William Ian McDonald, neurologist, made a significant contribution to the understanding of multiple sclerosis and the management of people affected by this disease. He was also an authority on nervous system diseases that affect the eye.

At the time of his death he was Emeritus Professor of Clinical Neurology at the Institute of Neurology at University College in London. He was also Consultant Physician Emeritus at the National Hospital for Neurology and Neurosurgery, Queen Square (London), a leading centre for academic and clinical neurology.

Over his distinguished career Professor McDonald received a raft of international honours. In 2000 he was given an honorary Doctorate of Science from the University of Otago where he studied in the 1950s. Though his career was played out on a world stage he always considered himself a New Zealander and visited regularly.

He was born in Wellington, then the family moved to Christchurch where he was educated at St Andrew's College. As a schoolboy he wrote an essay on the topic of the nervous system of the earthworm. Years later, when he an authority on nervous systems of a higher order, he told the story at lunch to an elderly Queen Mother. She snapped at him: "I didn't know they had one."

His first degree was a Bachelor of Medical Science in neurophysiology from Otago and, after finishing his medical training, he returned to the physiology department to complete a PhD. He applied the techniques of neurophysiology to the study of multiple sclerosis and in his PhD showed that the loss of myelin, the insulating sheath around human nerves, caused slowing and blocking of nerve impulses. Later, at Queen Square, he showed that myelin could be reformed, thus offering hope for multiple sclerosis sufferers in the future.

At Otago he was taught by Professor Archie Macintyre and Dr Keith McLeod. He was an excellent student, an engaging conversationalist and a clever mimic. Then and later he disliked sport, preferring to pretend he had no idea who the All Blacks were playing. He loved walking and music, and at one time he aspired to be a concert pianist. Much later in London he helped young New Zealand musicians find their feet. In London he rounded his life with cultural pursuits which he shared with his partner, Stanley Hamilton.

However his work was his focus. He was a consultant physician at Queen Square from 1966 to 1998 and was appointed Professor of Clinical Neurology there in 1974 and head of the Department of Neurology in 1995. From 1969 to 1996 he was Consultant Physician (Neurologist) at Moorfields Eye Hospital in London. He undertook many visiting professorships, gave named lectures and served many professional and university bodies.

He was president of the World Congress of Neurology, the European Neurological Society and the Association of British Neurologists. He was a scientific adviser to the National MS Society (USA) and chairman of the MS International Federation's Medical Advisory Board. From 1991 to 1997 he was editor of Brain magazine.

In the field of multiple sclerosis he made important contributions in the areas of genetics, physiology and most recently imaging. With the support of Britain's Multiple Sclerosis Society he pioneered the use of magnetic resonance imaging in improving diagnosis and understanding of the disease.