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Internal Medicine Joumai 2005; 35: 263-266
PERSONAL VIEWPOINT - CLINICAL TIPS
The rule of 4 of the brainstem: a simplified method
forunderstanding brainstem anatomy and brainstem vascularsyndromes
for the non-neurologistp. GATES
The Geelong Hospital, Barwon Health, Geelong, Victoria,
Australia
The rule of 4 is a simple method developed to help'students of
neurology'' to remember the anatomy of thebrainstem and thus the
features of the various brainstemvascular syndromes. As medical
students, we are taughtdetailed anatomy of the brainstem containing
a bewil-dering number of structures with curious names such
assuperior colliculi, inferior olives, various cranial nen'enuclei
and the median longitudinal fasciculus. In realitywhen we do a
neurological examination we test for onlya few of these structures.
The rule of 4 recognizes thisand only describes the parts of the
brainstem that weactually examine when doing a neurological
examina-tion. The blood supply of the brainstetn is such thatthere
are paramedian branches and long circumferentialbranches (the
anterior inferior cerebellar artery (AICA),the posterior inferior
cerebellar artery (PICA) and thesuperior cerebellar arter\' (SCA),
Occlusion of the para-median branches results in medial (or
paramedian)brainstem syndromes and occlusion of the
circumferen-tial branches results in lateral brainstem
syndromes.Occasionally lateral brainstem syndromes are seen
inunilateral vertebral occlusion. This paper describes asimple
technique to aid in the understanding of brain-stem vascular
syndromes.
Any attempt to over simplify things runs the risk ofupsetting
those who like detail and I apologise inadvance to the anatomists
among us, but for more than15 years this simple concept has helped
numerousstudents and residents understand, often for the firsttime,
brainstem anatomy and the associated clinicalsyndromes that
result.
In the rule of 4 there are 4 rules:1 There are 4 structures in
the 'midline' beginning withM
Correspondenee to: Associate Professor Peter Gates, Director of
Neurosdencc,
The Geelonf! Hospital, Bam-on Health, Geelong, I 'ie. 3220.
Australia.
Email: peterga(a),banvorifiealth.orf;.au
Received 28 Noi'ember 2003; accepted 24 March 2004.
Funding: None
Potential aiiijlicts of interest: None
2 There are 4 structures to the side beginning with S.3 There
are 4 cranial nerves in the medulla, 4 in thepons and 4 above the
pons (2 in the midbrain).4 The 4 motor nuclei that are in the
midline are thosethat divide equally into 12 except for 1 and 2,
that is 3,4, 6 and 12 (5, 7, 9 and 11 are in the lateral
brainstem).
If you can remember these rules and know how toexamine the
nervous system, in particular the cranialnerves, then you will be
able to diagnose brainstemvascular syndromes with ease.
Figure 1 shows a cross-section of the brainstem, inthis case at
the level of the medulla, but the concept of 4lateral and 4 medial
structures also applies to the pons,only the 4 medial structures
relate to midbrain vascularsyndromes.
The 4 medial structures and the associateddeficit are:1 The
Motor pathway (or corticospinal tract): contralateral weakness of
the arm and leg.2 The Medial Lemniscus: contra lateral loss of
vibrationand propriocepdon in the arm and leg.3 The Medial
longitudinal fasciculus: ipsilateral inter-nuclear ophthalmoplegia
(failure of adduction of theipsilateral eye towards the nose and
nystagmus in theopposite eye as it looks laterally).4 The Motor
nucleus and nerve: ipsilateral loss of thecranial nerve that is
affected (3, 4, 6 or 12).The 4 lateral structures and the
associated deficitare:1 The Spinocerebellar pathways: ipsilateral
ataxia of thearm and leg,2 The Spinothalamic pathway: contra
lateral alterationof pain and temperature affecting the arm, leg
and rarelythe trunk.3 The Sensory nucleus of the 5th: ipsilateral
alterationof pain and temperature on the face in the distributionof
the 5th cranial nen'e (this nucleus is a long verticalstructure
that extends in the lateral aspect of the ponsdown into the
medulla).4 The Sympathetic pathway: ipsilateral Homer's syn-drome,
that is partial ptosis and a small pupil (miosis).
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264 Gates
Figure 1 Cross-section of the brainstem (in this case
themedulla, but the same mle of 4 applies to the pons) showing the4
Midline structures and the 4 Lateral (Side) structures aspectof the
brainstem. The size of the coloured areas does notrepresent the
actual anatomical size, but are made large enoughto see and label.
1 MN, motor nucleus (3, 4, 6 or 12); 2 MLF,median longitudinal
fasciculus; 3 ML, medial lemniscus; 4 MP,motor pathway
(corticospinal tract); 5 SC, spinocerebellar;6 SP, spinothalamic; 7
SY, sympathetic; 8 SV, sensory- nucleusof 5th cranial nen'e.
(Adapted from figure 7.90, page 955,Gray's Anatomy, 37th edn; PL
Williams, R Warwick, MDyson, LH Bannister, eds. Churchill
Livingstone 1989)'
madulla
) Optic (iwver Optic tract3 Oculomatot
nerve4 Trochlaar
nervB5 Roots of
irigeminalnerve
6 AMuconinervg
T Roots ot lacminerve
8 Voslibulo.cochtearnerve
9 Giossopharyngealnerve
10 Rootlets otvagus nerve
11 Spinal root ofaccessorynervo
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The rule oj 4 of the brainstem 265
of longitude'. So far the lesion could be anywhere in themedial
aspect of the brainstem, although if the face isalso affected it
has to be above the mid pons, the levelwhere the 7th nerve nucleus
is.
The motor cranial nerve 'the parallels of latitude' indi-cates
whether the lesion is in the medulla (12th), pons(6th) or midbrain
(3rd). Remember the cranial nervepalsy will be ipsilateral to the
side of the lesion and thehemiparesis will be contralateral. If the
medial lemniscusis also affected then you will fmd a contra lateral
loss ofvibration and proprioception in the arm and leg (thesame
side affected by the hemiparesis) as the posteriorcolumns also
cross at or just above the level of theforamen magnum. The median
longitudinal fasciculus(MLF) is usually not affected when there is
a hemi-paresis as the MLF is further back in the brainstem.
The MLF can be affected in isolation 'a lacunarinfarct' and this
results in an ipsiiateral internuclearophthalmoplegia, with failure
of adduction (movementtowards the nose) of the ipsilateral eye and
leading eyenystagmus on looking laterally to the opposite side of
thelesion in the contra lateral eye. If the patient had
involve-ment of the left MLF then, on being asked to look to
theleft, the eye movements would be normal, but on lookingto the
right the left eye would not go past the midline,while there would
be nystagmus in the right eye as itlooked to the right.
Figure 3 shows the clinical features of the medialbrainstem
syndromes.
LATERAL BRAINSTEM SYNDROMESOnce again we are assuming that the
patient you areseeing has a brainstem problem, most likely a
vascularlesion. The 4 S's or 'meridians of longitude' will
indicatethat you are dealing with a lateral brainstem problemand
the cranial nerves or 'parallels of latitude' willindicate whether
the problem is in the lateral medulla orlateral pons.
A lateral brainstem infarct will result in ipsilateralataxia of
the arm and leg as a result of involvement of theSpinocercbcllar
pathways, contralateral alteration ofpain and temperature sensation
as a result of involve-ment of the Spinothalamic pathway,
ipsilateral loss ofpain and temperature sensation affecting the
face withinthe distribution of the Sensory nucleus of the
trigeminalnerve (light touch may also be affected with
involvementof the spinothalamic pathway and/or sensory nucleus
ofthe trigeminal ner\'e). An ipsilateral Horner's syndromewith
partial ptosis and a small pupil (miosis) is becauseof involvement
of the Sympathetic pathway. The powertone and the reflexes should
all be normal. So far all wehave done is localize the problem to
the lateral aspect ofthe brainstem; by adding the relevant 3
cranial nerves inthe medulla or the pons we can localize the lesion
to thisregion of the brain.
The lower 4 cranial nen'es are in the medulla and the12th nerve
is in the midline so that 9th, 10th and 11thnerves will be in the
lateral aspect of the medulla. Whenthese are affected, the result
is dysarthria and dysphagiawith an ipsilateral impairment of the
gag reflex and the
Add contralateratfacial weaknessif upper Pons or
MJdbrain
Add 3rd for midbrain
Add 6lh for Pons
Add 12th for medulla
Contra) ateralhemiparesis
arm and leg
Contralateral lossof vibration and
proprioception
Figure 3 The signs seen in medial (paramcdian)
brainstemsyndromes.
IpsilateralHomers
tpsilateral sensoryalteration of pain
and temperature
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266 Gates
palate will pull up to the opposite side; occasionallythere may
be weakness of the ipsilateral trapezius and/orsternocleidomastoid
muscle. This is the lateral medul-lary' syndrome usually resulting
from occlusion of theipsilateral vertebral or posterior inferior
cerebellararteries.
The 4 cranial nerves in the pons are; 5th, 6th, 7th and8th. The
6th nerve is the motor nerve in the midline, the5th, 7th and 8th
are in the lateral aspect ofthe pons, andwhen these are affected
there will be ipsilateral facialweakness, weakness ofthe
ipsilateral masseter and ptery-goid muscles (muscles that open and
close the mouth)and occasionally ipsilateral deafness. A tumour
such asan acoustic neuroma in the cerebello-pontine angle
willresult in ipsilateral deafness, facial weakness and impair-ment
of facial sensation; there may also be ipsilaterallimb ataxia if it
compresses the ipsilateral cerebellum orbrainstem. The sympathetic
pathway is usually too deepto be affected.
If there are signs of both a lateral and a medial (para-median)
brainstem syndrome, then one needs toconsider a basilar artery
problem, possibly an occlusion.
In summary, if one can remember that there are 4pathways in the
midline commencing with the letter M,4 pathways in the lateral
aspect of the brainstemcommencing with the letter S, the lower 4
cranial nervesare in the medulla, the middle 4 crania! nen'es in
thepons and the first 4 cranial nerves above the pons withthe 3rd
and 4th in the midbrain, and that the 4 motornerves that are in the
midline are the 4 that divide evenlyinto 12 except for 1 and 2,
that is 3, 4, 6 and 12, then itwill be possible to diagnose
brainstem vascularsyndromes with pinpoint accuracy.
REFERENCES1 C'bapter 7. Neurology. In: Williams PL, Warwick R,
Dyson M,
Bannister LH, eds. Gray's Anatomy, 37th edn. Edinburgh:Churchill
Livingstone; 1989; 860-1243.
Internal Medicine Journal 2005; 35: 263-266