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NEUROLOGICAL SIGN

Practical Neurology

10.1136/jnnp.2011.242222

Pract Neurol 2011; 11: 100105

Dermatomes and dogmaV Apok,1 N T Gurusinghe,2 J D Mitchell,3 H C A Emsley3

1Registrar in Neurosurgery, Royal

Manchester Childrens Hospital,

Manchester, UK

2Consultant Neurosurgeon,

Department o Neurosurgery,

Royal Preston Hospital, Fulwood,

Preston, UK

3Consultant Neurologist,

Department o Neurology, Royal

Preston Hospital, Fulwood,

Preston, UK

Correspondence to

Dr H C A Emsley, Department o

Neurology, Royal Preston Hospital,

Sharoe Green Lane, Fulwood,

Preston PR2 9HT, UK;[email protected]

DERMATOMES AND THEIRSIGNIFICANCELocalisation o sensory symptoms and signs

to specifc parts o the central and periph-

eral nervous systems is a signifcant part o

the neurological examination and diagnosticevaluationthe crucial where is the lesion?

question. Ever since the frst attempts at

mapping dermatomes in the late 19th cen-

tury, neurologists have used dermatomes in

their clinical diagnosis o radiculopathy and

in determining the level o spinal cord injury.

Neurosurgeons and neurophysiologists rely ondermatomes or intraoperative monitoring o

The concept o dermatomes came rom early attempts to correlate thephysiology o sensation with anatomy. There are various defnitions odermatomes and several maps in common use. While useul, dermatomesare subject to considerable variation between maps and, indeed, betweenindividuals. Anecdotally, precise dermatome distributions are generally regarded

by experienced neurologists with a degree o caution, being viewed as anapproximation. In this article, we consider the validity o the dermatome mapsand their background, as well as introducing a relatively recent evidence baseddermatome map.


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Apok, Gurusinghe, Mitchell, et al

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spinal cord unction through somatosensory

evoked potentials. And reliance is also placed

on dermatomes in the practice o regional

anaesthesia.

The concept o dermatomes originated in

early attempts to correlate the physiology o

sensory experience with an anatomical sub-

strate. Today, the term dermatome gener-

ally reers to an area o skin innervated by a

particular neural element, specifcally nerve

root, dorsal root ganglion or spinal segment.

Dermatomes are o course distinct rom the

areas o skin supplied by particular peripheral

nerves, these oten reerred to as the periph-

eral nerve felds (or cutaneous nerve distri-

butions). Despite the long tradition, and the

emphasis still placed on teaching dermatomes

to medical students, experienced neurolo-

gists, perhaps because they are well aware othe approximate nature o the various maps,

probably attach rather less signifcance to the

precise dermatome distribution than the cor-

responding myotomes in lesion localisation.

Alteration o sensation in a dermatome is a

sign about which neurologists are rightly cir-

cumspect. In this article, we will consider the

validity o dermatome maps, how they evolved

and draw attention to a recently devised evi-

dence based dermatome map.

VALIDITY OF CURRENTDERMATOME MAPSPerhaps surprisingly, the dermatome maps in

current use were largely constructed in the

early hal o the 20th century by Sir Henry

Head, Otried Frster, Jay Keegan and Frederic

Garrett. Currently, there are 14 dierent maps

in 13 dierent major texts.1 Even individual

texts (eg, Grays Anatomy) have variations

between dierent editions. The overwhelm-

ing message seems to be that these maps are

inaccurate, with a surprising lack o consensusabout the size and location o dermatomes.

This has much to do with the methodology

that was used to draw these maps.

The physiological means by which most

o the maps were derived did not take into

account various points o ambiguity, such as

the defnition and the nature o the neural ele-

ment being mapped. Compounding this ambi-

guity is the degree o variation and overlap

between adjacent dermatomes,between and

even sometimes within (eg, unilateral brachialplexus variant) individuals (box 1 reers to

the prefxed and postfxed brachial plexus as

an example o such variation2). There is some

agreement in the literature that dermatomes

in reality might be larger in area than those

shown in traditional texts and thereore have

a greater degree o overlap than originally

acknowledged. So a lesion o a nerve root

may produce a much smaller area o sensory

loss than a casual glance at a dermatome map

might suggest. The three most commonly re-

erenced dermatome maps in contemporary

anatomy texts are those o Head and Campbell

(1900), Otried Frster (1933) and Keegan and

Garrett (1947).

HOW DERMATOMES USEDTODAY CAME TO BEHenry Heads map

Henry Head (18611940), a physician at theRoyal London Hospital, published the frst

widely accepted dermatome diagram in 1900.

A voracious researcher into sensory localisa-

tion and pain in visceral disease, he sectioned

his own superfcial radial nerve and diligently

documented the developing sensory distur-

bance.3 His dermatome mapping work was

coauthored with Alred Walter Campbell

(18681937) and was largely based on draw-

ings and photographs o 450 patients with

herpes zoster eruptions.

4

The fnal product wasthe result o this large study o herpes zoster

patients as well as observations o patients

with spinal cord injuries and those with pain

due to visceral non-neurological disorders, in

whom Head described, or example, positions

over which the patient experienced pain in

Box 1 The prefxed and postfxed brachial plexus2

Anatomical sources o variation leading to deviation rom the expecteddermatome distribution, as well as dierences in motor supply o the upperlimb, include the prefxed and postfxed brachial plexus. Most standardmedical textbooks describe the brachial plexus as arising rom the lowerour cervical nerves and the frst thoracic nerve with an occasionalcontribution rom the ourth cervical and second thoracic nerve. A prefxedbrachial plexus has been described as one with a large contribution romthe ourth cervical nerve with or without a small contribution rom the frstthoracic nerve. A postfxed brachial plexus is one with a large contributionrom the second thoracic nerve and little or no communication with thefth cervical nerve. It is worth being aware that brachial plexus variationsare more the rule than the exceptionnot only in terms o unexpecteddermatome distribution but also because o the potential predisposition to

certain conditions, such as thoracic outlet syndrome.

A lesion of a nerve

root may produce a

much smaller areaof sensory loss than

a casual glance at

a dermatome map

might suggest


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that.my areas correspond to the supply not

o roots, but o segments o the spinal cord

rom which the roots in part arise.

Otried Frsters mapOtried Frster (18731941) was a German

neurologist who turned to neurosurgical prac-tice at the age o 40 years. Posterior rhizoto-

mies were being undertaken at the end o the

19th and start o the 20th centuries or the

treatment o spasticity, but ell out o avour

because o unacceptable adverse eects.

He developed his map by surgically isolating

single dorsal nerve roots ater sectioning the

dorsal nerve roots above and below the root

under investigation. These experiments were

modelled on those o Sir Charles Sherrington

(Nobel Prize winner or physiology and medi-

cine in 1932) who researched dermatomes inmonkeys with this same method o section-

ing nerve roots to isolate a single nerve root.

Frster extended these experiments to human

subjects stating, .I need not discuss circum-

stances under which such a selected proce-

dure may be undertaken.5

A signifcant aw in Frsters methodol-

ogy was his lack o consistent documentation.

There is little inormation provided on his

method o assessing and reporting on der-

matomes. Moreover, he also ailed to note thetime lapse between sectioning and dermato-

mal testing. This, in particular, may have had

signifcant implications as a result o physi-

ological processes such as Wallerian degen-

eration or even nerve regeneration, leading to

the possibility o shrinkage in the extent o

sensory loss over time.

Frster acknowledged the phenomenon o

dermatomal overlap and individual variation,

concluding that sectioning o a single nerve

root was never accompanied by sensory loss.

However, his maps were notable or not showing

portions o the limbs or the posterior trunk.5

Keegan and Garretts mapFinally, Keegan and Garrett proposed their map

in 1947, 13 years ater the frst reported case o

back and leg pain rom a herniated disc.7 Based

on their initial observations that herniated discs

compressing nerve roots were associated with

diminished sensation, they set about construct-

ing a dermatomal map based on observations

o hypoalgesia in patients with disc prolapse(165 cervical and 1264 lumbosacral, o which

gastric disturbancesthat is, areas o reerred

cutaneous tenderness or allodynia.5

Analysis o Heads work reveals limitations

in methodology and interpretation. His earlier

work on the L1 dermatome or instance was

based on a patient with frst and second lum-

bar vertebral ractures who had bilateral L1

nerve root involvement at surgery with bilat-

eral T12 nerve root sparing.6 The upper border

o sensory loss in this case was taken to be the

upper border o the L1 dermatome, although

the possibility o dermatomal overlap was

apparently not considered. L5 was determined

by analysing cutaneous tenderness in a patient

with an inamed right lobe o the prostate

gland; having already mapped S15 and L1 in

other patients, Head considered that the only

remaining area within this region o tender-

ness, the lateral aspect o the leg, on accounto its adjacency to the sacral skin segments,

must represent the L5 dermatome. This con-

cept o adjacent skin segments having adja-

cent root and spinal segments was not true

however or the L4 and S2 segments o his

map. The L4 dermatome was loosely derived

rom the observation o a single patient whom

Head claimed had a spinal cord injury although

no urther details are available on the nature

o the injury. Following this, he established the

L3 dermatome by elimination in a patient withherpes zoster in whom he deemed there to be

L3, L4 and L5 involvement. He conjectured that

the L3 dermatome must represent the area not

included in his previously determined L4 and

L5 dermatomes.6

A substantial drawback o Heads study o

herpes zoster cases was that histological con-

frmation o single dorsal root ganglion inam-

mation was apparently obtained in only 16 o

the 450 patients. Furthermore, among these

16, not all dermatomes were represented, there

were no recorded examples o C5 through C8or any root level below L1. Assumptions were

oten made about the precise dorsal root

involved in the production o the resulting

map, the frst to document the thoracic der-

matomes. It is also now known that herpetic

eruptions can aect several adjacent dorsal

root ganglia simultaneously. Furthermore, not

all the cutaneous fbres within a dorsal root

ganglion may be aected. Consideration o

these points highlights various aws in the

proposed map. Indeed Head himsel acknowl-edged the ambiguity o his fndings by noting


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its acknowledgement o common areaso signifcant overlapor example, C7overlaps considerably with C6 and C8, thedorsal surace o the hallux is commonlyinnervated by L5 but can also be suppliedby L4 and the S1 dermatome extends asar superiorly and posteriorly as the but-

tock and overlaps with S2. Aids to the examination of the peripheralnervous system12 is arguably one o themost authoritative yet accessible mono-graphs devoted to examination o theperipheral nervous system (see page 106or review o this book), its own rich his-tory recently revisited.11 It does indeedhighlight the problems o overlap andvariability aecting dermatome maps.For example, the current authorMichaelOBrienpoints out that while the usual

corresponding dermatomes or thumb,

28% and 56%, respectively, were confrmed at

operation).1 This map consisted o neat, non-

overlapping dermatomes, despite the authors

acknowledgement that dermatomes did over-

lap. These dermatomes almost always reached

the midline. Theirs is the most widespread der-

matome map in contemporary use but is argu-

ably the most awed o the three main maps

discussed thus ar. The site o nerve root com-

pression was based on myelographic rather

than operative fndings in the main. They also

claimed a high degree o reproducibility or

their map, with no more than 1 cm variation

between individuals. Importantly, Keegan sup-

ported the concept that intervertebral disc

compression o a single nerve root results in

an area o cutaneous sensory loss, contradict-

ing the work o Sherrington and Frster, per-

haps on account o the dierent physiologicalcharacteristics o nerve root compression as

opposed to sectioning.8

A NEW EVIDENCE BASEDDERMATOME MAPThere have been ew attempts at veriying

these original dermatome maps, especially in

more recent years:

O note is the prospective study o 403patients by Kortelainen.9 Pain reerral

patterns and neurological fndings werecharted. Radiological fndings (using CT)and surgical root irritation did not alwayscorrelate with anticipated pain reerralzones; this was a signifcant contributionto the mounting evidence o the limitedutility o dermatomes in clinical practice.

Nitta et al10 selectively blocked nerveroots with xylocaine injections underuoroscopic guidance in patients withradicular pain due to disc herniation. Theyconcluded that the characteristic L4, L5

and S1 dermatomes were only present inabout 80% o patients. The implicationwas that one in fve patients had inner-vation patterns other than those o thetraditionally described dermatomes.

In their comprehensive review exploringthe controversies o dermatome maps,Lee et al1 proposed an evidence baseddermatome map ormed rom the assimi-lation o previous maps (fgure); while theuse o the term evidence based may bestretching a point, this map is at least

an attempt to systematically distil thebest available evidence. It is notable or

Figure

The evidence based dermatome map representing the most consistent tactile dermatomal areas or

each spinal dorsal nerve root ound in most individuals, based on the best available evidence. The

dermatomal areas shown are not autonomous zones o cutaneous sensory innervation. Except across

the midline where overlap is minimal, adjacent dermatomes overlap to a large and variable extent.

Blank regions indicate areas o major variability and overlap. S3, S4 and S5 supply the perineum but

are not shown or reasons o clarity. Note consecutive dermatomes shown in bu or blue or clarity.

From Lee et al.1 Copyright Wiley-Blackwell (2008). This material is reproduced with permission o

Wiley-Blackwell, a subsidiary o John Wiley and Sons, Inc.


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Practical Neurology

10.1136/jnnp.2011.242222

DERMATOME MAPPING: ANEXERCISE IN FUTILITY?The greatest aw in seeking to map der-

matomes has been the assumption that the

correlation o CNS to skin is a direct and static

one. We now know that neural elements are

continuously being suppressed, acilitated andreorganised in a dynamic ashion. Moreover, as

ar back as 1893, Sherrington demonstrated in

his early experiments on monkeys that the dis-

tribution o sensory fbres is less dense towards

the periphery o a dermatome, hence maps can

only reect the regions o most intense cutane-

ous innervation. Almost a hundred years later in

1989, Moriishis work on cadavers demonstrated

the presence o intrathecal intersegmental con-

nections between the dorsal spinal rootlets,

with the greatest variation in the upper limb

dermatomes.13 This has eectively obviated theidea o dermatomes being the cutaneous repre-

sentation o dorsal root ganglia.

The most signifcant work in recognising the

vast complexity o cutaneous innervation was

by Denny-Brown and colleagues.1416 Through

their experiments on monkeys, they eectively

demonstrated that cutaneous innervation o

dorsal root ganglia is dierent to that o the

dorsal root. The role o adjacent dorsal root

ganglia and the spinal cord in determining the

cutaneous innervation o a given spinal nerveroot was recognised or the frst time. By mod-

iying Frsters method, they sectioned nerves

either proximal or distal to dorsal root ganglia

and studied patterns o cutaneous sensibil-

ity. Their most important fnding was that the

Lissauer tract (near the substantia gelatinosa

o the spinal cord) is a key mediator o dor-

sal root transmission. The medial and lateral

parts o this tract potentiate and inhibit sen-

sory impulse transmission, respectively. Hence

corresponding lesions result in dermatomal

shrinkage or expansion. In eect, this fnding

established that the previously accepted idea

o a direct correlation between neural element

and skin was an overly simplistic one.

That dermatomes can expand and shrink,

depending on the anatomical and physiologi-

cal characteristics o adjacent spinal cord seg-

ments and dorsal root ganglia, has led to an

emerging recognition o the dynamic nature o

cutaneous innervation. This work has yet to be

translated into clinical practice but it heralds a

new page in the history o attempts at under-standing cutaneous innervation.

middle fnger and little fnger are C6,C7 and C8, respectively, the index andring fngers are too variable to be clini-cally useul. Thus there must be someconcern that dermatome maps depictingthe distributions to the ends o the limbs(including the evidence based map) areawed in this respect. It is also importantto bear in mind that the evidence basedmap does not show the dermatomes asautonomous areas. OBrien reers to therebeing less, i any, overlap between non-consecutive dermatomes, and thereorethese boundaries give more reliable andclinically useul borders. In practice it isonly necessary to know the approximatecentre o a dermatome and, i appropri-ate, map the boundary with the principles

outlined in box 2 (based on the sensoryexamination sequence Aids12).

PRACTICE POINTS

Examination o cutaneous loss over dermatomes is necessary only whensuggested by the history.

Dermatome maps are approximations, subject to various methodological

weaknesses, and serve only as a guide. There is signifcant overlap between adjacent dermatomes. There is less, i any, overlap between non-consecutive dermatomes,

and thereore these boundaries give more reliable and clinically useulborders.

Box 2 Testing dermatomes

The history will usually determine whether examination o dermatomalsensation is required. The patient should be asked to indicate any area oaltered sensation, including its limits. It is usually not necessary to test all

dermatomes, with the examination ocusing instead on the region suggestedby the history. For sampling dermatomes, it is customary to move romdistal to proximal along the long axis o the medial and lateral borders othe limbs, and ascending vertically on both sides o the trunk. I there is areported area o sensory impairment to pinprick the examination shouldproceed rom the centre o the area o maximum abnormality towards thenormal area to defne the borders o the area o altered sensation. I there isan area o enhanced sensation, usually hyperalgesia, the examination shouldproceed in the reverse direction. The patient is asked to confrm that thestimulus is perceived as sharp in each dermatome. Temperature sensation,oten omitted i pain sensation is normal, is undertaken in a similarsequence. Usually the metal o the tuning ork is the most readily accessiblecold stimulus in the clinic. Arguably, light touch, tested with a wisp o

cotton wool or a light fnger touch on the skin, and otherwise ollowing thesame sequence, adds little additional inormation, although it is said that thearea o defcit can be somewhat larger than that to pinprick in dermatomalsensory loss.


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3. GreenbergSA. Henry Head (18611940). J Neurol2004;251:11589.

4. Head H, Campbell AW. The pathology o herpeszoster and its bearing on s ensory localization. Brain1900;23:353523.

5. GreenbergSA. The history o dermatome mapping.Arch Neurol2003;60:12631.

6. Head H. On disturbances o sensation with especialreerence to the pain o visceral disease. Brain

1893;16:1133.7. Mixter WJ, Barr JS. Rupture o the intervertebral discwith involvement o the spinal canal. N Engl J Med1934;211:21015.

8. Keegan JJ. Neurosurgical interpretation odermatome hypalgesia with herniation o thelumbar intervertebral disc. J Bone Joint Surg1944;26:23848.

9. KortelainenP, Puranen J, Koivisto E, et al. Symptomsand signs o sciatica and their relation to thelocalization o the lumbar disc h erniation.Spine1985;10:8892.

10. NittaH, Tajima T, Sugiyama H, et al. Study ondermatomes by means o selective lumbar spinal nerveblock. Spine1993;18:17826.

11. Compston A. Aids to the investigation o peripheralnerve injuries. Medical Research Council: Nerve

Injuries Research Committee. His Majestys StationeryOfce: 1942; pp. 48 (iii) and 74 fgures and 7 diagrams;with aids to the examination o the peripheral nervoussystem. By Michael OBrien or the Guarantors oBrain. Saunders Elsevier: 2010; pp. [8] 64 and 94Figures. Brain 2010;133:283844.

12. OBrien MD.Aids to the examination of the peripheralnervous system, 5th Edn. London: Saunders Elsevier(on behal o the Guarantors o Brain), 2010.

13. MoriishiJ, Otani K, Tanaka K, et al. The intersegmentalanastomoses between spinal nerve roots. Anat Rec1989;224:11016.

14. Denny-BrownD, Kirk E. Hyperesthesia romspinal and root lesions. Trans Am Neurol Assoc1968;93:11620.

15. KirkEJ, Denny-Brown D. Functional variation indermatomes in the macaque monkey ollowing dorsalroot lesions. J Comp Neurol1970;139:30720.

16. Denny-BrownD, Kirk EJ, Yanagisawa N. The tract oLissauer in relation to sensory transmission in thedorsal horn o spinal cord in the macaque monkey.J Comp Neurol1973;151:175200.

AND IN PRACTICE.There is no place or dogmatic adherence to

classical dermatome mapsrather, in the

teaching and practice o sensory examination,

we all need to be aware o the considerable

railties o the methods used to derive the

maps. They should serve merely as a guide, withthe examiner clearly understanding their many

limitations such as variation between individu-

als, as well as overlap between dermatomes. It

is however rereshing that an evidence based

approach has been taken, and arguably we

should be embracing more recent anatomical

work when considering dermatomes in clinical

practice rather than perpetuating the weak-

nesses o the original dermatome maps o

Head and Campbell, Frster, and Keegan and

Garrett, while at the same time acknowledg-

ing the crucial contributions o these earlierworkers.

ACKNOWLEDGEMENTSThis article was reviewed by Richard Hughes

and Michael OBrien, London.

Competing interests None.

Provenance and peer review Not commissioned;

externally peer reviewed.

REFERENCES1. LeeMW, McPhee RW, Stringer MD. An evidence-

based approach to human dermatomes. Clin Anat2008;21:36373.

2. PellerinM, Kimball Z, Tubbs RS, et al. The prefxedand postfxed brachial plexus: a review with surgicalimplications. Surg Radiol Anat2010;32:25160.

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