Chapter 12: The Vestibular System and Eye Movements

CLINICAL CASE | One-and-One-Half Syndrome

A 30-year-old woman suddenly developed double vision that became worse upon looking to the right. She also indicated that she was unable to look to her left. On examination, when asked to look to the left, her eyes remained fixed forward, as she described. She was unable to abduct the left eye, and the right eye, which normally adducts on looking to the left, remained fixed forward (Figure 12–1A). When she was asked to look to the right, her left eye did not adduct (Figure 12–1B).

She had an MRI that revealed a mid-pontine lesion located close to the midline, just under the floor of the fourth ventricle (Figure 12–1B1). Normal MRI is shown in Figure 12–1B2, and a myelin-stained section through the pons is shown in Figure 12–1B3. In addition to this lesion, the patient had additional white matter lesions. Based on these neurological and radiological signs, as well as additional laboratory tests, the patient was diagnosed with multiple sclerosis.

Based on your reading of this chapter, you should be able to answer the following questions.

1. Interruption of which components of the eye movement control circuit in this patient leads to the inability to look to the left?

2. Why does the left eye fail to adduct on looking to the right?

The lesion included the left abducens nucleus. This damaged abducens muscle motor neurons, thereby paralyzing that muscle. Adduction of the right eye was also absent. This is because the muscle control signals to look to the left originate in the left pons (Figures 12–1C and 12–7). When we want to look to the left, the paramedian pontine reticular formation, which receives commands from the cortex, sends signals to the left abducens nucleus. There are two neuron classes there: abducens motor neurons, which innervate the abducens muscle, and internuclear neurons, which project into the right MLF to command right medial rectus motor neurons to contract the medial rectus muscle to adduct the right eye. The lesion damages these neurons, as well as the motor neurons. Note that there is slight asymmetry to the left eye, showing a small amount of adduction, due to paralysis of the left lateral rectus muscle and the unopposed pulling of the intact right medial rectus muscle. Since multiple sclerosis is an inflammatory demyelinating condition, neurons are likely not degenerated but functionally impaired.

The lesion includes the MLF on the left side. Internuclear neurons from the opposite abducens nucleus project their axons into this MLF to reach left medial rectus motor neurons. Note that typically there is nystagmus, an abnormal oscillation or bobbing, of the abducting eye (see Figure 12–13).

Brust JCM. The Practice of Neural Science. New York, ...