The pupilloconstrictor muscle consists of cir-
cumferentially oriented muscle fibers that nar-
row the pupil when they contract, in the same
manner as the drawstring of a purse. The para-
sympathetic neurons that supply the pupillo-
constrictor muscle are located in the ciliary
ganglion and in episcleral ganglion cells within
the orbit. The preganglionic neurons for pu-
pilloconstriction are located in the oculomotor
complex in the brainstem (Edinger-Westphal
nucleus) and they arrive in the orbit via the
oculomotor or third cranial nerve. The pu-
pilloconstrictor fibers travel in the dorsomedial
quadrant of the third nerve, where they are
vulnerable to compression by a number of
causes (Chapter 3), often before there is clear
impairment of the third nerve extraocular
muscles. As a result, unilateral loss of pupillo-
constrictor tone is of great diagnostic impor-
tance in patients with stupor or coma caused
by supratentorial mass lesions.
Pharmacology of the Peripheral
Pupillomotor System
Because the state of the pupils is of such im-
portance in the diagnosis of patients with coma,
it is sometimes necessary to explore the origin
of aberrant responses. Knowledge of the phar-
macology of the pupillomotor system is es-
sential to properly interpret the findings.
82
The
sympathetic preganglionic neurons in the tho-
racic spinal cord are cholinergic, and they act
upon a nicotinic type II receptor on the sym-
pathetic ganglion cells. The sympathetic ter-
minals onto the pupillodilator muscle in the
iris are noradrenergic, and they dilate the pupil
via a beta-1 adrenergic receptor.
In the presence of a unilateral small pupil,
it is possible to determine whether the cause
is due to failure of the sympathetic ganglion
cells or is preganglionic. In the latter case, the
ganglion cells are intact, but not active. The
pupil can then be dilated by instilling a few
drops of 1% hydroxyamphetamine into the
eye, which releases norepinephrine from sur-
viving sympathetic terminals. Because the
postsynaptic receptors have become hypersen-
sitive due to the paucity of neurotransmitter
being released, there is brisk pupillodilation
after instilling the eye drops. Conversely, if the
pupil is small due to loss of postganglionic
neurons or receptor blockade, hydroxyam-
phetamine will have little if any effect. Post-
ganglionic failure can be differentiated from
receptor blockade (e.g., instillation of eyedrops
containing a beta blocker such as are used to
treat glaucoma) by introduction of 0.1% adren-
aline drops, which have direct beta agonist
effects. Denervated receptors are hypersensi-
tive and there is brisk pupillary dilation, but a
pupil that is small due to a beta blocker does
not respond.
The pupilloconstrictor neurons in the ocu-
lomotor complex use acetylcholine, and they
act on the ciliary and episcleral ganglion cells
via a nicotinic II receptor. The parasympathetic
ganglion cells, by contrast, activate the pupil-
loconstrictor muscle via a muscarinic choliner-
gic synapse. In the presence of a dilated pupil
due to an injury to the third nerve or the post-
ganglionic neurons, the hypersensitive recep-
tors will constrict the pupil rapidly in response
to a dilute solution of the muscarinic agonist
pilocarpine (0.125%). However, if the enlarged
pupil is due to atropine, even much stronger
solutions of pilocarpine (up to 1.0%) will be
unable to constrict the pupil.
CENTRAL PATHWAYS CONTROLLING
PUPILLARY RESPONSES
It is important to understand the central path-
ways that regulate pupillary light responses, be-
cause dysfunction in these pathways causes the
abnormal pupillary signs seen in patients with
coma due to brainstem injury.
Preganglionic sympathetic neurons in the
C8-T2 levels of the spinal cord, which regu-
late pupillodilation, receive inputs from sev-
eral levels of the brain. The main input driving
sympathetic pupillary tone derives from the
ipsilateral hypothalamus. Neurons in the para-
ventricular and arcuate nuclei and in the lat-
eral hypothalamus all innervate the upper
thoracic sympathetic preganglionic neurons.
83
The orexin/hypocretin neurons in the lateral
hypothalamus provide a particularly intense in-
put to this area.
84
This input may be important,
as the activity of the orexin neurons is great-
est during wakefulness, when pupillodilation is
maximal.
85
The descending hypothalamic in-
put runs through the lateral part of the pontine
and medullary brainstem tegmentum, where it
is vulnerable to interruption by brainstem in-
jury.
7
Electrical stimulation of the descending
sympathoexcitatory tract in cats demonstrates
56
Plum and Posner’s Diagnosis of Stupor and Coma
that it runs in a superficial position along the
surface of the ventrolateral medulla, just dor-
solateral to the inferior olivary nucleus.
86
Ex-
perience with patients with lateral medullary
infarction supports a similar localization in hu-
mans. Such patients have a central Horner’s
syndrome, which includes not only miosis and
ptosis, but also loss of sweating on the entire
ipsilateral side of the body. Thus, the sympa-
thoexcitatory pathway remains ipsilateral from
the hypothalamus all the way to the spinal
cord.
Other brainstem pathways also contribute to
pupillodilation. Inputs to the C8-T2 sympa-
thetic preganglionic column arise from a num-
ber of brainstem sites, including the Ko¨lliker-
Fuse nucleus, A5 noradrenergic neurons, C1
adrenergic neurons, medullary raphe seroto-
ninergic neurons, and other populations in the
rostral ventrolateral medulla that have not
been chemically characterized in detail.
8
As-
cending pain afferents from the spinal cord
terminate both in these sites as well as in the
periaqueductal gray matter. Brainstem sympa-
thoexcitatory neurons can cause pupillodilation
in response to painful stimuli (the ciliospinal
reflex).
10
They also provide ascending inhib-
itory inputs to the pupilloconstrictor neurons
in the midbrain. As a result, lesions of the
pontine tegmentum, which destroy both these
ascending inhibitory inputs to the pupillo-
constrictor system and the descending excit-
atory inputs to the pupillodilator system, cause
the most severely constricted pupils seen in
humans.
Preganglionic parasympathetic neurons are
located in the Edinger-Westphal nucleus in
primates.
87,88
This complex cell group also
contains peptidergic neurons that mainly pro-
vide descending projections to the spinal cord.
In rodents and cats, most of the pupillocon-
strictor neurons are located outside the Edin-
ger-Westphal nucleus, and the nucleus itself
mainly consists of the spinally projecting pop-
ulation, so that extrapolation from nonprimate
species (where the anatomy and physiology of
the system has been most carefully studied)
is difficult.
The main input to the Edinger-Westphal
nucleus of clinical interest is the afferent limb
of the pupillary light reflex. The retinal gan-
glion cells that contribute to this pathway be-
long to a special class of irradiance detectors,
most of which contain the photopigment me-
lanopsin.
89
The same population of retinal gan-
glion cells that drives the pupillary light reflex
also provides inputs to the suprachiasmatic nu-
cleus in the circadian system, and in many cases
individual ganglion cells send axonal branches
to both systems. Although these ganglion cells
are activated by the traditional pathways from
rods and cones, they also are directly light sen-
sitive, and as a consequence pupillary light re-
flexes are preserved in animals and humans
with retinal degeneration who lack rods and
cones (i.e., are functionally blind). This is in
contrast to acute onset of blindness, in which
preservation of the pupillary light reflex im-
plies damage to the visual system beyond the
optic tracts, usually at the level of the visual
cortex.
The brightness-responsive retinal ganglion
cells innervate the olivary pretectal nucleus.
Neurons in the olivary pretectal nucleus then
send their axons through the posterior com-
missure to the Edinger-Westphal nucleus of
both sides.
90
The Edinger-Westphal nucleus
in humans, as in other species, lies very close to
the midline, just dorsal to the main body of the
oculomotor nucleus. As a result, lesions that
involve the posterior commissure disrupt the
light reflex pathway from both eyes, resulting
in fixed, slightly large pupils.
Descending cortical inputs can cause either
pupillary constriction or dilation, and can ei-
ther be ipsilateral, contralateral, or bilateral.
91
Sites that may produce pupillary responses are
found in both the lateral and medial fron-
tal lobes, the occipital lobe, and the temporal
lobe. Unilateral pupillodilation has also been
reported in patients during epileptic seizures.
However, the pupillary response can be either
ipsilateral or contralateral to the presumed
origin of the seizures. Because so little is known
about descending inputs to the pupillomotor
system from the cortex and their physiologic
role, it is not possible at this point to use pu-
pillary responses during seizure activity to de-
termine the lateralization, let alone localiza-
tion, of the seizure onset. However, brief,
reversible changes in pupillary size may be due
to seizure activity rather than structural brain-
stem injury. We have also seen reversible and
asymmetric changes in pupillary diameter in
patients with oculomotor dysfunction due to
tuberculous meningitis and with severe cases
of Guillain-Barre´ syndrome that cause auto-
nomic denervation.
Examination of the Comatose Patient
57