Thallium
In the first 48 hours after serious thallium exposure, the affected individual will usually experience nausea, vomiting, and diarrhea. Within a few days, symptoms of nervous system damage become apparent. These symptoms can include pain, loss of reflexes, convulsions, muscle wasting, headaches, numbness, dementia, psychosis, and even coma. After two to three weeks, characteristic changes are seen in the bases of hair shafts, and there is hair loss (alopecia). Finally, after around three weeks post-exposure, heart rhythm disturbances may occur.
Chemical Poisoning
As an organic farmer, I know a bit about the symptoms of chemical poisoning wrt herbicides, pesticides and insecticides. Many of these chemicals can cause symptoms similiar to Susan's (if exposed to in large quantities, or over a period of time). Earaches and tinnitus are common symptoms in over-exposure to many of these chemicals. Particularly carbamates like aldicarb, carbaryl and carbofuran. Interesting to consider, since Josh was so into his lawn and garden.