
** Pain
** Abnormal metabolism
** Toxaemia
** Electrolyte imbalance such as hypocalcaemia, hypomagnesaemia, and hypokalaemia.
* Also in
** Diabetes mellitus
** Uraemia
** Tetanus
** Lead poisoning.

=====Pathology=====

* loss of smooth muscle tone leads to a flaccid bowel.
* Bowel is distended with fluid.

=====Pathogenesis=====

* Intestine susceptible to neurogenic damage during an operation.
* Peristalsis fades away over a few days producing paralytic (adynamic) ileus.
* Particularly occurs if bowel handled roughly, or if serosa gets cold and dry at surgery.
* Very difficult to start peristalsis again but will sometimes respond to pharmacological or electrical stimulation.
* The horse is very susceptible, and the dog is somewhat suscpeitble.

====Dysautonomia====

* Most notably affects horses and cats.

=====Equine dysautonomia, or grass sickness=====

* Most prevalent in the UK and western Europe.
** Common in wetter areas, e.g. the South West.
* Seen in horses out at pasture in late summer and autumn.
* Usually affects young adults.
** 6-7 years old.
* '''Clinical'''
** Acute oneset:
*** Muscular tremors
*** Abdominal pain
*** Does not eat
*** Constipation
*** Become severly tympanic in acute cases
*** Dull and restless
*** Avoid swallowing
*** Salivate excessively
** Degenerative lesions are seen in the autonomic nerve ganglia, including enteric plexuses
** May either:
*** Progress rapidly to death
*** Take a slower clinical course.
**** Eat a bit, but food drops out of mouth
**** Go on to die slowly.
** Some horses recover
*** This is very unlikely, and the condition is usually fatal.
** Clinically difficult to diagnose - signs are confined to the gut.
*** Easy to diagnose on post mortem
* '''Pathology'''
** [[Forestomach - Anatomy & Physiology|Stomach]] and [[Small Intestine - Anatomy & Physiology|small intestine]] large amounts of contain watery yellow fluid.
*** There is an abrupt change in the [[Large Intestine - Anatomy & Physiology|large intestine]], where no fluid is present.
**** [[Large Intestine - Anatomy & Physiology|large intestine]] has very dry mucoid contents.
* '''Pathogenesis'''
** Due to functional obstruction at ileocaecal valve and a degree of paralytic ileus of the [[Small Intestine - Anatomy & Physiology|small intestine]].
** The exact cause is unknown, but a type of bacterial or fungal toxin which may damage autonomic nervous system ganglia may be involved.
*** ''[[Clostridium botulinum]]'' is thought to be involved.
* A similar condition seen in hares
** Certain yeares almost seem to have outbreaks.
* Certain pastures at certain times of year produce grass sickness quite often.
** A definitive diagnosis must be made - if the condition is due to the grazing we need to know.
*** E.g. if on livery or stud grazing, may put people off going there.
* 'Diagnosis'''
** At post mortem look for degenerative changes in coeliaco-mesenteric ganglia - need to examine histologically.
*** Ganglia are peanut sized and found in perirenal fat between adrenal gland and the aorta.

=====Feline dysautonomia, or Key-Gaskell Syndrome=====
[[Image:Ba 250 07.jpg|thumb|right|Histological section of degenerate neuron in feline dysautonomia(Courtesy of Susan Rhind)]]
* Occurs mostly in the UK and continental Europe.
* Is also of unknown aetiology. Suggested causative factors include:
** Environmental toxins
** Infectious agents
** Botulinum toxins .
* Clinical signs:
** Anorexia
** Depression
** Bradycardia
** Decreased lacrimation,
** Altered pupillary dilataion,
** [[Megaoesophagus]]
** Constipation.
* Degenerative lesions of autonomic nerve ganglia can be seen.
* Also occurs in the [[Key-Gaskell Syndrome|oesophagus]].
[[Category:Intestine - Physical Disturbances]]
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