INO

Question 1

INO description

Answer 1

A lesion of the medial longitudinal fasciculus (MLF, the area carrying internuclear neurones between 6th and 3rd nerves) resulting in a palsy of the medial rectus muscle and a dissociated gaze-evoked nystagmus, greatest in the abducting eye.

Question 2

INO pathway

Answer 2

Anatomical Pathway
The sixth nerve nucleus acts as the final common pathway for horizontal gaze.
Axons from interneurones travel in the MLF and cross the midline to synapse with the contralateral third nerve subnucleus supplying the medial rectus muscle.
Axons affected in INO.

Question 3

Types of INO

Answer 3

Unilateral INO:
Affects interneurones from one sixth nerve nucleus, resulting in impairment or loss of adduction of the affected medial rectus on attempted conjugate gaze.
The saccadic, pursuit and vestibulo-ocular systems are all affected.
Milder cases show a reduction in the peak saccadic velocity of the affected medial rectus.
Associated with abducting nystagmus of the other eye.
A skew deviation may be seen in unilateral INO and the hypertropic eye is usually on the same side as the INO

Bilateral INO:
Affects interneurones running in the MLF from both sixth nerve nuclei.
Loss of adduction of each eye on attempted contralateral gaze.
Abducting nystagmus present in each eye on lateral gaze.
Retained convergence, though often asymmetric.

Question 4

Aeitiology of INO

Answer 4

Younger patients: Commonly caused by multiple sclerosis; may be the presenting feature of the disease.
Older patients: Often due to small blood vessel occlusion; usually unilateral.
Tumours can cause bilateral lesions, though the commonest causes are multiple sclerosis and basilar artery branch occlusions

Question 5

Features of INO

Answer 5

Diagnostic Features:
Limitation of the adducting eye and abducting nystagmus of the contralateral eye.
Don’t normally complain of diplopia
Preservation of convergence and the range of adduction with reduced medial rectus saccadic velocity.
SKEW deviation with ipsilateral hypertropia

Theories for Abducting Nystagmus:
Adaptive change to medial rectus weakness.
Increase in convergence tone.
Interruption of descending pathways.
A form of gaze-evoked nystagmus.

In bilateral INO, there is a disturbance of vertical gaze and up-beating nystagmus.

Wall-eyed BIL INO = XOT with lesions in midbrain MLF when both MR subnuecli affected

Question 6

Dx tests and recovery in INO

Answer 6

Diagnosis
Suspected whenever there is an acquired limitation of adduction.
XOT or XOP which increases on attempted horizontal gaze affected side
Saccadic movements are usually impaired more than pursuit movements.
Optokinetic nystagmus will be impaired, difficult to tell.
Hess chart can record progress.
If medial rectus contractions in response to both convergence and conjugate gaze are equally affected, the lesion is more likely more likely to be in the upper part of the MLF close to the third nerve nucleus.
Posterior INO when convergence is lost, and anterior INO, in which convergence is retained.

Natural History
Recovery is likely in cases due to multiple sclerosis or vascular causes.
INO secondary to tumour may not recover and may progress.

Question 7

Diff Dx in INO

Answer 7

Myasthenia:
Can mimic INO closely.
Signs include variability, preserved saccadic velocity, ptosis, and Cogan’s lid-twitch sign.
Orbital Trauma:
Blow-out fracture of the medial wall may cause medial rectus underaction.
Duane’s Retraction Syndrome:
Restriction of abduction and palpebral changes on horizontal gaze.
Infranuclear Medial Rectus Palsy:
Extremely rare; loss of convergence and absence of abducting nystagmus differentiate from INO.

Question 8

Mx in INO

Answer 8

MX: Unilateral INO rarely causes diplopia in the primary position but can result in symptoms on lateral gaze, and nasal sector occlusion to the spectacle lens of the affected eye can help relieve symptoms. Prisms not effective.