An aneurysm pressing on the third nerve compresses the pupillary fibres that run on its outer surface, so a dilated pupil is a warning that the next event could be rupture. A normal pupil does not reliably exclude compression in partial, early, evolving or painful palsies. NICE NG228 (2022) treats a thunderclap headache as a red flag for subarachnoid haemorrhage: it describes a headache peaking within about 1–5 minutes, and notes that only a minority of such headaches turn out to be SAH. Giant cell arteritis can cause permanent visual loss, and the BSR 2020 guideline notes that most visual loss happens before treatment starts, so do not wait for blood results when the story fits. Posterior circulation strokes often present with diplopia and vertigo and are missed without a structured neurological examination. In an SCA case, name the emergency you are excluding and give a clear disposition: 999, same day, or urgent.

Monocular diplopia is usually an optical problem in one eye. The pinhole test is a quick confirmation you can do in the room, and the patient can usually be seen by an optometrist rather than an ophthalmologist. Binocular diplopia means the two eyes are no longer aligned: a nerve, muscle, orbit or brainstem problem, so it needs a neurological and orbital assessment. Asking whether the images are side by side or one above the other, and in which direction of gaze they are furthest apart, helps describe the pattern before you examine — precise localisation is a specialist task. A childhood squint that decompensates later in life is a common, benign cause of intermittent double vision, but it should be a diagnosis of exclusion.

An isolated sixth or fourth nerve palsy in someone over 50 with diabetes or hypertension is often microvascular, but this cannot be safely diagnosed in primary care — do not assume it because of vascular risk factors; all new binocular diplopia needs prompt specialist assessment. The same sign in a younger person, or in anyone with headache or papilloedema, may be the first sign of raised intracranial pressure or a skull base lesion. The "rule of the pupil" in third nerve palsy helps but is not absolute, because early or partial compressive palsies can spare the pupil. That is why specialist review is needed even when the pupil looks normal. Myasthenia is frequently missed because the examination can be normal when the patient is rested. Ask specifically about symptoms that are worse at the end of the day. Acetylcholine receptor antibodies are found in only about half of people with purely ocular myasthenia, compared with around 80–85% of those with generalised disease, so a negative test does not exclude it.

The pupil examination can change management to emergency referral, so document it explicitly: size, symmetry and reaction. Examining eye movements with the patient telling you where the images are widest helps you describe the abnormality; precise localisation is a specialist task. A formal orthoptic assessment later measures the deviation. A full neurological examination picks up the brainstem signs that turn an "isolated" palsy into a stroke or demyelination. Temporal artery examination takes seconds and can prevent blindness from missed giant cell arteritis. Fundoscopy is worth attempting because papilloedema makes a sixth nerve palsy an emergency, but an undilated view in general practice is limited, so it cannot rule raised pressure out.

ESR and CRP are needed quickly when GCA is possible in anyone over 50 with new binocular diplopia, but the BSR 2020 guideline is clear that treatment should not wait for results or biopsy when giant cell arteritis is strongly suspected. Biopsy or ultrasound should still be arranged promptly because steroids gradually reduce their yield. Vascular angiography (CT or MR) is how a compressive aneurysm is excluded. It belongs in the acute hospital pathway, not a routine outpatient request. Whether a presumed microvascular palsy in an older person with vascular risk factors needs immediate imaging varies between specialists. The safe primary care approach is to discuss or refer every new binocular diplopia and let the ophthalmologist or neurologist decide on imaging. Glucose/HbA1c and BP may be checked where a microvascular palsy is possible, but they do not establish the diagnosis; lipids belong to longer-term cardiovascular risk management.

For giant cell arteritis with ischaemic or visual features, start high-dose oral glucocorticoid straight away per the local GCA pathway; IV treatment is considered only through the specialist/acute pathway. Oral treatment should not be delayed while IV is arranged. Same-day specialist assessment is needed because the other eye is at risk. Many areas now route new binocular diplopia through an urgent eye clinic or neuro-ophthalmology service rather than general neurology, so know your local pathway. A new palsy should not simply be watched for 12 weeks in primary care without specialist contact.

Covering one eye removes double vision immediately. Patients are often relieved to be told this is safe in adults and does not harm the covered eye. The driving rules are a legal duty: GOV.UK states that drivers must tell DVLA about diplopia and can be fined up to £1,000 if they don't, and GPs should advise and document. GCA glucocorticoid dosing follows the local GCA pathway. Vascular risk review after specialist confirmation follows the patient's existing disease-specific guidance.

Microvascular cranial nerve palsies share their risk factors with stroke and heart disease, so the consultation is an opportunity for cardiovascular prevention as well as symptom relief. Patients often underestimate how much double vision affects everyday tasks like stairs, pouring drinks and crossing roads. Practical safety advice and a falls assessment for older people are part of good care. In an SCA consultation, the driving conversation is often the hardest part. Explain the reason, acknowledge the impact on independence, and agree a plan for getting around in the meantime.

The key follow-up principle is that a confirmed microvascular palsy should improve, not worsen, and follow-up timing is set by the specialist pathway. Progression, a new nerve involved or a change in the pupil suggests a compressive or inflammatory cause that needs urgent imaging, even if the first assessment was reassuring. Giant cell arteritis needs long-term shared care because relapses are common during tapering, and both the disease and its treatment carry risks. Written or recorded safety-net advice with named symptoms is easier for patients to act on than a general instruction to return if worse. It is also the part of the record that matters most if things go wrong.