Which red flag symptoms in a patient with double vision warrant immediate referral to secondary care?

Red flag symptoms in a patient presenting with double vision that warrant immediate referral to secondary care include:

• New-onset squint (which can present as double vision) occurring together with ataxia, vomiting, or headache, requiring immediate referral to acute paediatric services (applicable to children but indicative of serious neurological pathology) ¹.
• Paralytic squint, which should prompt urgent neurological assessment even if other signs of raised intracranial pressure are absent ¹.
• Sudden-onset limb or facial weakness, which may accompany neurological emergencies and requires urgent investigation and referral ¹.
• Sudden-onset acute vestibular syndrome not explained by benign causes, which may be associated with neurological emergencies and requires immediate referral ¹.
• Any new neurological signs accompanying double vision, such as persistent facial numbness or abnormal neurological signs, should prompt urgent neuroimaging and referral ¹.

In summary, double vision accompanied by signs of raised intracranial pressure (headache, vomiting, ataxia), paralytic squint, sudden neurological deficits, or other abnormal neurological signs are red flags necessitating immediate secondary care referral ¹.