IMAGING STRABISMUS

Diagnostic scanning with magnetic resonance imaging (MRI) has dramatically altered current understanding of the anatomy and function of the extraocular muscles (EOM), Joseph L Demer MD, PhD told delegates attending the 2015 Congress of the European Society of Ophthalmology (SOE) in Vienna, Austria.

“In the 21st Century we have available to us MRI imaging of the actual anatomy of the EOM, which allows us to directly observe the function of the EOM. Orbital imaging is now available in living people at near microscopic resolution in the orbit,” he said.

Advances in imaging techniques have also enabled discovery of the extraocular muscle pulleys, which have fundamentally changed understanding of the actions of the muscles, said Dr Demer, Professor of Ophthalmology at Jules Stein Eye Institute and Professor of Neurology at University of California, Los Angeles, USA.

“Imaging can demonstrate EOM function as well as anatomy, and can therefore distinguish neurological from mechanical causes of strabismus, and mechanical causes are a substantial proportion of all forms of strabismus,” he said.

PRACTICAL APPLICATIONS

There are numerous practical clinical applications of lessons derived from orbital imaging, Dr Demer said. “It allows us to make diagnoses and design surgeries on a rational basis that should give us better outcomes for our patients.”

Dr Demer said that while MRI has become the standard of care in many medical and surgical fields, ophthalmology
has lagged behind in realising the enormous potential of this imaging technology.

“Orthopaedic surgeons, for instance, make very effective and complete use of MRI imaging in their surgical planning and diagnostic studies. As ophthalmologists we do make extensive use of imaging techniques for the anterior and posterior segments of the eye, including ultrasound, optical coherence tomography and fluorescein angiography. But as ophthalmologists we have not really taken full advantage of the capabilities of imaging of the back of the eye, of those anatomical structures behind the eye in the orbit,” he said.

UNIQUE CHALLENGES

Imaging the living eye in all its dynamic complexity brings its own unique challenges, said Dr Demer.

“By optimising techniques to minimise the movements of the eye, controlling fixation with targets and using sequences like T2-weighted fast spin-echo MRI imaging, we can now explore the functional anatomy of the orbit,”he said.

MRI imaging has already made a major contribution to understanding the functional anatomy of the orbit, thanks to the discovery of the connective tissue pulleys within the orbit, said Dr Demer. The existence of these pulleys has profound implications for the kinematics or rotational properties of the globe, all of which can be summarised in the active pulley hypothesis, he added.

“The active pulley hypothesis proposes that the global layers of muscles do insert on and rotate the globe whereas the orbital layers of muscles do not. The orbital layers move the pulleys anteriorly and posteriorly during contraction and relaxation,” he said.

Imaging can also demonstrate directly the function of the EOM, said Dr Demer. For instance, recent work by Dr Demer and his colleague Robert A Clark has helped to define a metric for the contractility of the EOM.

“A contracting extraocular muscle has an increase in volume and a relaxing one has a decrease in volume. By examining a mid-posterior region of the orbit between 8mm and 14mm posterior to the optic nerve junction, we can measure volume changes within that region which are very effective indicators of the contractility of the EOM,” he said.

This type of metric can be useful clinically in the diagnoses of certain conditions such as superior oblique palsy, he said.

Dr Demer concluded his lecture with an appeal to the assembled ophthalmologists. “Please consider incorporating MRI imaging of the orbit and the EOM into your practices in addition to the type of imaging that you have been doing all along for the retina, the cornea and the anterior segment.”

Joseph L Demer: jdemer@jsei.ucla.edu