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).

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

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

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