A 28-year-old woman presents with sudden-onset blurred vision in the left eye for 2 days, accompanied by pain on eye movements. On examination, visual acuity is 6/36 in the left eye. Fundoscopy reveals hyperaemia and blurring of the optic disc margins with peripapillary flame haemorrhages. The right eye is normal. What is the most likely diagnosis?

## Clinical Diagnosis: Papillitis ### Key Distinguishing Features **Key Point:** Papillitis is unilateral optic neuritis with inflammation of the optic disc head, presenting with acute vision loss, pain on eye movement (retrobulbar inflammation), and disc hyperaemia with peripapillary haemorrhages. ### Differential Diagnosis: Papilloedema vs Papillitis | Feature | Papillitis | Papilloedema | |---------|-----------|---------------| | **Onset** | Acute (hours to days) | Insidious (weeks to months) | | **Vision** | Markedly reduced (6/36 or worse) | Normal or mild blurring initially | | **Pain** | Present (pain on eye movement) | Absent | | **Laterality** | Unilateral | Bilateral | | **Disc appearance** | Hyperaemic, blurred margins, flame haemorrhages | Swollen, blurred margins, no haemorrhages initially | | **Pupil response** | Relative afferent pupillary defect (RAPD) | Normal | | **Associated features** | Retrobulbar inflammation | Signs of raised ICP (headache, vomiting, CN VI palsy) | ### Clinical Pearl **Clinical Pearl:** Pain on eye movement is a hallmark of optic neuritis (papillitis) and is due to inflammation of the optic nerve sheath. This is **absent in papilloedema**, where the optic nerve is compressed but not inflamed. ### High-Yield Facts **High-Yield:** Papillitis is often associated with demyelinating diseases, particularly multiple sclerosis (MS). In a young woman with acute unilateral vision loss and optic neuritis, MRI brain and spine should be performed to assess for demyelinating lesions. **Mnemonic:** PAPILLITIS = **P**ain + **A**cute vision loss + **P**eripapillary haemorrhages + **I**nflammation + **L**ateral (unilateral) + **L**oss of colour vision + **I**ncreased disc hyperaemia + **T**ransient visual obscurations absent + **I**ntracranial pressure normal + **S**wollen disc ### Pathophysiology Papillitis results from inflammation of the optic nerve head, typically demyelinating in nature. The inflammatory process causes: 1. Optic nerve oedema and hyperaemia 2. Peripapillary haemorrhages and exudates 3. Rapid loss of visual function 4. Pain due to inflammation of the dura and nerve sheath ### Investigation - **Visual fields:** Central scotoma (typical of optic neuritis) - **Colour vision:** Red desaturation - **MRI brain and spine:** To detect demyelinating lesions and assess MS risk - **OCT:** Shows optic nerve swelling acutely, then optic atrophy chronically [cite:Neuro-Ophthalmology, Kanski & Bowling 8e Ch 8]