From the looks of this article, it seems that they are denied quite a bit, for "not medically necessary." 
http://homecaremag.com/hcexperts/reimbursement-denials/tens-device-denials-201004/

Here's a post from our forum earlier on TENS:
http://medicalassociationofbillers.yuku.com/topic/7627/TENS-unit-billing-help

I posted the LCD from the NHIC site.  Who is your Medicare Part D Carrier?  You might want to look on their site for an LCD, as to how to bill for this. 
http://www.medicarenhic.com/dme/medical_review/mr_lcds/mr_lcd_current/L11506_2009-12-01_PA_2009-12.pdf

Here's my carrier's (Noridian Admin Svcs) LCD on TENS. https://www.noridianmedicare.com/dme/coverage/docs/lcds/current_lcds/transcutaneous_electrical_nerve_stimulators.htm%3f

Melinda Brown, CMBS
Ins Biller