Heterochromia iridis
 

This is another physical sign where a good observation can pay dividend at the beginning of the examination.  The heterochromia is usually best seen in a well-lit environment. There are many causes of heterochromia iridis,  however, if the heterochromia forms the main part of the examination (for example 'Look at this patient's eye and do  the necessary examination') the most likely case is siderosis bulbi. Remember that the abnormal eye can be the eye  with either the darker as in siderosis bulbi or the lighter iris for example congenital Horner's syndrome or Fuchs'  heterochromic iridis. The patient has heterochromic iridis.  Further examination  (if you can choose the type of examination then choose pupillary examination or slit-lamp examination. In pupillary  examination look .for congenital ptosis or siderosis bulbi; in slit-lamp examination look for siderosis bulbi in which  there are usually signs of penetrating injury or Fuchs' heterochromic iridis): look for any signs of mild ptosis or anisocoria on the side of the lighter iris for congenital Horner's syndrome mention that you like to test the patient's visual acuity and pupil response (in siderosis bulbi the darker eye has poor vision and the pupil is usually dilated and does not react to light). Also mention that you like to examine the eye for any retained foreign body or signs of previous penetrating injury. The optic disc may  show glaucomatous damage. slit-lamp examination for diffuse keratitic precipitate, flare and posterior subcapsular cataract in Fuchs'  heterochromic iridis.

Questions:

1. What are the causes of heterochromic iridis?

Answer
The best ways to classify heterochromic iridis is to divide the abnormal iridis into those where the lesion is found in the hyperchromic iris from those in hypochromic iris.
Heterochromia may be a congenital condition in which the iris has either too much pigment or too little pigment.
Causes of hyperchromic iris:
iris naevus
drug-induced as in unilateral use of latanoprost
rubeotic iridis
siderosis bulbi
Causes of hypochromic iris:
chronic inflammation such as Fuchs' heterochromic cyclitis
congenital Horner's syndrome
trauma

2. What is the feature of ERG in siderosis bulbi?

Answer
In siderosis bulbi, the ERG initially shows increased a-wave and normal b-wave. With time, the amplitude of b-wave decreases. Eventually, the ERG becomes extinguishes.

3. What are the signs of siderosis bulbi?

Answer
iris heterochromia
pupil dilatation and poor reaction to light
cataract
brown deposits on the anterior lens
vitreous opacities
peripheral retinal pigmentation which eventually progresses to diffuse retinal pigmentation
narrowed retinal vessels
optic disc atrophy
secondary glaucoma due to accumulation of iron in the trabecular meshwork

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A patient with left-sided siderosis bulbi. Note the dark iris and dilated pupil.
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Heterochromic iridis in a patient with congenital Horner's syndrome. Note the
miosis and mild ptosis on the side with lighter iris.
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