MENINGES

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The spinal cord and spinal nerve roots are surrounded by three meninges[1]: one pachymeninx[2] (dura mater) and two leptomeninges[3] (arachnoid and pia mater). They are continuous with the meninges of the encephalon through the foramen magnum. At the level of the spinal nerves, the duramater becomes the epineurum, and the leptomeninges the perineurum and endoneurum.  

 

The spinal dura mater is the outermost meninx. It is made up of two layers: the outer layer (or periosteal dura mater) and the inner layer. The two layers fuse at the level of the intervertebral foramen, foramen magnum, and along the floor of the vertebral canal within C1 and C2 vertebrae. The caudal prolongation of the dura mater attaches to the first caudal vertebrae and forms the spinal dura mater filament.

The epidural ligaments or meningovertebral ligaments anchor the inner layer of the dura to the outer layer or periosteal layer. In dogs, the ventral epidural (meningovertebral) ligament extends throughout the length of the vertebral canal[4]. In the cervical region, the dura is attached to the ligamenta flavum with the posterior cervical epidural ligaments or dorsal meningovertebral ligaments[5]. However, as the two layers of the dura mater are attached one another at the floor of the vertebral canal at C1 and C2, and the ventral vertebral venosus plexuses are big at the cervical region, the ventral epidural ligament is more difficult to distinguish. These ligaments prevent damaging the spinal cord when the column is flexed by keeping the spinal cord and meninges in place. The cervical spine lengthens during neck flexion. However the inner layer of the dura mater, attached to the outer layer (periosteal layer) and the ligamenta flava, migrates within the spinal canal. When a disc protrusion is present, the mobility of the dura may be impeded increasing the tension of the dura resulting  in pain because the dura in sensory innervated.

At the intervertebral foramina there are also intraforaminal ligaments that connect the periosteum and the transforaminal ligaments to the nerve root and vessels to protect the nerve roots mechanically when stretching. These ligaments surround the root radially. The foraminotomy affects the

ligamentous structures and may be responsible for postoperative pain[6].

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The space between the outer layer or the periosteal dura mater and the inner layer of the dura mater is called epidural space. It contains he epidural fat, the interarcuate vein, the dorsal longitudinal ligament[7] and the intercapital ligament[8] and the dorsal and ventral internal vertebral venous plexuses. The dorsal internal vertebral venous plexus is very thin and easily to find at the cervical region. The ventral internal vertebral venous plexus is formed by two large veins which lie in the dorsal surface of the vertebral bodies. In the cranial cavity the outer and the inner layers of the dura mater are firmly adhered one another leaving no epidural space. They only detach one another to locate the venous sinuses and arteries. When removing the encephalon, the outer layer remains attached to the bones but the inner layer comes out easily. These two layers remain attached at the foramen magnum and at the floor of the vertebral canal in the atlas and axis.

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The arachnoid is located between the dura and the pia mater. It is in close contact with the dura mater because the pressure of the cerebrospinal fluid, that occupies the subarachnoid space, pushes the arachnoid outwards.

 

The dura mater and the arachnoid form a single sac that contains the spinal cord and the proximal portions of the spinal roots. The distal portions of the spinal roots, including the spinal ganglion, are surrounded by independent sleeves of dura and arachnoid. 

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The pia mater surrounds the spinal cord, the roots of the spinal nerves and the initial portion of the terminal filament. The pia mater is continuous with the arachnoid at the level of the terminal filament and, by strands of collagen fibers, across the whole subarachnoid space.

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The septum posticum is a condensation trabeculae between the arachnoid and the pia mater on the dorsal middle plane. It has been described in humans extending totally or partially along the length of the spinal cord. However it is not mentioned in classical veterinary neuroanatomy books concerning dogs and cats.

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Some mammals develop only few arachnoid trabeculae others develop closely packed, fused bands, strengthened with fibrous tissue especially in the dorsal midline[9].

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It has been proposed that derangements or condensations of these leptomeningeal trabeculae of the so called septum posticum may play a role in the development of subarachnoid cysts[10],[11] and be considered in differential thoracic myelopathies[12],[13],[14].

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The denticulate ligament is a bilateral thickening along the pia mater, from spinal segment C1 up to spinal segment L5 or L6. In 25% of dogs, it ends between segments L7 and L6 (Page 824 of "Miller's anatomy of the dog " by Evans, H. E. 3rd ed.). The denticulate ligament has lateral extensions, located between the spinal roots, which connect with the dura mater through the arachnoid. These help to maintain the spinal cord inside the sac formed by the dura mater and the arachnoid.

 

Innervation of the spinal meninges

 

Before entering the intervertebral foramen, the spinal nerves give off a meningeal branch. They innervate the meninges, and the facet joints, the anulus fibrosus and the intervertebral disk.

 

 

[1] From the Greek meninx, "membrane".

[2] Thick meninge.

[3] Thin meninges.

[4] Kent et al. Anatomic description of clinical relevance of the meningovertebral ligament in dogs. JAVMA 687-694, Vol 255, 6, 2019

[5] In chapter 3, The vertebral column and spinal meninges by Gulgun Kayalioglu, in The Spinal Cord, 2008.

[6] Akdemir, G. J. Thoracic and lumbar intraforaminal ligaments. Neurosurg. Spine. 13:351-355, 2010.

[7] The dorsal longitudinal ligament runs along the floor of the vertebral canal from the dens of the axis to the caudal vertebrae.

[8] In between the ribs extending from the level of the second pair up to the tenth.

 

[9] CASE REPORT Recurrent spinal arachnoid cyst in a cat Schmidt, M.C., Schachenmayr, W., Thiel, C., Kramer, M. Journal of Feline Medicine and Surgery (2007) 9, 509-513

[10] Brauss, H. Anatomie des Menschen, Centrales Nerven- system. Berlin: Springer Verlag (1932). Klika, E. The ultrastructure of the meninges in vertebrates. Acta Universitas Carolina Medicinae 13, 53-71 (1967).

[11] Nickel, R., Schummer, A., Seiferle, E. Lehrbuch der Anatomie der Haustiere. Nervensystem, vol. IV, Sinnesorgane, Dru ̈sen: Parey Verlag (1992)

[12] The Neuroimaging Spectrum of Septum Posticum Derangement and Associated Thoracic Myelopathy. Hakky, M.M., Justaniah, AI., David, C., French, R..J, Martin, D., Kwok, N., Erbay, S.H. J Neuroimaging. 2015 Sep­Oct;25(5):818­23

[13] Perret, G., Green, D., Keller, J. (1962). Diagnosis and treatment of intradural arachnoid cysts of the thoracic spine. Radiology 79, 425-429

[14] Lake, P.A., Minckler, J., Scanlan, R.L. (1974) Spinal epidural cysts: theories of pathogenesis. Journal of Neurosurgery 40, 774-778.