Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Apr. 30, 2022	Apr. 30, 2021	Apr. 30, 2020
Operating Loss Carryforwards [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ (124,000)
Change in valuation allowance	11,200,000	11,200,000
Provision for unrecognized tax benefits	0	$ 0	$ 0
Annual 382 Limitation [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	300,000
Net operating losses	83,000,000.0
R&D Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carry forward amount	1,500,000
Tax credits to utilize	100,000
Credit to expire unutilized	1,400,000
U.S. Federal Income Tax [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	6,000,000.0
U.S. Federal Income Tax [Member] | Annual 382 Limitation [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	300,000
Net operating loss unutilized	76,700,000
U.S. Federal Income Tax [Member] | Carried Forward Indefinitely [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	11,700,000
U.S. Federal Income Tax [Member] | Carried Forward Indefinitely [Member] | Not Subject To Annual 382 Limitation [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	$ 11,700,000
U.S. Federal Income Tax [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carry forwards expiration period	2036
State Income Tax [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	$ 14,900,000
State Income Tax [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carry forwards expiration period	2036
U.K. Income Tax [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards	$ 74,000,000.0