Legal Case Summary
R v Hennessy [1989] 1 WLR 297
Diabetes and defences – automatism or insanity in hyperglycaemic cases
Facts
The appellant (a diabetic) was apprehended while driving a stolen car. He later collapsed at the police station it became apparent that he was having a diabetic episode and that he had not taken his insulin for several days, at least in part because of emotional turmoil due to being left by his wife. Evidence was given that hyperglycaemia (which is what was afflicting the defendant) resulted in drowsiness and possibly degradation of one’s ability to understand what is happening around them, as well as of their physical and mental abilities. Further, emotional states such as anxiety and depression can exacerbate the effect. He was charged with both theft (contrary to the Theft Act 1968 c.60, s.1) and driving while disqualified (contrary to the Road Traffic Act 1988 c.52, s.103). The appellant did not recall taking the car and at trial relied on the defence of automatism. The trial judge ruled that automatism does not apply to his condition and that he should have relied on a plea of insanity instead. The appellant was convicted.
Issues
On appeal against the conviction, the issue was whether the automatism defence had been available to the appellant or whether insanity is the correct defence for diabetics suffering a hyperglycaemic episode.
Decision/Outcome
The trial judge had correctly assessed that insanity is the correct defence to apply to a hyperglycaemic episode, since this is caused by the underlying condition of diabetes and can properly be described as a disease of the mind. Further, automatism refers to situations where the defendant’s actions are involuntary. R v Quick [1973] QB 910 distinguished.
Updated 20 March 2026
This summary accurately reflects the decision in R v Hennessy [1989] 1 WLR 297 and the legal principles it established. The case remains good law. The distinction drawn between sane automatism and the insanity defence in the context of hyperglycaemia (high blood sugar caused by the underlying diabetic condition) continues to represent the established position in English criminal law, and the contrast with R v Quick [1973] QB 910 (which concerned hypoglycaemia caused by external administration of insulin) remains authoritative.
Readers should be aware that the law of insanity in English criminal law has been the subject of law reform proposals for some years. The Law Commission published a report on Criminal Liability: Insanity and Automatism in 2013, which criticised the continued application of the M’Naghten Rules (including in cases such as Hennessy) and proposed reform. However, as of the date of this note, no legislation implementing those proposals has been enacted, and the legal position described in the article therefore remains current law. Students should keep an eye on any future legislative developments in this area.
