## Facts

- The defendant suffered from diabetes and experienced hyperglycemia resulting from a failure to take insulin.
- Medical evidence established that untreated hyperglycemia impairs mental clarity and reasoning.
- The defendant’s involuntary state during the alleged offense was attributed to this untreated medical condition.
- The court evaluated whether the defendant’s state arose from an internal factor (illness) or an external factor (outside influence).
- The case contrasted internal bodily conditions, such as diabetes, with external factors, such as medication.

## Issues

1. Whether hyperglycemia caused by failing to take insulin constitutes an internal or external factor for legal purposes.
2. Whether the defendant’s mental state due to hyperglycemia meets the requirements for the insanity defense under the M'Naghten Rules.
3. How to distinguish between conditions warranting the insanity defense and those supporting automatism.

## Decision

- The court held that involuntary states resulting from internal factors, such as hyperglycemia due to diabetes, are governed by the insanity rules.
- Hyperglycemia in this instance qualified as a mental disorder under the M'Naghten Rules.
- The insanity defense was found applicable because the internal medical condition impaired the defendant’s reasoning.
- The court distinguished this case from Quick, where hypoglycemia from an external factor (insulin) allowed for the defense of automatism.

## Legal Principles

- Conditions with internal causes, such as illnesses or endogenous factors, fall within the legal definition of insanity.
- External factors, such as the influence of medication, may support a defense of automatism instead.
- The M'Naghten Rules require impairment of reasoning attributable to a mental disorder for insanity to apply.
- The nature of the condition—internal versus external—is important in determining the appropriate legal defense.

## Conclusion

The case confirms that hyperglycemia from diabetes is classified as an internal cause for insanity under the M'Naghten Rules, providing a clear distinction between insanity and automatism based on the origin of the condition, and offering guidance for assessing criminal responsibility in cases involving involuntary states.


R v Hennessy [1989] 2 All ER 9 (CA)