I have several toe nails with Beau's line and other nasty changes. I just found out that it's likely due to what is suspected as Psoriatic Arthritis in my case. I thought my neuropathy was playing a role...and I still think it may be partially resonsible.

There are many systemic diseases that can cause nail changes. MrsD's link was quite helpful to detail the different variations and how they are related to various medical conditions.