The eyes and visual system

Neuromuscular junction Myasthenia gravis Variable ptosis that fatigues, may be associated diplopia and facial weakness Myopathic Myopathies, especially myotonic dystrophy Usually symmetrical. Features of associated muscle disease Mechanical Aponeurotic dehiscence (common in the elderly) The tarsal plate is separated from the levator muscle

Exophthalmos is usually a feature of hyperthyroidism but may indicate orbital disease. Enophthalmos is a feature of Horner’s syndrome. Lid retraction is seen in hyperthyroidism and in rare upper brain stem lesions.

Remember false eyes can be cosmetically effective – a pitfall in exams.

Examination of the pupils

First look at the pupils. Are they the same size? Examination then aims to assess the afferent (optic nerve) and the efferent (parasympathetic via 3rd nerve constricting and sympathetic dilating) elements of the pupillary reflexes.

Ask the patient to look into the distance and shine a light twice in each eye in turn. First, look at the response in the eye into which you are shining the torch (the direct response), and then at the response in the other eye (the consensual response). Then ask the patient to look at your finger held 15 cm from the patient’s face and look at the pupils for brisk constriction – the accommodation response. If no response is obtained from shining a light into the eye, but there is a normal response on accommodation, this is called an afferent pupillary defect and indicates significant optic nerve disease. Other abnormalities are summarized in Table 2.

When a bright light is shone into the eye with an intact optic nerve, there is a brisk constriction of both pupils. When an optic nerve is partly damaged, the constriction to the same stimulus is less brisk. If the light is moved rapidly from a normal eye to one with an abnormal optic nerve, the pupil of the damaged eye continues to dilate. This is because the relatively weaker stimulus to constriction transmitted by the impaired optic nerve does not overcome the relaxation following a powerful stimulation on the other side. This is called a relative afferent pupillary defect.

Examination of acuity

This is best done using a Snellen’s chart; alternatives include a near-vision chart. Refractive errors are best corrected using the patient’s glasses but can be improved using a pin hole. The result of the Snellen’s chart is expressed as two numbers: the distance from the chart (usually 6 m or 20 ft) and the distance at which the letter the patient could read should be read at. For example, 6/6 – letter read at the correct distance; 6/60 – the patient could read a letter at 6 m that could normally be read at 60 m. If the patient cannot read any letters, see if he or she can count fingers (abbreviated to CF), see hand movements (HM) or only has perception of light (PL).

Acuity can be affected by lesions anywhere in the visual pathway from ocular causes such as corneal lesions, cataracts and macular degeneration, through optic nerve disease and hemianopias involving the macula, to cortical blindness. Cortical blindness results from bilateral occipital lesions and is associated with normal pupillary responses and a normal eye examination.

Examination of visual fields

The visual fields are described from the patient’s point of view. When looking out of the right eye, the right half of the vision is the temporal field and the left the nasal field, and vice versa from the left eye.

The visual fields are examined by covering one of the patient’s eyes, asking the patient to fixate on your opposite eye and then bringing an object in from the periphery to find the edge of the patient’s vision (Fig. 1). This is done slightly differently depending on the type of object at which the patient is asked to look. The visual field for a large moving object is normally much larger than for a small red object. It is easiest to screen the visual fields using a crude stimulus, for example a wiggling finger, and then refine any defect with a smaller object, usually a small, red hat-pin head.