Long-term Debt - Additional Information (Detail) (USD $)	9 Months Ended	1 Months Ended	9 Months Ended	0 Months Ended	3 Months Ended		9 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2014	May 31, 2011 MidCap Financial SBIC, LP [Member]	Sep. 30, 2014 MidCap Financial SBIC, LP [Member]	May 13, 2013 Comerica Bank [Member]	Sep. 30, 2014 Comerica Bank [Member]	Sep. 30, 2013 Comerica Bank [Member]	Sep. 30, 2014 Comerica Bank [Member]	Sep. 30, 2013 Comerica Bank [Member]	May 13, 2013 Comerica Bank [Member]	Sep. 30, 2014 Comerica Bank [Member] Tranche Two [Member]	Sep. 30, 2014 Comerica Bank [Member] Series E Convertible Preferred Stock [Member]	Sep. 30, 2014 Comerica Bank [Member] Common Stock [Member]	Sep. 30, 2014 MidCap Loan [Member]	Sep. 30, 2013 MidCap Loan [Member]	Sep. 30, 2014 MidCap Loan [Member]	Sep. 30, 2013 MidCap Loan [Member]
Debt Instrument [Line Items]
Loan and security agreement, borrowed amount		$ 6,000,000							$ 4,500,000
Loan and security agreement, floor interest rate		2.00%
Loan and security agreement, interest rate spread		8.00%					4.00%
Loan and security agreement, variable interest rate basis			LIBOR
Loan and security agreement, interest rate description			The stated rate of LIBOR, at a 2% rate floor, plus 8% spread per annum.
Final payment of loan and security agreement, percentage		3.00%
Final payment of loan and security agreement		180,000
Loan and security agreement, repayment description			The loan was payable in monthly installments beginning September 2013 through August 2014.				The terms allow for an interest only period of 15 months, and the remaining principal and interest will be repaid starting September 2014 over a nine-month period (24 months in total).
Loan and security agreement, repayment start date		2013-09
Loan and security agreement, repayment end date		2014-08
Venture debt loan, LIBOR period							30 days
Venture debt loan, basis points					2.50%		2.50%
Debt instrument term				24 months
Venture debt loan, interest period				15 months
Venture debt loan, interest rate					7.25%		7.25%
Due period for repayment of principal and interest				9 months
Debt instrument maturity amount in 2014							4,500,000
Debt instrument maturity amount in 2015							0
Venture debt loan, Description							Upon (i) Comerica’s receipt of evidence satisfactory to Comerica that the 1,000 patient interim analysis in the REGULATE-PCI study is successful and performed by April 30, 2014 and (ii) our completion of the IPO and receipt of net proceeds of at least $50.0 million prior to June 30, 2013, we had the option to borrow an additional $4.0 million in the second tranche, or (“Tranche Two”). Since the latter of the Tranche Two conditions was not satisfied, Tranche Two is solely at the discretion of Comerica.
Debt security, option to borrow additional loans										4,000,000
Loan and security agreement, interest expense					82,000	138,000	245,000	211,000					0	0	0	325,000
Issuance of warrants to purchase of stock											156,250	9,356
Warrant price per share											$ 0.72	$ 12.02
Transaction costs attributable to debt	$ 85,000