When ‘Normal’ isn’t Normal!

You correctly identify Horner's syndrome. To ensure a comprehensive evaluation, you consider the wide range of potential causes. Which of the following options correctly categorizes a Pancoast tumour as a cause of Horner's syndrome?

The CT angiogram is performed and reported normal. You contact the on-call stroke consultant and ask him kindly to review the images. He reviews the images and states, "I can see some luminal narrowing of the left internal carotid artery, but this can be normal. I recommend discharge of the patient." You are concerned because simple luminal narrowing does not fully explain the patient's Horner's syndrome based on your anatomical knowledge.

What is the most appropriate next action?

Following your decision to admit the patient, a formal multidisciplinary team (MDT) review the next morning involving senior Radiology, Neurology, and Vascular Surgery consultants confirms the diagnosis: left internal carotid artery dissection with an overlying mural thrombus. The radiology report is formally amended to reflect these findings. The patient is continued on aspirin with plans for stroke clinic follow-up.

Reflecting on this case, what is the most critical anatomical principle that explains why "luminal narrowing" alone was an insufficient explanation for this patient's Horner's syndrome?