lacey spears of scottsville , kentucky , was found guilty last month of second-degree murder in the death of garnett-paul spears .
the 27-year-old spears was found guilty of force-feeding heavy concentrations of sodium through the boy 's stomach tube .
judge who sentenced spears on wednesday said she suffers from a mental illness and said the crime was still ` unfathomable in its cruelty '
she showed no emotion when she was sentenced .