Sensory dysfunction is claimed for this stage which may precede the first clinical signs of dementia by up to ten years. Most notably the sense of smell is lost. The loss of the sense of smell is associated with depression and loss of appetite leading to poor nutrition. It is suggested that this dysfunction may come about because the olfactory epithelium is exposed to the environment. The lack of blood-brain-barrier protection here means that toxic elements can enter and cause damage to the chemosensory networks.