Ocular melanosis (OM) is a blue-gray and/or brown lesion of the conjunctiva that can be separated into benign conjunctival epithelial melanosis (BCEM) and primary acquired melanosis (PAM), of which the latter is considered a risk factor for uveal melanoma. The disease is caused by an increase of melanocytes in the iris, choroid, and surrounding structures. Overproduction of pigment by these cells can block the trabecular meshwork through which fluid drains from the eye. The increased fluid in the eye leads to increased pressure, which can lead to glaucoma. In humans, this is sometimes known as pigment dispersion syndrome. Benign Conjunctival Epithelial Melanosis
BCEM, also referred to as conjunctival hypermelanosis, complexion-associated melanosis, or racial melanosis, is a non-cancerous lesion of the conjunctiva that is more commonly found in dark-skinned individuals (over 90% of lesions are found in black persons and around 5% in white persons). It is due to excess production of melanin in the setting of a normal number of melanocytes in the conjunctiva. It appears very early in life and the pattern does not seem to change upon reaching adulthood. There can be asymmetrical involvement of the eyes, and lesions are usually described as flat, brown, and patchy areas of pigmentation. Primary Acquired Melanosis
PAM is a potentially cancerous lesion of the conjunctiva, which has a higher risk of transforming into a malignant melanoma in white persons. Nearly 75% of all melanomas that arise from the conjunctiva have been found to have occurred in the setting of PAM.
Limitations of eye movements are confined to abduction of the affected eye (or abduction of both eyes if bilateral) and the size of the resulting convergent squint or esotropia is always larger on distance fixation - where the lateral recti are more active - than on near fixation - where the medial recti are dominant. Abduction limitations that mimic VIth nerve palsy may result secondary to surgery, to trauma or as a result of other conditions such as myasthenia gravis or thyroid eye disease. In children, differential diagnosis is more difficult because of the problems inherent in getting infants to cooperate with a full eye movement investigation. Possible alternative diagnosis for an abduction deficit would include:
1. Mobius syndrome - a rare congenital disorder in which both VIth and VIIth nerves are bilaterally affected giving rise to a typically expressionless face. 2. Duane syndrome - A condition in which both abduction and adduction are affected arising as a result of partial innervation of the lateral rectus by branches from the IIIrd oculomotor cranial nerve. 3. Cross fixation which develops in the presence of infantile esotropia or nystagmus blockage syndrome and results in habitual weakness of lateral recti. 4. Iatrogenic injury. Abducens nerve palsy is also known to occur with halo orthosis placement. The resultant palsy is identified through loss of lateral gaze after application of the orthosis and is the most common cranial nerve injury associated with this device.