Mild latent squints can sometimes go undetected until a period of stress or perhaps excessive reading precipitates symptoms of eyestrain and headache. The effort to maintain both eyes in line causes the symptoms. The latent deviation could be inward or outward but because most people's eyes tend to assume a slightly divergent position when completely at rest, a degree of latent divergence (exophoria) is almost the rule and of no significance. Vertical muscle imbalance is less well tolerated and even a slight deviation can cause symptoms. Small but significant degrees of vertical muscle imbalance are seen in otherwise normal individuals who show a marked difference in refractive error between the two eyes or in those with facial asymmetry. The provision of a small prism incorporated into the spectacle lenses of such patients can produce dramatic relief, but we must always remember that the appearance of an ocular muscle imbalance might be the first indication of more serious disease. A small vertical deviation can be the first sign of a tumour of the lacrimal gland or thyrotoxic eye disease and a wide range of investigations might be needed before one can be satisfied with the excellent but sometimes deceptive results of symptomatic treatment.
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