Unequally sized pupils in combination with a drooping eyelid on the side of the smaller pupil and decreased sweating on the same side of the face is known as Horner’s syndrome, named for Johann Friedrich Horner, a Swiss ophthalmologist who wrote up a case in 1869. When present, Horner’s syndrome indicates interruption of the sympathetic nervous system on that side of the body and is still a valuable tool in modern diagnosis.

The sympathetic nervous system helps govern various functions outside conscious control, like pulse, blood pressure, sweating, etc. The portion of the sympathetic pathway influencing the eyes and face follows a convoluted pathway that starts in the brain and flows down through the brainstem to the spinal cord. At the base of the neck, the pathway passes outward from the spinal cord and through the top of the lung. From there it rises through the neck again and into the head where it finally reaches the eye and face. A pair of otherwise identical sympathetic pathways serves each side of the head.
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While Horner’s observations were valid and the syndrome has been known by his name ever since, he was not the first to recognize this condition. Instead, an American physician by the name of William Keen first diagnosed a case of “Horner’s syndrome” in an injured Union soldier during the American Civil War. The soldier, Edward Mooney, had been shot through the right side of his neck at the battle of Chancellorsville.

In 1864, along with fellow physicians, Silas Weir Mitchell and George Morehouse, Keen published a small book, “Gunshot Wounds and Other Injuries of the Nerves,” that included Mooney’s case report under the title “Wound of the Sympathetic Nerve.” Fresh out of medical school when he entered military service, Keen made the diagnosis upon recognizing the similarities between the soldier’s face and that of a cat illustrated in a textbook of physiology. In 1905, near the end of Keen’s career as a pioneering neurosurgeon, the College of Physicians of Philadelphia published his reminiscences about the case:

The first nervous case that I remember was a very remarkable one, and the first of its kind ever recorded. It occurred while I was executive officer at the Satterlee Hospital, West Philadelphia. As executive officer it was my duty to assign new patients to the wards, and also to transfer the cases in the specialties, such as the eye, nervous diseases, and injuries, etc., to the special hospitals. One morning, as I sat at my desk, a soldier applied for assignment. On looking up at him I said to myself: ‘You are Dalton’s cat.’

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