Doctors mainly prescribe antispasmodics or spasmolytics for irritable bowel syndrome (IBS). This variety of drug has two subclasses.
Anticholinergic agents, such as Buscopan or Merbentyl, can also be used for diverticular disease. They work by blocking the transmission of nerve signals to the walls of the intestine, which helps to relieve pain as well as relaxes the bowel walls. Anticholinergics are best reserved for acute symptoms and are not to be used long term, largely because they come with a load of side-effects, including dizziness, problems with passing urine and blurred vision.
Direct-acting smooth-muscle relaxants act directly on the bowel walls to relieve the spasms caused by IBS. This category of drug includes Spasmonal (or alverine), the drug your mother was prescribed. Although these are available without a prescription, they are not without side-effects. According to the Medicines Compendium (Datapharm Communications, 2002), smooth-muscle relaxants can cause nausea, dizziness, itching, rash and headache.
Although Spasmonal has not been associated with blurred vision, as have its close relatives the anticholinergics, it is certainly prudent to suspect a possible link.