Dr. Wu addresses an important clinical point on the association between pain and inflammation of the optic nerve. Importantly, the optic nerve has no nociceptive afferents. Consequently, all pain perceived by a patient suffering from inflammation of the optic nerve is defined as referred pain. To answer Dr. Wu's question, we have therefore summarised the nociceptive afferents of the anatomical structures adjacent to the optic nerve (Figure 1). As first observed by Whitnall,1 the closest anatomical relationship between the optic nerve sheath, which contains pain fibres of trigeminal origin, is with the superior and medial recti. The optic nerve sheath adheres to these muscles in the orbital apex. Whitnall's hypothesis was therefore that traction of the optic nerve sheath explains worsening of pain with eye movements.1 This lead Lepore to prospectively test Whitnall's hypothesis.2 As a rule of thumb, magnetic resonance imaging (MRI) confirmed that localisation of retrobulbar neuritis to the posterior orbital apex most frequently led to pain.
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