Changes in gastric motor function related to ageing
The stomach is responsible for the accommodation of ingested food and fluids, their mixing with digestive enzymes, and grinding of solids into small particles.48 Liquids empty preferentially in a mono‐exponential fashion for low‐nutrient liquids and a more linear pattern as their energy density increases, while solids empty at a linear rate after an initial lag phase. Gastric emptying reflects the coordinated motor activity of the stomach and duodenum, which is controlled by feedback from neural and humoral signals generated by the interaction of nutrients with the small intestine. As a result, the rate of energy delivery to the small intestine is relatively constant and essentially independent of the ingested nutrient load. Small‐intestinal feedback may be modulated by previous patterns of nutrient intake so that gastric emptying is retarded in starvation, while emptying of glucose is more rapid after dietary glucose supplementation. The proximal region of the stomach, comprising the fundus and much of the gastric body, relaxes after meal ingestion and acts as a reservoir for the solid component of the meal, while liquids begin to empty. Tonic contraction of the proximal stomach also generates a pressure gradient to assist gastric emptying. The distal stomach, comprising the antrum and pylorus, is responsible for grinding solids and generating flow across the pylorus – the latter is predominantly pulsatile rather than continuous, and retrograde flow occurs frequently. Contractions of the stomach are linked to an underlying electrical rhythm of about three cycles per minute, generated by a gastric pacemaker. During fasting, the stomach and small intestine undergo cyclical activity about every 90 minutes, known as the migrating motor complex (MMC); it consists of motor quiescence (phase I), irregular contractions of increasing frequency (phase II), and a brief (5–10 minutes) period of regular contractions that propagate distally and serve to empty the stomach of indigestible solids and sweep the small intestinal lumen of debris (phase III). After a meal, the MMC is interrupted by integrated motor activity of the stomach and small intestine that regulates gastric emptying and facilitates digestion and absorption.
Table 17.2 Causes of delayed gastric emptying.
| Acute |
| Drugs (opiates, anticholinergics, L‐dopa) |
| Postoperative ileus |
| Viral gastroenteritis |
| Metabolic (hyperglycaemia, hypokalaemia) |
| Critical illness |
| Chronic |
| Idiopathic/Functional dyspepsia |
| Diabetes |
| Post‐surgical (including fundoplication) |
| GORD |
| Chronic liver disease |
| HIV infection |
| Other endocrine and metabolic (hypothyroidism, chronic renal failure, anorexia nervosa) |
| Muscular and connective tissue diseases (myotonic dystrophy, muscular dystrophy, dermatomyositis, systemic sclerosis, amyloidosis, tumour‐associated) |
| Neurological (central nervous system disease, spinal cord injury, chronic idiopathic intestinal pseudo‐obstruction, idiopathic autonomic degeneration) |
Table 17.3 Medications that affect gastrointestinal motility.
| Decreased motility or gut transit |
| Opiates |
| Anticholinergics |
| GLP‐1 receptor agonists |
| L‐dopa |
| Tricyclic antidepressants |
| Calcium channel antagonists |
| Nitrates |
| Phosphodiesterase type 5 inhibitors (e.g. sildenafil) |
| Clonidine (an α‐2 agonist) |
| Sumatriptan (a 5‐HT‐1P agonist)a |
| Increased motility or gut transit |
| Metoclopramide |
| Domperidone |
| Erythromycin (a motilin agonist)b |
| Prucalopride |
| Beta blockers |
| Selective serotonin reuptake inhibitors |
| Cholinesterase inhibitors |
| Excess thyroxine |
a Relaxes the gastric fundus and slows gastric emptying but increases oesophageal motility.
b Stimulates gastric emptying but slows small‐intestinal transit.
Scintigraphy remains the gold standard for measurement of gastric emptying,49 and the use of two isotopes allows both the solid and liquid components of a meal to be studied (Figure 17.6). Regional meal distribution can also be evaluated by defining regions of interest within the stomach. Ultrasonography and13C isotope breath tests are alternative methods of measuring gastric emptying, although the former is restricted to liquid meals. Manometry is used predominantly for research purposes to record the frequency, amplitude, and organisation of lumen‐occlusive contractions in the antrum, pylorus, and duodenum. Proximal gastric relaxation in response to a meal can be evaluated in the laboratory using an electronic barostat; the volume of air required to maintain a fixed pressure in an intragastric bag is used as an index of proximal gastric tone. Single‐photon emission computed tomography (SPECT) is a nuclear medicine technique representing an alternative means of evaluating gastric volumes. The electrical rhythm of the stomach may be recorded from cutaneous electrodes, although the clinical significance of abnormalities of this electrogastrogram (EGG) is unclear since they are not specific for particular gastrointestinal disorders.
There is little information about the effects of ageing on the mechanics of the stomach, but fasting and postprandial antral motility did not differ between patients age 18–39 and those 40–69 years who were being investigated for unexplained gastrointestinal symptoms.50 Proximal gastric compliance is unchanged in the fasting state in the healthy elderly compared to the young, but the perception of gastric distension is diminished,15 akin to the reduction in visceral sensitivity evident in the oesophagus and stomach. Moreover, proximal gastric accommodation to a meal is delayed compared with young controls, which might contribute to early satiation. Conversely, the antrum is more distended after a nutrient drink in the healthy elderly,51 and antral width correlates with satiation and satiety in both young and old subjects. EGG recordings are similar in healthy old and young subjects, with subtle differences in response to nutrients.