Latent heliotropism

• heliotropism

The cover photographs and illuminating essays that accompany each new issue of the BJO remind us that ocular anatomy and physiology can only be fully understood as a function of evolution. I have become convinced that many evolutionary clues to human movement disorders lie buried within the ancient biology literature. For example, the torticollis that accompanies congenital homonymous hemianopia may attest to the primitive role of vision in establishing baseline muscle tone. Humans born with homonymous heminanopia maintain a curious head turn away from the side of the seeing visual field.^1,2 A patient with a congenital right homonymous hemianopia will tonically turn the head away from the seeing left visual field and towards the right shoulder, while the eyes are maintained in a leftward deviated position within the orbits.^1,2 Since visual fixation remains on the object of regard even when the head is turned, this torticollis does not seem to serve any obvious compensatory function for vision.³ Patients with congenital homonymous hemianopia are generally unaware of their abnormal head posture and are unable to explain why they maintain it. In patients with acquired homonymous hemianopia, torticollis does not manifest unless the causative hemispheric injury occurs in infancy.^3,4

Proposed explanations assume that the torticollis must serve a compensatory function for visual orientation or navigation. It has been suggested that the head turn may be an adaptive response to frontalise the visual environment relative to the body, that it may permit the patient to use saccades to increase the effective visual field during ambulation, or that it may serve to minimise a subclinical nystagmus that is damped in one lateral field of gaze.^1–4

Torticollis is a manifest tonus imbalance of the neck muscles. Tonus describes the …