168 Neuroanatomy: Draw It to Know It
Pontine Syndromes ( Advanced )
Case I
Patient is found in an apparent comatose state. Exam
reveals normal pupil reactivity, normal vertical eye move-
ments, volitional blinks, complete bilateral face and
body paralysis, normal sleep–wake states, and an absent
gag refl ex.
First, draw an axial section through the pons and
establish the anterior–posterior plane of orientation;
this is an axial, anatomic view of the pons, with the ante-
rior surface of the brainstem at the bottom of the page
and the posterior surface at the top. Th en, separate the
basis from the tegmentum. Next, in the basis, draw the
scattered descending corticonuclear (aka corticobulbar)
and corticospinal tracts. Paralysis of the body and of the
lower cranial nerves (tongue movements, gag , and swal-
low) results from damage to the descending corticospi-
nal and corticonuclear tracts.
Next, draw the reticular formation in the pontine teg-
mentum; the normal sleep–wake states are maintained
because the majority of the reticular formation is spared.
Now, draw the abducens nucleus of cranial nerve 6.
And then draw the facial nucleus of cranial nerve 7 and
show that cranial nerve 7 forms an internal genu around
the abducens nucleus, which creates a bump in the fl oor
of the fourth ventricle, called the facial colliculus.
Paralysis of the face results from destruction of the exit-
ing facial motor nerve fi bers. Th e facial nucleus, itself,
lies within the dorsal pons and is spared.
Next, show the pontine circuitry for horizontal eye
movements. First draw the paramedian pontine reticular
formation (PPRF) in the paramedian ventral pontine
tegmentum. Th en, show the medial longitudinal fascic-
ulus (MLF) in the contralateral dorsal tegmentum.
Indicate that the PPRF stimulates the abducens nucleus,
which sends eff erent nerve fi bers through the medial
pons to produce ipsilateral eye abduction. Th en, also
show that the abducens nucleus sends ascending
interneuronal fi bers up the contralateral MLF, which
innervate the oculomotor nucleus and cause the ipsilat-
eral eye (the eye contralateral to the abducens nucleus)
to adduct. Paralysis of horizontal eye movements in this
case results from destruction of the PPRF; note that
although most of the reticular formation is spared, this
small portion is injured. Volitional vertical eye move-
ments are spared because the center for volitional verti-
cal eye movements lies within the midbrain (above the
level of the lesion).
Th e patient’s ability to blink results from the ability
to elevate and retract the upper eyelids through spared
third nerve innervation of the levator palpebrae and
through third nerve relaxation, which passively closes
the eyelids. Orbicularis oculi is required for forced eyelid
closure; it is innervated by the facial nerve, which is
injured in this syndrome.
Finally, encircle the pontine basis and ventral parame-
dian pontine tegmentum and indicate that injury here
produces the aforementioned constellation of symptoms,
called locked-in syndrome. In locked-in syndrome, the
pontine basis and ventral tegmentum are injured, causing
devastating paralysis, which is oft en misperceived as coma
when in reality consciousness is preserved.^2 , 3 , 5 , 7 , 8 , 14 – 16Case II
Patient presents with slurred speech and clumsiness of
the right hand. Exam reveals impaired smile on the right;
dysarthria; dysphagia; loss of fi ne motor movements in
the right hand; and mild weakness of the right arm with
normal right leg strength.
Draw another axial section through the pons and estab-
lish the anatomic right–left planes of orientation. Now,
separate the basis from the tegmentum. Indicate that within
the basis of the pons, from medial to lateral, lie the face,
arm, and leg fi bers. Encircle the face and arm fi bers and
show that injury here results in dysarthria-clumsy hand
syndrome: a syndrome of contralateral face and upper
extremity weakness with preserved lower extremity strength
due to restricted paramedian pontine injury.^2 , 3 , 5 , 7 , 8 , 12 , 15 , 17