The olfactory nerve is more likely to be damaged in head trauma and occupying tumors, and once damaged it will be difficult to return to normal without recovery within a short period of time. The probability of injury to the olfactory nerve is about 3% to 10% in patients with head trauma. Many patients who have suffered direct violence to the head tend to have an abnormal sense of smell, and the olfactory nerve is avulsed at the site where it passes through the sieve plate because of the external force, and is often accompanied by a fracture of the paranasal sinuses. About one-third of patients with occipital injuries resulting in contusion of the base of the frontal lobe of the brain in the contralateral region develop hyposmia or loss of sense of smell. Abnormalities of the olfactory nerve can also occur when some tumors compress or invade the olfactory nerve. Because of the fragility of the olfactory nerve, if the sense of smell is not fully restored within two to three months of injury, it will be difficult to return to normal in the future. Before the recovery of the sense of smell, there may be abnormalities in the sense of smell, such as the patient may smell the odor of burnt feathers.