Conjoined Nerve Roots

The spinal cord ends in the upper part of the lower back, and below it the canal contains a bundle of nerve roots, the cauda equina, which descends and then peels off in pairs, one root to the left and one to the right at each level, to leave the spine through small bony tunnels called the foramina. Each root carries sensation and power to a particular territory of the leg. In the usual arrangement, every root has its own dural sleeve, a thin tubular cuff of the membrane that wraps the nerves, and exits at its own level in an orderly, predictable pattern.

A conjoined nerve root departs from this pattern. Here, two neighbouring roots share a common origin from the dural sac, or travel together inside a single common dural sleeve, for part of their journey before separating to reach their own exit points. In some variants the two roots leave through a single foramen together, leaving the adjacent foramen empty; in others they remain bound together within one sheath low against the sac. The defining feature, across all the descriptions in the literature, is that two roots that should be separate are instead joined, sharing a sheath or an origin at some point along their course from the sac to the foramen.

It is worth stating plainly that this is a developmental variant present from birth. It reflects the way the nerve roots and their coverings formed during early development, and it is fixed and unchanging throughout life. It is not a consequence of wear, of injury, of a slipped disc or of any process that has gone wrong, and it is not progressive. A person may live an entire lifetime with a conjoined root and never know of it.

Most conjoined nerve roots produce no symptoms whatsoever. They are, in the language of radiology, an incidental finding: something noticed in passing on a scan obtained for an unrelated reason, of no consequence to the person's health in itself. A conjoined root does not, of itself, cause back pain, leg pain, weakness or any of the symptoms that people associate with the spine. It is simply a different but functioning arrangement of normal nerves.

This framing matters because the term can sound alarming. The word anomaly, often used in the literature, means only a variation from the most common pattern, not an abnormality in the sense of disease. Anatomical variants of this kind are common throughout the body; the human plan allows a good deal of harmless variation in how vessels, muscles and nerves are arranged. A conjoined nerve root sits comfortably within that category. The nerves it contains work normally and supply their territories in the leg as they should.

The reason a conjoined root is discussed at all in a surgical context is not that it threatens health, but that it changes the local anatomy that a surgeon would encounter were an operation on that part of the spine ever to be required. Its significance is anticipatory and practical: it is something best known about in advance, so that it is recognised for what it is and treated with appropriate care. In the absence of any plan for surgery, a conjoined root identified on a scan is generally regarded as a curiosity to be noted rather than a finding that calls for action.

Conjoined nerve roots are uncommon, and the figures quoted for how often they occur vary strikingly depending on how they are looked for. This variation is itself one of the most instructive features of the subject, because it shows how much harder these variants are to see on a scan than to find at dissection.

The lowest figures come from imaging and from operations. On routine magnetic resonance imaging and computed tomography, the reported prevalence ranges widely but is often quoted from about a quarter of one per cent up to around the mid-teens of per cent, with many series clustering at the lower end. Older studies using myelography, a contrast-based X-ray technique, reported figures of roughly four per cent. When the finding is counted only as cases noticed incidentally during operations, the figure falls to around one to one and a half per cent, reflecting how often these roots are simply not seen until the surgeon is upon them.

The highest figures come from cadaveric studies, in which the spine is carefully dissected and every root can be traced by eye. Here the reported prevalence rises considerably, with figures ranging from around eight or nine per cent up to as high as thirty per cent in some dissection series. The gap between the cadaveric figure and the imaging figure is the heart of the matter: it tells us that conjoined roots are a good deal more common than scans suggest, and that many go unrecognised on imaging. Taken together, the literature describes a variant that is genuinely uncommon but considerably under-detected, present in perhaps a few per cent of people overall, and seen far less often on scans than it truly exists.

Conjoined nerve roots can in principle involve any pair of adjacent roots, and rare examples have been described in the neck as well as the lower back. In practice, however, the overwhelming majority occur in the lower lumbar and sacral region, and the level most frequently involved is the junction between the last lumbar vertebra and the sacrum, where the fifth lumbar and first sacral roots are joined. The level just above, involving the fourth and fifth lumbar roots, is the next most commonly reported.

There is a straightforward reason this lower region dominates the descriptions. It is the most densely packed and anatomically crowded part of the nerve-root anatomy, and it is also by far the most common site of disc problems and of lumbar surgery. A conjoined root in this region is therefore both more likely to exist and more likely to come to attention, because it is precisely here that scans are most often performed and operations most often undertaken for unrelated degenerative conditions.

To describe these variants consistently, surgeons and radiologists use a classification first set out by Neidre and MacNab, who reviewed a small series of surgical cases and grouped the lumbosacral root anomalies they encountered into categories. Their scheme remains the most widely cited framework for the subject.

In broad terms, the classification distinguishes three families of anomaly. The first and most relevant is the conjoined root itself, in which two roots share a common dural sheath at some point along their course. This conjoined category is usually divided further: in one subtype the two roots arise from a common origin high on the sac, sharing a sleeve, while in another the origins are very close together low on the sac without quite sharing a single sheath. The second family covers roots that leave through a shared foramen, with one variant in which two roots exit one foramen and leave a neighbouring one empty, and another in which two roots share a foramen while all the others remain occupied. The third family describes an anastomosis, a small connecting branch that joins two adjacent roots while they otherwise run their separate courses.

More recent authors have proposed refinements and additions to this scheme, including a confluent variant in which contributions from two adjacent roots merge into a single root before leaving through one foramen, and amendments intended to make the classification more useful for keyhole and endoscopic operating. The detail of these subtypes is of interest chiefly to the surgical and radiological team, for whom the precise pattern influences how the anatomy is approached. For the purpose of understanding, the essential point is simply that conjoined describes roots that share a sheath, and that the various subtypes capture different ways in which adjacent roots can be joined.

In the large majority of people, a conjoined nerve root is entirely silent and produces no symptoms across a lifetime. It is, however, recognised in the literature that a conjoined root can occasionally be associated with symptoms, and it is worth understanding how this is thought to come about, while keeping it in proportion against the many silent cases.

Because two roots share a confined space at the conjoined level, that space is more crowded than usual. If a disc bulge, an osteophyte or age-related narrowing develops at the same level, the reduced room to spare means the joined roots may be more readily irritated or compressed than separate roots would be. When symptoms do arise, the pattern described tends to differ from that of a straightforward slipped disc. Some series report that people with a symptomatic conjoined root are relatively more likely to describe a heaviness, aching or cramping in the leg brought on by walking and eased by rest, a pattern resembling neurogenic claudication, rather than the sharp, clearly defined radiating pain of a disc pressing on a single root. A mixed picture, with elements of both, is also described. None of this is a means of self-diagnosis; it is simply an account of patterns reported in groups of patients.

It bears repeating that the relationship between the variant and any symptoms is loose and often coincidental. A conjoined root seen on the scan of a person with back or leg pain may have nothing to do with that pain, which may arise from quite separate degenerative changes. The variant is best thought of as a piece of background anatomy that is usually irrelevant to how a person feels, and only occasionally a contributing factor.

One of the most important practical points about a conjoined nerve root is that, on imaging, it can closely imitate a herniated disc fragment, and this resemblance can mislead. Where a normal root leaves the sac as a thin, well-defined structure, a conjoined root forms a larger, more rounded mass of nerve tissue against the sac and within the lateral part of the canal. On a single cross-sectional image, this bulkier soft-tissue shadow can be misread as a piece of disc that has slipped out and is pressing on the nerve.

The consequences of this confusion run in two directions. If a conjoined root is mistaken for a disc fragment, a person might be thought to need an operation to remove a herniation that does not in fact exist. More seriously, if the variant is not recognised before surgery and a surgeon, expecting to find and remove a soft disc fragment, encounters instead a bundle of nerve tissue, there is a risk of injuring the very nerve the operation was meant to protect. This is the central reason the variant matters, and it is why radiologists and surgeons place such weight on distinguishing the two. The reassuring counterpoint is that the features that separate a conjoined root from a true disc fragment are increasingly well described, so that with awareness and the right images the distinction can usually be made.

Magnetic resonance imaging is the principal and most accurate non-invasive means of identifying a conjoined nerve root, and a number of recognised signs help to flag the variant on the standard cross-sectional images that are obtained routinely.

On the axial images, those that slice across the spine, several appearances have been described that should raise suspicion. The asymmetry of the front corners of the dural sac, sometimes called the corner sign, reflects the way a conjoined root drags the sheath out to one side. A crescent of fat sitting between the sac and the joined root, the fat crescent sign, marks the plane where the variant departs from the sac. And the way the whole affected root can be followed running in parallel at the level of the disc, the parallel sign, helps to identify it. On the sagittal images, taken from the side, an asymmetry sometimes called a shoulder sign has been described where a joined root is compromised by a disc.

Where the standard images leave doubt, additional techniques can be brought to bear. Coronal images, reconstructed in the plane of the back, and specialised sequences such as magnetic resonance neurography that display the nerves directly, increase the chance of identifying the variant. Computed tomography combined with a contrast study, CT myelography, can be valuable where the magnetic resonance findings are inconclusive. The recurring theme across this literature is that recognising a conjoined root often depends on a degree of suspicion on the part of the reporting radiologist, who can then look specifically for these signs and, where needed, request the extra images that confirm the finding. Even so, the literature is candid that the variant is sometimes missed despite good imaging, which is why intraoperative awareness remains important.

The whole clinical significance of a conjoined nerve root converges on a single point: it should be recognised before any operation on the lower lumbar spine. Identifying the variant in advance allows the surgical team to plan around it, and it is this planning that protects the nerve.

The vulnerability arises because a conjoined root is tethered and relatively immobile. A normal root can be gently moved aside to give access to a disc behind it. Two roots bound together in a shared sheath are fixed in position and far less forgiving of being retracted; an attempt to move them, or worse to separate them, risks tearing the dura and stretching or injuring the nerve. Added to this is the crowding of the shared space, which leaves little margin for instruments, and the resemblance to a disc fragment, which can invite exactly the wrong manoeuvre. For all these reasons the literature consistently identifies a conjoined root, when unanticipated, as a recognised cause of dural tears and of nerve injury during disc and decompression surgery.

When the variant is known about beforehand, none of this need follow. The team can choose an approach that avoids unnecessary handling of the joined roots, can plan to reach a disc from a different angle or side, can select techniques and instruments suited to the cramped anatomy, and can proceed with deliberate care wherever the roots lie. Reports describe operations completed safely and with good outcomes precisely because the conjoined root was identified in advance and respected. Where it is discovered only during surgery, experienced teams describe modifying their technique on the spot, but the safer position by far is to have known of it from the start. This is why a careful preoperative scan, read with the possibility of the variant in mind, is the cornerstone of managing a conjoined nerve root.

It is important to be clear that the literature on conjoined nerve roots consists almost entirely of case reports, small case series, cadaveric studies, imaging studies and narrative reviews. There are no large trials, and there is no formula that dictates what should be done. The descriptions that follow report what the literature observes in general terms, not guidance for any individual, and any decision in practice rests with the treating team and the person concerned.

For a conjoined root found incidentally on a scan, with no related symptoms and no operation in prospect, the literature describes no treatment at all. The finding is noted, its presence is recorded so that it is known to any future surgical team, and the person is reassured that it is a harmless variant. Nothing more is required, because there is nothing to treat.

Where a conjoined root lies at a level that, for separate degenerative reasons, is being considered for surgery, the emphasis falls entirely on recognition and planning rather than on treating the variant itself. The literature stresses thorough preoperative imaging, a high index of suspicion in reading it, and where helpful the additional sequences and contrast studies that confirm the anatomy. It describes selecting a surgical approach that minimises handling of the joined roots, including reaching a disc from the opposite side or by an angle that keeps clear of the tethered nerves, and where appropriate choosing approaches that come at the disc space from the front or the side rather than disturbing the roots from behind. These are descriptions of strategies reported in the surgical literature, not recommendations for any case.

In short, the evidence points towards no intervention for the silent incidental variant, and towards careful recognition and individualised surgical planning where an operation at the affected level is independently required, all within a framework of assessment and shared decision-making by the treating team.

The outlook associated with a conjoined nerve root is, for the great majority of people, entirely benign. As an incidental finding it carries no implications for health, no tendency to worsen and no need for monitoring or treatment; it is simply a variant arrangement of working nerves that happens to have been seen on a scan. Most people who have one will never know, and it will never trouble them.

Where the variant does acquire significance, it does so almost entirely in the context of surgery on the affected part of the spine, and even there the outlook is favourable when the finding is recognised in advance. The published experience describes operations carried out safely, and good recoveries achieved, when a conjoined root is identified beforehand and the surgical plan is shaped around it. The unfavourable scenarios in the literature are largely those in which the variant was unanticipated, and these are precisely what careful preoperative imaging is intended to prevent.

The consistent themes across the field are therefore reassuring: a conjoined nerve root is a harmless and unchanging quirk of anatomy; it is usually silent and discovered by chance; its chief importance is to be seen and respected before lumbar surgery so that it is not mistaken for a slipped disc or inadvertently injured; and with that recognition, the great weight of reported experience is of safe and successful outcomes. Any individual interpretation of the finding is a matter for assessment by the treating team rather than something that can be read from the label alone.

• —Trimba R, Spivak JM, Bendo JA. Conjoined nerve roots of the lumbar spine. *Spine J.* 2012;12(6):515–524. DOI: 10.1016/j.spinee.2012.06.004. PMID: 22749652
• —Neidre A, MacNab I. Anomalies of the lumbosacral nerve roots. Review of 16 cases and classification. *Spine (Phila Pa 1976).* 1983;8(3):294–299. PMID: 6623195
• —Lotan R, Al-Rashdi A, Yee A, Finkelstein J. Clinical features of conjoined lumbosacral nerve roots versus lumbar intervertebral disc herniations. *Eur Spine J.* 2010;19(7):1094–1098. DOI: 10.1007/s00586-010-1329-6. PMC2900016
• —Arboleda R, Spohn A, Pfeiffer B, Gubler D. Intraoperative discovery of a conjoined nerve root during transforaminal lumbar interbody fusion: a case report. *J Am Osteopath Acad Orthop (JAOAO).* 2025;IV(2). DOI: 10.70709/3hmz0k5td2
• —Böttcher J, Petrovitch A, Sörös P, et al. Conjoined lumbosacral nerve roots: current aspects of diagnosis. *Eur Spine J.* 2004;13(2):147–151. PMID: 14634853
• —Song SJ, Lee JW, Choi JY, et al. Imaging features suggestive of a conjoined nerve root on routine axial MRI. *Skeletal Radiol.* 2008;37(2):133–138. DOI: 10.1007/s00256-007-0403-6. PMID: 18087746
• —Burke SM, Safain MG, Kryzanski J, Riesenburger RI. Nerve root anomalies: implications for transforaminal lumbar interbody fusion surgery and a review of the Neidre and MacNab classification system. *Neurosurg Focus.* 2013;35(2):E9. DOI: 10.3171/2013.5.FOCUS13157
• —Zhang B, Zhou Q, Wang H, et al. An amendment to the Neidre and MacNab classification system for lumbosacral nerve root anomaly and its implication in percutaneous endoscopic lumbar discectomy. *World Neurosurg.* 2018;111:e756–e764. DOI: 10.1016/j.wneu.2017.12.156. PMID: 29233749
• —Sharma A, Singh V, Agrawal R, et al. Conjoint nerve root: an intraoperative challenge in minimally invasive tubular discectomy. *Asian Spine J.* 2021;15(4):545–549. DOI: 10.31616/asj.2020.0250. PMC8377216
• —Kuroki H, Nagai T. Conjoined nerve root in a patient with lumbar disc herniation accompanied by a lumbosacral spine anomaly: a case report. *J Med Case Rep.* 2023;17(1):82. DOI: 10.1186/s13256-022-03749-1. PMC9960683
• —Takagi Y, Yamada H, Ebara H, et al. Conjoined lumbosacral nerve root: a case report. *J Med Case Rep.* 2024;18(1):91. DOI: 10.1186/s13256-024-04415-4
• —Lightsey HM, Xiong GX, Schoenfeld AJ, Simpson AK. Microendoscopic decompression of conjoined lumbosacral nerve roots. *BMJ Case Rep.* 2022;15(3):e248680. DOI: 10.1136/bcr-2021-248680. PMC8928263
• —Zickler CL, Martinez Elejalde J, Landa-Galindez A. An incidental diagnosis of conjoined nerve root after a work-related back injury: a case report. *Cureus.* 2024;16(9):e68436. DOI: 10.7759/cureus.68436. PMC11445797
• —Heo DH, Kim SW, Chang HH, Park CK. Usefulness of oblique lumbar magnetic resonance imaging for nerve root anomalies and extraforaminal entrapment lesions. *Asian Spine J.* 2018;12(3):423–427. DOI: 10.4184/asj.2018.12.3.423
• —Yaman O, Ozer A. Novel L5–S1 interbody fusion technique for root anomalies or abnormal root configurations of L5–S1 foramens. *J Craniovertebr Junction Spine.* 2018;9(3):163–167. DOI: 10.4103/jcvjs.JCVJS_33_18
• —Aota Y, Saito T, Uesugi M, et al. Conjoined lumbosacral nerve roots compromised by disc herniation: sagittal shoulder sign for the preoperative diagnosis. *Spine (Phila Pa 1976).* 2007;32(26):E839–E841. PMID: 18157722
• —Scuderi GJ, Vaccaro AR, Brusovanik GV, et al. Conjoined lumbar nerve roots: a frequently underappreciated congenital abnormality. *J Spinal Disord Tech.* 2004;17(2):86–93. PMID: 15260090
• —Chotigavanich C, Sawangnatra S. Anomalies of the lumbosacral nerve roots. An anatomic investigation. *Clin Orthop Relat Res.* 1992;278:46–50. PMID: 1563168
• —Kadish LJ, Simmons EH. Anomalies of the lumbosacral nerve roots. An anatomical investigation and myelographic study. *J Bone Joint Surg Br.* 1984;66(3):411–416. PMID: 6725353
• —Song SJ, Choi JY, Lee JW, et al. Conjoined nerve roots of the lumbar spine — imaging review. Radiopaedia.org reference article (Schubert R, Campos A, Sharma R). DOI: 10.53347/rID-16025
• —Saberi H, Sharifkashani S. Conjoined lumbosacral nerve roots: direct demonstration on MR neurography. *Clin Imaging.* 2015;39(2):258–262. DOI: 10.1016/j.clinimag.2014.10.012. — NEEDS LOGIN
• —White JG, Strait TA, Binkley JR, Hunter SE. Surgical treatment of 63 cases of conjoined nerve roots. *J Neurosurg.* 1982;56(1):114–117. PMID: 7054409
• —Postacchini F, Urso S, Ferro L. Lumbosacral nerve-root anomalies. *J Bone Joint Surg Am.* 1982;64(5):721–729. PMID: 7085697

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.