The word "atrophic" means something that has become smaller or weaker than it should be. It is often used in medical contexts to describe a condition where a tissue or an organ has shrunk or deteriorated.
Symptoms of atrophic gastritis are similar in nature to other gastritis disorders. (wagwalking.com)
The etiology of atrophic gastritis is not certain though it tends to occur more frequently in older dogs and dogs with acid reflux disease. (wagwalking.com)
However, some elderly patients with atrophic gastritis require doses much higher than the RDA to avoid subclinical deficiency [38]. (ods.od.nih.gov)