Pathology: Why do the symptoms of Moebius occur?

Because of the wide variation in the symptoms of Moebius syndrome, the pathologies of the patients vary widely. The main pathological features are often found in abnormalities of the brain stem. Biopsies and imaging studies of Moebius syndrome patients have shown calcifications at the level of abducens (#6) and facial (#7) nuclei in the brain stem.

The above image is borrowed from Yale University - Cranial Nerves.

Many "mission critical" functions such as breathing and maintaining a proper heart rate also have their basis in the brain stem, and it is important to realize how close together the nuclei are to each other. Damage to one area has a great potential for widespread effects.

Image scanned from McNeill, M. E. (June 1997) Neuroanatomy Primer

Specifically, notice how close the abducens nucleus (in red, at the middle of the pons) is to the facial nucleus (in green, just below and to the left of the abducens nucleus).  Just from the proximity of these two nuclei it is easy to understand why damage to the tegmentum of the pons would produce palsies of both the abducens nerve and the facial nerve. Also, the solitary fasciculus, which has respiration rates as one of its functions, is in close proximity to the abducens and the facial nuclei. This might explain why some Moebius syndrome patients have breathing abnormalities.

Moebius syndrome patients with palsy of the abducens cannot move their eyes past the midline. This occurs because the abducens nerve innervates the lateral rectus muscle, a muscle that moves the eye.  When the lateral rectus muscle contracts, it draws the eyeball outward, away from the nose. Also, because the lateral rectus muscle is not innervated by the abducens, the muscle atrophies, creating an imbalance with the strong, normal medial rectus muscle. This imbalance is the main cause of convergent strabismus (being cross-eyed).

For more information on the functions of the cranial nerves, see Yale University - Cranial Nerves.

For an explanation of what causes the pathology of Moebius syndrome, see the next page: Etiology