Surgeons may have rethink the way they operate on children with crossed eyes, or strabismus. This is thanks to the discovery by Joseph Demer, Jules Stein Eye Institute at the University of California at Los Angeles of the orbital pulley system. Using MRI and cadaver dissection, Demer found that each extraocular muscle consists of a global layer contiguous with the tendon and that inserts into the eyeball, along with a similar-sized orbital layer forming the extraocular muscle's pulley, which controls the eye's movement. Demer described the orbital pulley system at a recent conference sponsored by the New York Academy of Sciences at Case Western Reserve University in Cleveland. "This is a revolution in how we think the muscles rotate the eyes," commented conference co-chair R. John Leigh, neurologist at University Hospitals of Cleveland and the Louis Stokes Veterans Affairs Medical Center. "We have to rethink entirely the way the brain programs the muscles to contract." The upshot, he believes, is that surgeons, instead of shortening or lengthening muscles at the back of the eye, will relocate misplaced pulleys at the side of the eye. Relocation of congenitally misplaced pulleys is expected to be especially important for certain forms of strabismus that need surgical correction: the A and B patterns. "In the B pattern, the eyes separate when you look up and come together when you look down, and in the A pattern the eyes separate when you look down and come together when you look up."
New Gene, Protein for Crossed Eyes
Congenital fibrosis of the extraocular muscles (CFEOM) consists of several distinct disorders, but in each case infants are born with eyes that face in various directions and do not work together to create binocular vision. Droopy eyelids, or ptosis, is frequently seen with this condition. In her lab at Harvard University, Elizabeth Engle succeeded in cloning the so-called Arix gene and its protein for one form of congenital fibrosis of the extraocular muscles, which she terms CFEOM 2. This is an autosomal recessive disorder, as reported at the recent New York Academy of Sciences meeting on eye movement disorders at Case Western Reserve University and in publication (M. Nakano et al., "Homozygous mutations in ARIX (PHOX2A) result in congenital fibrosis of the extraocular muscles type 2," Nature Genetics, 29:315-20, November 2001.). In an interview during the meeting, Engle remarked that CFEOM 2 occurs in many families in Turkey and Saudi Arabia, where there are many consanguineous marriages. The other two disorders, CFEOM 1 and 3, however, are autosomal dominant, and Engle's lab is trying to locate the various genes and proteins involved in these disorders, by working with extended families with these conditions. Research has shown that the alpha motor neurons in the midbrain do not send their messages to the oculomotor muscles in these disorders, as well as in a related condition called Duane syndrome, which, unlike the others, is often accompanied by other birth defects such as agenesis of a limb. Engle relates that in CFEOM l the eyes are fixed a downward position, so that the patients often develop cervical spine problems because they have to bend backward to see. In CFEOM2, she said, the eyes are fixed outward, and the patients often have droopy eyelids as well. CFEOM3 is a bit more variable, both in its manifestations and in terms of its variable penetrance, seeming at times to skip a generation. With this disorder, eyes may be outward, or inward, or one can be outward and one inward. Sometimes the only clinical manifestation is ptosis.
