Head posture and disorders of binocular vision

Vision is undoubtedly our most precious sensory modality, allowing us to recognize objects, navigate our surroundings, and enjoy the many aesthetic qualities provided to us by images and colors. Unsurprisingly, our body has automatic and largely unconscious mechanisms which compensate for deficits in vision.

Specifically, disorders of binocular vision, which entail any difficulty in the eyes moving together, are often dealt with by changes in head posture. These adjustments, often subtle, can prevent the onset of double vision, a common symptom of binocular dysfunction, but also provide relief from eyestrain. The kind of head adjustment depends on the kind of binocular dysfunction.

Superior oblique palsy

Each eye is connected to six muscles which are responsible for moving it around in its socket. The superior oblique muscle takes an unusual course, hanging off a small pulley-like ligament before attaching to the upper side of each eye. The activity of the superior oblique is threefold: it turns the eye down (depresses), turns the eye out (abducts), and rotates the eye towards the nose (intorts).

The nerve which supplies the superior oblique muscle is called the trochlear nerve, a name which derives from the Latin word for pulley, and it is also the fourth out of twelve cranial nerves. The trochlear nerve is unique in two respects: of all the cranial nerves, it takes the longest path from the brainstem to its target, and it contains the smallest number of neural connections.

Due to these factors, the trochlear nerve is highly susceptible to damage. Superior oblique palsy (SOP) describes any dysfunction of the superior oblique muscle which is caused by a weakening or complete paralysis of the trochlear nerve. Although SOP most often results from head trauma, it can also be congenital. The most common symptom of superior oblique palsy is diplopia, or double vision, caused by the misalignment of the two eyes, particularly when a person reads.

A common compensatory measure found in cases of superior oblique palsy is a head tilt, with the head tilted towards the shoulder of the unaffected side. To understand why this occurs, consider a case of SOP of the right eye, which results in the eye having difficulty intorting. When the head is tilted towards the left shoulder, gravity causes an intorsion of the right eye, so that the superior oblique muscle of that eye will be relaxed. Meanwhile, the superior oblique muscle of the left eye will be activated in order to prevent its extorsion due to gravity.

The relaxation of the superior oblique muscle of the right eye during a left-shoulder tilt can resolve double vision and also reduce eyestrain. The head tilt is a classic finding associated with SOP, particularly the congenital kind.

Duane’s syndrome

Although superior oblique palsy results in a difficulty in moving the eye in certain directions, it doesn’t typically result in a noticeable misalignment. By contrast, some disorders of binocular vision result in a deviation of the eye which is visible. Horizontal deviations in particular are some of the most visible misalignments, and are in fact the most common form of binocular dysfunction. Strabismus is the name given to any misalignment of the eye.

Duane’s syndrome is a kind of congenital strabismus which affects the horizontal axis, and it comes in three variants. The first variant is the most common, and involves an inability for an eye to abduct, or turn away from the body’s midline.

The compensatory head posture used to treat this variant of Duane’s syndrome is a turning towards the affected eye. To understand why, consider a case of left-eye Duane’s syndrome, in which the left eye has difficulty abducting. If a person wanted to look off to their left, they would experience double vision, since the left eye would be unable to abduct as it should. However, when the head is turned to the left, the person can see more of their left side without having to abduct their left eye.

Duane’s syndrome is caused by rare genetic variations which result in entire cranial nerves being absent, causing a differential ennervation of the muscles responsible for eye movement.

Nystagmus

To call nystagmus a disorder of binocular vision is to use the term very liberally. However, since it results in an inability for the two eyes’ line of sight to be in alignment, this classification isn’t completely unwarranted.

Nystagmus refers to any condition of involuntary eye movement which results in reduced vision. It has a striking clinical presentation: a person will be unable to keep their eyes still on a target, as they will make slow displacements followed by fast twitches, appearing to dance in their sockets. Nystagmus is caused by the influence of the vestibular system, which normally provides useful input to the occulomotor (eye-moving) system.

To deal with nystagmus, many people find what is called a ‘null point’. This refers to a specific posture of the head which results in the lowest intensity of irregular eye movement. This posture isn’t always obvious, and is simply the point at which the pathological inputs from the vestibular system are at their most minimal.

In some cases, the null point occurs in central gaze, when a person is looking straight ahead. When the null point is some head posture away from center gaze, eye muscle surgery or prism lenses can be used to shift this null point towards central viewing.