There are times in my working life when I have absolutely no idea what to do. Fortunately they don't occur too often or I'd probably be sacked, but I had one such moment yesterday. And I still can't work out what I should have done.

I saw a patient on Friday morning who was a lovely lady with learning difficulties. People with such challenges in their life are not uncommon in our screening programme, and I'm generally able to screen them without too much trouble. In fact, one of the most enjoyable appointments I ever had was with a patient who had the mental age of a child, and insisted on me photographing her carer too. Never has a consulting room been filled with so much laughter and carefree chatter.

This particular patient was also a cheerful soul, and insisted on shaking my hand and smiling in a friendly manner. But in addition to her learning difficulties, she was also profoundly deaf, and could only communicate through sign language. Naturally she had a carer with her who translated for me, and I carried out the VA with no problems at all. She was quite happy to have the eye drops too. In fact, the first half of the appointment went without a hitch. The problem was the photos.

Unfortunately the lady lacked the mental capacity to understand that she needed to look directly at the light. So I had to direct her. Picture the scene: I have a lady positioned with her head on the chin rest, able to see nothing but the lens of a camera in front of her, and I want to give her an instruction. She can't 'hear' that instruction unless she moves her head to look at her carer, but if she moves her head, that instruction no longer applies.

For example, I need her to look a little more to the right. So the carer taps her on the shoulder and signs the instruction to her. By which time she's looking in an entirely different direction and the instruction is no longer relevant.

We tried this for more than twenty minutes. The carer attempted to explain that she needed to look at the light, but she struggled to understand, and once in position at the camera, she couldn't 'hear' a thing without moving her head. On a few occasions, she would look in the right direction by chance, but her eyes would be half shut. I wanted to tell her to keep looking in that direction and to open her eyes a little wider, but in order to do so, she would need to look elsewhere, and I'd lose my shot.

We tried it with the light, with my finger, and with the carer trying to point, but all to no avail. I managed to capture a few semi-acceptable images, mostly due to luck and persistence, but after twenty-five minutes, the previously smiley and relaxed patient was becoming stressed and irritable, and I felt we'd pushed her to her limits of co-operation. I decided to let her go.

So here's the question: how do you say "Stay where you are" to someone who can only hear you by moving?