riMLF: rostral interstitial nucleus of medial longitudinal fasciculus, iNC: interstitial nucleus of Cajal, CCN: caudal central subnucleus, MLF: medial longitudinal fasciculus, III N: oculomotor nucleus, VI N: abducens nucleus
, IO: intraocular muscles, SR: superior rectus muscle, MR: medial rectus muscle, Lid: Levator palpebrae superioris muscle, IR: inferior rectus muscle, and pupil: sphincter pupillae muscles.
For conjugate horizontal eye movements to occur, motor neuron axons from the abducens nucleus
innervate the ipsilateral lateral rectus muscle, causing abduction.
It results from an absent or dysplastic abducens motor neurons with aberrant innervations of the lateral rectus muscle by the oculomotor nerve, from failure of normal development of the pontine abducens nucleus
or nerve resulting in failure of the normal innervation of the lateral rectus muscle on the affected side.
A unilateral INO is due to the interruption of the ipsilateral MLF after it has crossed the midline caudally in the pons from its site of origin in the contralateral abducens nucleus
. In a review of 410 cases of INO, the commonest aetiology was infarction followed by multiple sclerosis and other unusual causes like trauma, tentorial herniation, vasculitis, tumour, infections (tuberculosis, brucellosis, neurocysticercosis, syphilis), lymphoma, aneurysm and AV malformation.
Co-activation of these 2-neuron groups within one abducens nucleus
is thus associated with horizontal movements of both eyes towards the same side.
They bend around the abducens nucleus
to form the first genu of the facial nerve.
Developmental differences also exist for Abd motoneurons, as the abducens nucleus originates from embryonic rhombomeres (Rhs) 5 and 6 for most vertebrates, including lampreys, teleosts, birds, and reptiles (1), and exclusively from Rhs 5 in frogs (16) and mammals (1).
In particular, a distinct subgroup of neurons largely ventrolateral to the abducens nucleus (Fig.
The abducens nucleus, located in the pons, is of central importance in the control of horizontal gaze as it governs conjugate movements of both the ipsilateral lateral rectus and contralateral medial rectus muscles (Fig 3).
The abducens nucleus receives input from several sources.
A similar case was reported by Sans Beom Han (10) where a metastatic mass in the facial colliculus of lower pons involving abducens nucleus resulting in Gaze palsy.
(10.) Presumed Metastasis of Breast Cancer to the Abducens Nucleus Presenting as Gaze Palsy.