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Refer this article as: Cabanis, E., The Neuro Ocular Plane (NOP), Points de Vue, International Review of Ophthalmic Optics, N69, Autumn 2013

The Neuro Ocular Plane (NOP)

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A double natural cephalic reference, that of the head posture in Homo sapiens, standing, looking straight ahead, it is the neuroanatomy of visual pathways, from the "cornea to the calcarine fissure".


The French Ophthalmology Society (SFO) elected as its annual reporter E. Hartmann on "Radiography in ophthalmology. A clinical atlas" (1936), then H. Fischgold et coll. (1966) for "Neuroradiological exploration in ophthalmology" and, for 1996, the author of this article, for "Imaging in ophthalmology," the 3rd phase in this 30 year cycle, due to chance and the need for the development of X-rays into digital neuroimaging (X-ray and magnetic scanner, MRI) [1, 2], Fig.1.

Fig. 1: The control panels in a 3Tesla MRI room, in front of the Faraday cage (dappled).

Focussing on digital technology, which came into being in 1972 (X-ray scanner), this report summarises 40 years of progress to date, that of the new digital anatomy (2008, MRI, Fig. 1), both normal and pathological, of visual pathways in "Homo Sapiens". An axial section (horizontal) of the head containing the optic nerve, from its papilla through to the optic canal, performed by my friend Professor Ugo Salvolini (Universita di Ancona, 1st X-ray scanner in Italy), "spreads out" as far as possible the intra-orbital segment of the two optic nerves, in primary gaze position, excluding the "partial volume effect" (Fig.2).

Fig. 2: Initial observation of the NOP in an adult using an X-ray brain scanner (1973). In primary gaze position, the axial section and thick (6mm) cephalic transverse section contains, from front to back, the relative hyperdensities of the 2 crystalline lenses, of the heads of the two optic nerves and of the 2 optic canals.

The transversal diameter of the optic nerve " in vivo " became measurable. The first "NOP" section, thick axial (6 mm), 1 year after the presentation of the X-ray scanner invention by Godfrey Newbold Hounsfield (1972) in London (Nobel Prize for Medicine in 2003), provided the first maximum axial vision of the eyeball (increased in myopics). The "NOP" was born. Five years later, as head of the neuroimaging department at the Quinze-Vingts National Ophthalmology Hospital I confirmed this section on the new ND 8000 scanner (Thomson CGR) which had been evaluated for 4 years in the factory (Fig. 2).

After the first "NOP" publication by the Société Anatomique de Paris (1978) Professor A. Delmas was kindly informed "Dear Friend, your work is reminiscent of Broca‘s visual plane, I‘ve checked". Both delighted with this first scientific validation and furious at having missed his first centenary reference, before the author became professor of neuroimaging and radiology at the Pierre et Marie Curie Paris 6 University (and associate Anatomy Professor), he was to contribute actively to the book entitled "Paul Broca géant du 19e siècle" (Paul Broca, giant of the 19th century) [3]. An anatomist and anthropologist, Broca wrote in 1873 "(…) The head is horizontal when a person is standing and looking towards the horizon. That is the natural direction of the gaze (…)". The 1976 annual report of the French Ophthalmology Society (SFO), 762 p. and 257 co-authors, devotes 83 p. (324-407) to chapter 2 "The twelve anatomies of in vivo visual pathways" for 4 reasons.
1. Anatomy is "plural", from microscopic anatomy to surgical anatomy.
2. The power of digital tools (X-ray scanner then MRI, image processing and nuclear imaging), in terms of both sensitivity and spatial resolution, leads to a proliferation of in vivo results leading to chemistry and therefore molecular anatomy and genomics.
3. MRI has provided the fourth dimension, sagittal, frontal and oblique (3D) to horizontal bidimensional exploration (2D) of the head.
4. Logically ordered, normal results make their mark, validated by the perspective of half a century and hundreds of thousands of clinical observations. The notion of "space and "cephalic references" in digital anatomy, in vivo , is therefore the first of the 12 approaches to the head, a spherical shape with two orthogonal diameters, one, horizontal, of the sensory relays containing the neuronal vision pathway and the other perpendicular to the previous one, containing the oculomotor pathways, from the cortex to the cerebral trunk. Since the nineteen fifties, stereotactic neurosurgery teaches rigorous spatial identification for the cerebrum and the diencephalon. On his model of the optic pathways, Henry Hamard modelled in white the horizontal optic pathways, orthogonal to the arterial vascular and cervico-encephalic axes and to the direction of the cervical spine (Fig. 3). Added to this is the oculomotor organisation, orthogonal to the optical pathways, axial and transversal (like the horizontal section obtained by X-ray scanner, if the head is correctly placed in the machine).

Fig. 3: Top left the anatomical diagrams describing the human body, Homo sapiens standing, "looking towards the horizon". Top middle, skull without a mandible (removed) placed on a board with 2 needles stuck into the 2 optic canals at the back and the 2 centres of the orbital surfaces at the front, virtual diagram of vision parallel to the horizontal board. Top right, Paul Broca. Bottom, model of
the optic pathways, in white, orthogonal to the cervical spine and the arterial axes.

1. Historically, orientation planes of the head were firstly those of its skeleton, the skull, at the origins of anthropology and human and compared animal palaeontology, from Daubenton (1764) to Virchow- Hoelder (1850), and then from A. Delmas and B. Pertuiset in orbitomeatal planes (1959) [3] or the bicommissural CA-CP planes of Talairach and Szikla (1949-1977) [4], to the vestibular plane of Dr Perez dissecting the semi-circular canals of the inner ear (1982) [5], and the various orientations of the dry skull (and then in vivo using standard X-ray and vascular and ventricular neuroradiology) (Fig. 4).

Fig. 4: Display of several cephalic orientation planes on a median cephalic section of the head (MRI), with the NOP defining the horizontal. Top left, NOP with CA-CP (white anterior and posterior commissure – mammillary body), CP-MB chiasmatic point-mammillary body), OM (orbitomeatal). Top right, bicommissural verticals (ACV and PCV). Bottom left, the NOP horizontal. Bottom right, Orbitomeatal plane (+ 20° over the previous one).


2. The axial plane of NOP visual pathways from the X-ray scanner (1973) to Paul Broca (1873), meets the orbital definition (X-Ray scanner, MRI, other axial photonic imaging of the head awaited): "Plane of horizontal section of the head, of millimetre thickness (5 to 1) which, in any position of the gaze, includes, symmetrically sectioned from front to back, the 2 crystalline lenses according to their longest axis, the 2 optic nerve heads and the 2 optic canals" [1] (Fig. 2). The NOP therefore includes the 3h-9h horizontal meridian of the emmetropic eyeball, it is the horizontal meridian plane of the orbital pyramid whose apex is at the orbit orifice of the optic canal. This plane leads to axial exploration of the optic nerves using the X-ray scanner and MRI, avoiding the "partial volume effect" which hinders exploration of the canalicular and intra-orbital segment of the 2 optic nerves. 120 years earlier P. Broca wrote "(…) The head in the direction it is during life, when it is balanced on the spine and the patient is looking straight ahead … on the dry skull (…) The direction of this horizontal visual axis (…) a line which, starting from the optic aperture, will pass through the orbital opening …", a skull positioned on the craniostat is fitted with two orbital needles (Fig. 3, 4) [2]. This "intuition on skeleton" (the skull) confirmed, 113 years later, by X-ray scanner and MRI of the "head" (the contents, brain), is therefore confirmed as the "new plane" of vision and visual pathways, by multiple biometric, orbital and maxillofacial works, with 3 contributions made by MRI in 1984: 1. confirmation in vivo of axial and transversal layout of the visual pathways, 2. increased justification of a cephalic spatial reference within a poly-dimensional anatomic technique, 3. imagination of a new plane, vertical this time, the Transhemispheric Oblique Neuro-Ocular Plane, complementary since it is an oblique vertical of the head (see above). This Fig.3 shows on a sagittal MRI section of the head, strictly oriented in the NOP, the NOP, OM and AC-PC. This horizontal aspect of the visual pathways shows, like corporal anatomy overall, very slight individual variability due to age first (angulation of the chiasma in children) and the ethnicity (brachycephaly v. dolichocephaly). From "the cornea to the calcarine fissure" the NOP therefore contains the sensorial pathways of vision. This axial and transversal layout of optical pathways, shown clearly in descriptive neuroanatomy and everyday in vivo MRI, as well as in functional MRI and neurotractography, is particularly well suited to exploration by X-ray scanner and MRI.

As an illustration, Fig.6 shows the angular difference of functional postures (therefore anatomical sections) of the cephalic orientation of the two people. The angular difference on OM (+ 20°), is therefore compensated horizontally, i.e. if both subjects lift their chin by 20°. They are then standing to attention, looking straight forward towards the horizon. The black line on the door behind the two subjects in profile, shows this (NOP), fixedly angled at 7° on Francfort and vestibular skeletal planes (6°5). The NOP MRI (Fig. 5), with comparative anatomical control (in cadaver) checks that the NOP contains the visual pathways, from the cornea to the calcarine fissures, at the same heights as the optic canals, from the mesencephalon and even from the culmen of the vermis cerebellum, in the falcotentorial angle. Two points should be underlined here as they are essential: 1. The NOP is orthogonal in the direction of the cerebral trunk on the sagittal sections of the MRI, which contains the corticospinal or pyramidal tract. 2. The NOP is therefore perpendicular to the floor of the fourth ventricle. All this brings us back to the intuition for which Broca could have had no other proof than a skeleton and two knitting needles: "The head is horizontal when a man is standing, looking straight ahead towards the horizon. This is natural direction of the gaze". The book mentioned at [1] refers to the practical application of installing the patient in the tunnel of the machine which, it would appear here, is quite unexpected for the reader.

Fig. 5: In vivo and in morte, NOP of visual pathways 3D referencing of the head) (X-ray scan and MRI) shown here by a red line on the face of the bald headed man with a moustache. The so-called "Francfort" planes (+ 7°, below, in black) OM (and AC-PC) used in traditional radiology and stereotactic neurosurgery (in red + 20° below). In MRI recognition of grey matter (cortex, nuclei) and white matter, left, right, confirms the anatomic correlation of the visual pathways, "from the cornea to the calcarine fissure".

Fig. 6: In vivo, respecting the NOP means people can see one another and speak to each other. The horizontality defined by the black line placed on the door (behind the two people in profile) positions the NOP, fixed at 7° on the Frankfort and vestibular skeletal planes (6°5). When lifting their chins to 20°, the 2 men stand to attention, looking towards the horizon with an angular difference (+ 20°) compared to the OM.

3. The oblique trans-hemispheric neuro-ocular plane or OTNOP, the oblique vertical cephalic reference (Fig. 7). Beyond the horizontal plane of the X-ray scanner, MRI shows the 3 dimensions of the head and their digital reconstruction. The creation of oblique sections, in every spatial plane, was soon achieved. Now, this type of "oblique" anatomy is without reference system in the classic anatomical books. These works are restricted to the usual 3 planes, OX, OY, OZ. A reference system would therefore appear to be even more important in this circumstance of oblique vertical exploration, using the NOP. The intra-orbital optic nerve is then the reference from its intra-ocular system through to the optic canal, whatever the position of the gaze. Another reference comes in, the presence of the foramen magnum and of the atlanto-axial joint in the "OTNOP" section, because it follows the vertical meridian of a globe, the optic nerve, the chiasmatic decussation and the contralateral strip, down to the contralateral occipital section of the globe observed. This is an "oblique vertical section plane of the head, of millimetre thickness (1 to 5) which, in any "indifferent" position of the gaze, includes: the crystalline lens according to its large vertical axis, the head of the homolateral optic nerve, the homolateral optic canal and the foramen magnum above the odontoid apophysis of the axis (C2)" (Fig. 7) [1].

Fig. 7: Oblique trans-hemispheric neuro-ocular plane (OTNOP): left, trajectory of the sections used and the result, right, compare with the median sagittal plane of the head with MRI.

The plane is limited by the angular geometry of the direction of the optic nerve and it is difficult to obtain both the crystalline lens and the head of the optic nerve in the same plane since the latter passes, in fact, through the macula. The skeletal fixedness of the OTNOP on the cervicooccipital hinge in MRI has been shown in 41 European patients of average age, 39 of whom had the same anatomical layout of the anterior visual pathways. In the NOP, the direction of the 2 optic nerves, from the head to the optic canal, is crossed through in the middle with the superior projection of the odontoid apophysis. Electronic superimposition of the references obtained in the NOP (odontoid apophysis at the front and foramen magnum at the back) leads one to observe that the vertical projection of the direction of the 2 optic nerves occurs exactly on the vertical up from the odontoid apophysis of the axis (C2). Reference must be made here to former, known correlations existing between cervicooccipital biomechanics and the constraints of the oculocephalogyric reflex. The functional fixedness of this projection is interesting. The OTNOP acts as an oblique functional and descriptive vertical anatomical reference of the head.


"Bios (life) and metron (measurement) meet once the references have been fixed. Between 1974 and 1995, from the X-ray scan to MRI, work proceeded and was verified" [1, 2]. This field alone is summarised here.
1. Angular biometry of the NOP of the Francfort skeletal plane (NOP/FR) = 7° (average m = 6°49’ and σ = 2°38’) (see details of the 4 groups of measurements 1977-1982).
2. Angulation of the NOP on the vestibular plane (Perez, Delattre and Fenart) and on the OM/CA-CP plane is measured on average at 28°35’ (σ = 5°13’) in 52 young adults. Added to this is a notion of parallel between the NOP and Broca‘s alveolar-condyl plane, found in a "bite" (Fig.5, pencil bitten by the model). All the skeletal data confirms the fixedness of orientation of the visual plane on the skeleton of the head. The OM/CA-CP parallel agrees with the NOP/OM-CA-CP angulation of an average of 20° (and not of 15° or 10° as has been stated in literature). Visual cephalometry and its foremost practical application, oculo-orbital topometry, is therefore based on a certainty, that of the anatomical correlations established between the spatial orientation of the brain (visual pathways) and of its skeleton (the bony globe of the skull). Fig.3. resumes the fixedness of the NOP on Francfort, the vestibular plane, the ocular globe, topometric reference sphere (neuro-ocular index and dissociation of populations with papilledemas by HIC, in the middle, left and centre). The facial contour achieved based on the NOP by X-ray scan models the end appearance of the ocular vestibulography used on board the European space laboratory (Nov.-Dec. 1983).
3. Biometrics, oculo-orbital and facial topometry, exophtalmometry
3.1. Definitions of distances and indices, normal readings in an emmetropic patient, in the NOP by X-ray scan the letter "o" indicating the standard gap per calculated average. Fig.5 shows the oculo-orbital contours and measurements established on the axial section of the NOP by X-ray scan in an emmetropic adult (1978-1983). The methods used are indicated in the book referred to [1].

The series are normal, adults and children, pathological in dysthyroid ophthalmopathy. The contour on the console or work station of the X-Ray scanner or MRI provides these detailed measurements (Fig. 7). First contour: line joining the anterior point of the 2 external orbital pillars in the NOP. Since this is a thick section (6mm) it is not a line but, by definition, a plane. The readings indicated below refer to Figure 8. The External Bi-Canthal Distance (EBCD) measures the distance between the two external orbital pillars (m = 97.52 mm, σ = 4.43). The Inter-Ocular Distance (IOD) = distance between the central point of the 2 crystalline lenses (m = 63.73 mm, σ = 3.62). The Maximum Inter-Plane Distance (MIPD) measures the distance between the 2 external orbital walls in view of their possible temporal convexity (m = 28.7 mm, σ = 2.67). The External Ante-Bicanthal segment (EABC) measures the distance between the PEBC and the tangent at the anterior corneal hyperdensity (m = 15.89 mm, σ = 1.96). The Retro External Bi-Canthal Segment (REBC) of the ocular globe measures the distance between the PEBC and the tangent at the posterior coroid-scleral hyperdensity, close to the head of the optic nerve. The Maximum Axial Length (MAL) of the globe measures the distance between the tangent at the anterior corneal hyperdensity and the tangent at the posterior coroid-scleral hyperdensity, close to the head (centro-ocular perpendicular to the PEBC) (m = 24.19 mm, σ = 1.03). The transversal diameter of the optic nerve (DON) is measured at the mid-section of its intra-orbital segment (m = 3.5 mm, σ = 0.5). The transversal Diameter of the Right Internal Muscle (DRIM) measures the maximum interval separating its medial and lateral sides. The Cantho-Bicanthal Distance (CBCD) measures the interval separating the cutaneous surface of the internal canthus, at the front, from the external bicanthal plane at the back (measurement of the thickness of soft areas). The Apex Temporal Distance (ATD) measures the interval separating the tangency points of the Anterior Temporal Plane (ATP) with the temporal cavities. The External Bicanthal Plane – Temporal Apex (BPTA) measures the interval between the External Bicanthal Plane (EBCP) and the Anterior Temporal Plane (ATP). The establishment of biometric indices according to H.V. Valois (the shortest distance related to the longest multiplied by one hundred) establishes classifications around the average and variance limits at 2 σ. Thus, it may be recalled that Retzius‘ horizontal cranial index offers segmentation between the "mesocephalic skull", the "doichocephalic skull" and the "brachycephalic skull". The first index established is still the most important because it is in everyday, systematic usage. This is the Ocular- Orbital Index (OOI) or exophtalmometry index, which relates the AEBC segment to the MAL (m = 65.44, i.e. 65% of the length of the globe, in adults, projecting out of the PEBC (Fig. 8).

Fig. 8: Exophtalmometry by axial section (MRI or X-Ray scan), anterior visual pathways, from crystalline lens – optic canal.

The figure of 68% in one of the first series corresponded to an error of including patients with ametropia. The Neuro-Ocular Index (NOI) relates the diameter of the intra-orbital optic nerve at its mid section to that of the ocular globe (m = 14.8 mm, σ = 0.74) [6]. The histogram in figure 9 isolates the significant difference of the 2 populations, with and without papilledemae [7]. The External Bicanthal Ocular Index (EBCOI) relates the External Ante-Bicanthal segment to the External Retro- Bicanthal segment (m = 1.91). The Inter-Ocular Distance Index (IODI) relates the Inter-Ocular Distance (IOD) to the External Bicanthal Distance (EBCD) (m = 65.35). The Inter-Pupil Distance would therefore appear to correspond, on average, to two thirds of the External Inter-Canthal Distance. The Teleorbitism Index (TOI) relates the Maximum Inter-Plane Distance (MIPD) to the External Bicanthal Distance (EBCD) (m = 29.42). Synthesis work in ocular-orbital biometry [8] relates the thousands of measurements, tables and numerous inter-correlations of the characters seen earlier. Only some of these are related here. The right/left symmetry of measurements, which presents a high correlation coefficient, a reflection of binocular vision (for MAL R/L r = 0.9512, for EABC R/L, r = 0.9619). Orbital Depth (Depth R/L, r = 0.9489), will be looked at later. The position of the ocular globe explains the high index correlation (for IOD/EBCD, r = 0.8753, for IOD/MIPD, r = 0.7572, for IOD/MIPD, r = 0.7805). These are transversal indices. In the sagittal plane a negative correlation is observed between the Ante-Bicanthal segment of the ocular globe and the Orbital Depth (r = -0.5027). The Orbital Depth related to its aperture angle shows high correlation (r = 0.6110). The nature (matching, anatomical closeness…) of significant correlations, like their multi-factor analysis completes the statistical work referred to earlier [8]. Correlation with Hertel‘s exophtalmometry is established [9].
3.2. Maxillofacial biometry in the NOP, by X-ray scan and embedded ocular facial contouring [10]. The quality of the previous statistical correlations resulted in a request to use NOP references for the acquisition of facial contouring by X-Ray scanner as from 1980. This contouring produces a large scale ocular globe, a key factor in an ocular stimulator-recorder used on board the space shuttle (Space Lab European Research, 1983). The practical creation of the equipment was entirely satisfactory. A horizontal dento-maxillofacial biometric application for the X-Ray scan in the NOP was quickly sought [11]. A population of 76 patients was therefore studied, presumed healthy for the anatomical region under consideration, and aged between 19 and 82 years, with an average cephalic index = 78 (74/84). 7 measurements were established, 4 linear and 3 angular, on the cranial base. The Inter- Pterygoid distance (IPD) measures the gap between the anterior extremity of the 2 pterygoid apophyses (m = 36 mm (31/48). The Inter-Styloid Distance measures the gap between the base of the 2 styloid apophyses (m = 76 mm (89/63). The Inter-Condylar Distance (ICD) measures the gap between the central point of the two mandibular condyles on the temporal articular facet (m = 103 mm (93/116)). The Extreme Inter- Zygomatic Distance (EIZD) measures the longest transversal zygomatic diameter (m = 117 mm (110/120)). 3 angle measurements complete the series. The Sagittal Plane Condyle Angle (SPCA) measures the orientation of the condyle on the median sagittal plane (m = 63°5’ (R), 66°8’ (L)). The Sagittal Plane – Ramus of the Mandible Angle (SPRMA) measures the orientation of the mandible ramus angle (m = 14°5’ (R), 12° (L)). The Angle of the Posterior Wall of the Maxillary Sinus (APWMS) measures the orientation of the posterior-external wall of the maxillary sinus on the sagittal plane (m = 38°9’ (R), 43°3’ (L)). 3.3. Exophthalmometry and dysthyroid ophthalmopathy: from I-III grading to the De Saint-Yves syndrome [12]. Dysthyroid ophthalmopathy was the first practical field of application of ophthalmometry in the NOP (Fig.10). In 1978, it was shown that the cephalic fixedness of the visual pathways plane enables quantification of ocular-orbital topographical normality in adults. The Ocular-Orbital Index (OOI) is used to establish 4 topometric classes.

Fig. 9.

Beyond normality (60 < IOO < 70), a grade I axile exophthalmia is confirmed in the value: 70< IOO < 100. Grade II is defined by OOI = 100, that is to say the tangency of the posterior pole on the External Bicanthal Plane (EBCP) and grade III by a value of IOO > 100, that is to say by the projection of the posterior pole of the globe out of the External Bicanthal Plane. This is therefore, strictly speaking, an "exorbitism". Figure 10 reminds us that, although exophthalmia can be stated "absolutely" (increase in the value of the OOI), in one of the 2 eyes, and in a "relative" way from one eye to the other (difference of the OOI and millimetre difference in the EABC segment), the inversion of the OOI index in newborns and the very old must be remembered (maximum enophthalmia with OOI of 30 %). An ocular dystopia moves the horizontal ocular meridian of the NOP vertically. This situation does not prevent recognition of the plane itself, with the approximation becoming firstly clinical-cutaneous (lateral markers) and then anatomical on the X-Ray or MRI scan image. The symmetry of the external orbital pillars, optic canals and lateral masses of the ethmoid enable recognition of the visual plane. Shifting of the globe is then easily measured on the succession of section planes. For the past 30 years (1983), MRI has undertaken vertical and oblique exophthalmometry, that of the OTNOP (Fig. 11).

Fig. 10: Grades of exophthalmia (dysthyroid ophthalmopathy).

Fig. 11: Clinical application of the OTNOP: the direct view of the 4 segments of the optic nerve (intra-ocular, intra-orbital, intra-canal and intra-cranial intracisternal), offers varied semiological diagrams, showing the diameter and nerve signal: atrophia, SEP plaque, vascular accident, intrinsic and extrinsic tumour pathology, dilation of spaces by HIC.

Results quantified as normal and variants are the object of research work (unfortunately now halted) that was to give an answer, by the vertical plane of the MRI, to ocular-orbit biometry in case of vertical movement of the globe (process occupying the space adjacent to a horizontal wall or a malformation syndrome, for example). Evolutive monitoring under medical treatment or after surgery requires precise biometry in a strict NOP only. Whence the obligation of using MRI for therapeutic monitoring, the repetition of the examination, in circumstances of cephalic tilting and acquisition parameters permitting anatomical comparisons. It is necessary to carry out an initial pre-therapeutic examination to act as an undisputed anatomical reference, which will become a medico-legal obligation. This truth of ophthalmometry by X-Ray or MRI scan represents the fulfilment of the following observations: reality and fixedness of the NOP, reality, fixedness and symmetry of ocular-orbit biometry in normal adults (conditions of emmetropia and binocular vision). Between 1980 and 1982, 432 observations of dysthyroid ophthalmopathy (amongst 11 000 measured by X-Ray scan) were brought together after the publication of a preliminary series of 60 cases [12]. In collaboration with N. Newman, B. Illic, T. Laroche and S. Liotet, various series permitted the definite validation of exophthalmometry and a better knowledge of the mechanisms of endocrine ophthalmopathy. It was biometric and anatomic comparisons in patients followed and treated for Basedow‘s disease and in patients consulting primarily for isolated exophthalmia or inaugural oculomotor disorder that resulted in further knowledge. The name of "De Saint Yves syndrome" was suggested in view of the anatomic and biometric observation of axial, unilateral or bilateral exophthalmia, still unrecognised initially and clinically, before any biological verification. It is still awaiting a nosology framework, based on the biometric observations made by X-Ray or MRI scan. It is an isolated exophthalmia, often barely visible clinically (1 to 2mm), with a normal muscular volume and increased volume of the intra- and extraconic fatty compartments. Mr de Saint-Yves, the first ophthalmologist surgeon, describes in his treaty published in 1773, i.e. 67 years before Basedow and 64 years before Graves, the existence of a fatty issue when an inferior palpebral incision is made in an exophthalic and tachycardic patient. This was the object of a presentation made to the National Academy of Medicine, for which a prize was awarded [12]. It will be remembered that physiological enophthalmia observed at the extreme ages of life is caused by the low relative volume of intraorbital, retrobulbar intraconic and extraconic fatty compartments. The close hormonal dependency of the intra-orbital lipocyte explains the frequency of dysthyroid exophthalmy, as well as the first application of quantitative orbital-ocular biometry by X-Ray scan in exophthalmometry. This biometric data is used by E. Modigliani in MRI, with correlation of endocrinological therapeutic monitoring.
4. Ocular-orbital growth, strabology: orbit angles and depths by X-Ray scan Our ophthalmologist colleagues have shown that a significant ocular-orbital biometric difference can be explained by the occurrence of an acquired organic unilateral amblyopia (traumatic cataract), with converging strabism before puberty and divergent afterwards. Details of the results of this work are not included here. [1]. The growth of the normal globe is shown by ultrasound scan, as indicated in the book referred to [1]. This anterior-posterior axial measurement of the ocular globe, in utero, reproduces the exponential shape of the growth graphs of the foetus exactly, from the age of 3 months to birth and then from birth up to the age of 9 years [1]. Today these direct linear measurements are accessible by MRI of the foetus in utero with high anatomic resolution enabling on its own the recognition of the existence of a congenital malformation syndrome. The FO report by H. Mondon and P. Metge already mentioned [9] provides a table of the average of the linear, angular and index measurements in myopia. Observation of a dominant posterior expansion of the ocular globe is the main result of the study. Measurements of the orbital volume by X-Ray scan, from living to fossil skull, provide useful data on the growth of the orbital volume, from birth to the age of 20, of a factor 4 approximately, along with a constancy in orbital volume in recent paleanthropians ("Ferrassie I", "Cro-Magnon", "La Chapelle aux Saints I"). Dynamic muscular biometry (IRMOD) in NOP and OTNOP, in adults, is revealed in muscular and angular detail, with calculation of the globe‘s centre of rotation, in the aforementioned book. Reference must also be made to the work done by A. Roth and C. Speeg-Schatz in oculomotor surgery and strabology [15].
5. Direct recognition of an optical neuropathy (tumour, vascular accident, genetic congenital atrophy of the optic nerve, …), either directly or by intracranial hypertension, is another major application of this work. In the 3 planes which they themselves therefore define, the 2 intra-orbital segments of the optic nerves become the "key" to encephalic exploration, to its inflammatory effects (S.E.P.), tumours, and degenerative effects (glaucoma and rarefaction of neuro-optical neuro-tractography.
6. Biometry of intracranial and encephalic visual pathways, sectional and vascular descriptive anatomy by X-Ray scan and MRI, anatomy of development (embryology) and of growth, velocimetric, then molecular and genetic circulatory anatomy are the chapter headings that conclude the study of these twelve anatomies. The density of illustrations, from sectional anatomy to 3D anatomy, explains why it is not possible to report on this material here, nor even to condense it within the space available. The reader is invited to refer to the work mentioned on numerous occasions [1], another document is in the course of preparation.



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Refer this article as: Cabanis, E., The Neuro Ocular Plane (NOP), Points de Vue, International Review of Ophthalmic Optics, N69, Autumn 2013

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