Thoracic disc prolapse is rare an event due to the bony thoracic cage permitting limited movements, the anterior-posterior direction of the apophyseal joints, and relatively small size of thoracic disc. The first description can be traced back to 1838 by key et al and the incidence was described ass 0.04 % of disc prolapses. Incidence of clinically significant disc is1/year/10 lakh populations. It is found more common in people carrying loads in the back.  There is no sex predilection.  Both occupation and trauma appear to be related only incidentally in most cases, although trauma sometimes seems to be an important precipitating and aggravating factor.

The common site is in lower third of thoracic level, the most common being at T11 level.  Multiple level protrusions are even rarer. Only a couple of sequestrated thoracic disc is reported. 

Pathophysiology:

The thoracic canal is small with little leeway between the thoracic disc and spinal cord.

The thoracic cord is restrained from backward displacement by the dentate ligaments.

Circulation to the lower thoracic region is precarious and largely due to single artery of Adamkiewicz which usually arises between T8 and L4 on the left side in 60% of cases.

The combination of mechanical and vascular damage may account for the severe neurological deficit and poor post operative recovery.

Vascular damage may account for a higher clinical level that seem inappropriate for the level of radiological lesion.

Clinical features:

The history is usually vague misleading. The common symptoms are axial pain (77%) radicular pain (64%), signs of cord symptoms (59%) and sensory loss (36%);myelopathy are more with central herniation. 

The pain may be unilateral or bilateral and generally mild or moderate. Patients with bilateral pain tend to progress rapidly towards a transverse myelitis. Pain may be absent.

Subjective sensory changes in association with minimal motor deficit are highly suggestive. Sensory symptoms may be segmental, unilateral, or bilateral. It usually begins peripherally and ascends gradually. Decreased pain and temperature as well as hyperesthesia and paresthesia are common.

Bladder disturbances and impotence, trohic ulcers are late features. 

History of spontaneous remission as exacerbation of symptoms is not as common as with lumbar and cervical disc prolapses. The majority give a history of many years duration.

Upper thoracic disc prolapses may present with brachialgia or horner’s syndrome.

Mild scolosis or kyphosis may be present. 

Investigations: Plain x-ray may show reduced disc space with bony spurs. Prolapsed disc may be calcified and seen in plain x-ray.  CT and CT myelography have become obsolete now. MRI has revolutionized the management of thoracic disc prolapse. Accuracy of MRI is more than 95%, depicting the extent, relationship to cord & root and differentiating from tumors ossified posterior longitudinal ligament and calcified ligamentum flavum. Absence of CSF both anterior and posterior to the cord indicates mechanical significant compression. Thus MRI is an
excellent screening and diagnostic investigation of choice.	Thoracic disc--MRI	Thoracic disc-CT

Treatment: 

Asymptomatic or incidental thoracic disc does not require treatment except for a regular follow up.

Radiological evidence of cord and / or root compression with corresponding clinical features mandates early surgical decompression.

Routine laminectomy has been associated with great risk.

Many approaches have been described, depending on the level of heniation, laterality of  disc prolapse, and single or multiple levels of disc prolapses and most of all, surgeon's familiarity. The latest addition is an endoscopic approach. The goals of these approaches being visualization of the herniated disc without retraction of the already deformed cord as it can not tolerate any additional deformation.

Thoracic disc surgery is described as the most devastating of all the disc surgeries. The preoperative diagnosis and levels must be accurate in lateral & anterolateral approaches. Thoracic canal being narrow, thoracic cord is very sensitive to cord compression & very susceptible to vascular compromise. Drilling helps in decompression without introducing instruments in the tight canal.  Dissection should be carried out under good visualization with a microscope.

Costotransversectomy (lateral extrapleural approach):

A right sided approach is preferred to avoid artery of Adamkiewicz unless there are  lateralizing features. The patient in the partial decubitous position with a 30-degree elevation, a long curved paraspinal incision is made. The muscles are retracted and the rib to be removed is identified. The intercostal neurovascular complex is separated from its inferior. The head and neck of the rib along with a part of the shaft is removed and the intercostal vein and arteries are followed to the nerve root foramen. The parietal pleura is separated from the adjacent ribs  and spines. Parts of the pedicles are removed with a drill and the dura and disc are identified and the disc is removed. Prior to closure, the lungs are inflated. Chest tube may not be necessary.

This approach gives a better access to the spinal canal than the following transthoracic approach which is more popular.

Transthoracic approach:

It is the most popular approach.

A standard right posterior thoracotomy is made.                                                                                                                  The corresponding rib is either removed or retracted and neurovascular bundle is followed to the intervertebral foramen.      The pleura is reflected; the pedicles are removed; and the disc is removed as in costotransversectomy approach. The parietal pleura is sutured over the vertebral body and chest tubes are placed. Prior to closure, the lungs are inflated.

Central disc herniations between T-2 and T-5 may require an anterior trans-sternal approach, the lower extent of this approach is limited by the aortic arch.

Posterolateral approach:

A laminectomy is performed with a high speed drill. A portion of the lateral wall of the spinal canal is drilled away, if necessary through an horizontal skin incision at the level of the disc. The aim is  to get a lateral approach to the disc which is removed.         

Transpedicle approach:                  

Through a midline incision, the paravertebral muscles are retracted far enough to expose the facet joints. The facets and the pedicle of the vertebra caudal to the disc are removed. The interspace is entered, and the disc is removed. If necessary, laminectomy is performed after disc excision.

It is a simple procedure and the results are encouraging. 

Endoscopic discectomy is being employed in certain centers; discectomies, corpectomies, and instrumented fusions have been performed thoracoscopically.

Surgical Results & Complications

Best results are obtained in patients with only radicular  pain with or without mild signs of myelopathy.

Severe preoperative deficit long duration of symptoms carries a poor prgnosis.

Various reports suggest satisfactory pain relief in 79%; improved myelopathy in 71 to 97%; improvement in sphincter functions in 60%.