Naming the side:
A
right INO is one in which there is an impairment of adduction of the right eye
SIGNS AND SYMPTOMS
There is a painless
onset of visual disturbance, but often no diplopia in primary gaze. There will be horizontal diplopia in lateral gaze. The
patient will manifest an adduction deficit on the involved side and a nystagmus of the fellow eye in extreme abduction.
Occasionally,
the condition is bilateral with medial rectus palsy and adduction deficit in each eye and nystagmus upon abduction in both
eyes (bilateral internuclear ophthalmoplegia, or BINO) While there appears to be medial recti palsy, most patients will be
able to converge (posterior INO). In some cases, the patient will not be able to converge (anterior INO).
PATHOPHYSIOLOGY
To produce synchronous eye movements, cranial nerves III, IV and VI communicate through the medial longitudinal fasciculus
(MLF), the neural pathway connecting the cranial nerve nuclei responsible for eye movements. In INO, a lesion disrupts this
pathway, preventing communication between cranial nerves.
For example,
for a patient to gaze to the left, the left supranuclear control center of horizontal eye movements [paramedian pontine reticular
formation (PPRF)] must signal the left CN VI nucleus to turn the left eye outwards. At the same time, the PPRF must signal
the right CN III nucleus, via the right MLF, to simultaneously turn the right eye inwards. A lesion of the right MLF would
not allow the neural impulse to reach the right medial rectus. In this case, the left eye would abduct, but the right eye
would not adduct. Further, the left eye would go into an abducting nystagmus.
Most lesions of the MLF are located in the pons, or caudal mesencephalon. Thus, patients with INO or BINO will be able
to converge (posterior INO). However, if the lesion affects the MLF within the mesencephalon and involves the CN III nucleus,
then the patient will not be able to converge (anterior INO).
Possible
causes of INO:
- multiple sclerosis
- brainstem infarction
- brainstem and fourth ventricular tumor
- viral infection
- trauma
- syphilis
- Lyme disease
- drug intoxication (phenothiazines and tricyclic antidepressants)
- subdural hematoma
Unilateral INO is
caused by virtually any pathology affecting the brain stem and is sometimes seen in drug-induced coma. Bilateral INO in younger
subjects is usually due to MS.
Typically, multiple sclerosis
causes a bilateral presentation, whereas ischemic vascular infarction causes a unilateral episode. Also, myasthenia gravis
can produce a pseudo-INO/BINO with a motility pattern identical to true INO.
MANAGEMENT
Manage INO/BINO by identifying the underlying cause, and
then obtaining appropriate medical treatment. In cases of ischemic vascular infarction, the motility pattern returns to normal
over time. Appropriate testing includes MRI of the brainstem, FTA-ABS, VDRL, Lyme titre, fasting blood glucose, complete blood
count with differential, blood pressure measurement, and toxicology screen.
Points to remember
- Remember that myasthenia gravis can
mimic the motility pattern of INO/BINO.
- In younger patients, the etiology
of INO is most commonly multiple sclerosis. In fact, INO is the most common ocular motility dysfunction in MS. Approximately
92 percent of patients who develop INO from demyelinization develop MS.
In older patients who develop INO, the most common etiology
is ischemic vascular infarction. Beyond MRI studies, these patients need medical evaluation for ischemic vascular diseases
such as diabetes and hypertension. These cases typically resolve over time.
