Spinal AVMs are localized collections of blood vessels, often abnormal in structure and number, representing an anomaly of the spinal circulatory system with altered haemodynamics. 

Epidemiology: 

They are rare with an incidence of 3.3 to11% of spinal SOLs with a male preponderance.

They usually present in the 4^th or 5^th decade. Usually they extend over 4 or 5 segments and as a rule they are located posterior or posterolateral in the caudal spinal canal.

Dural AVMs have preference for the thoracic and thoracolumbar areas. 

Classification and pathophysiology: 

There are two main types: 

A) Dural AVMs: 

The nidus is embedded in the dural sheath of the nerve root.

They do not interfere with the blood supply of the spinal cord and hence, believed to be acquired anomalies. They are low flow shunts in the proximal dura of the nerve root, the adjacent spinal dura or both.

It is supplied by a dural artery and empties into the coronal venous plexus on the surface of the spinal cord leading to venous hypertension and serpentine transformation of the coronal venous plexus.

As the medullary arterial supply is different arterial steal is uncommon. Spinal cord ischaemia, cell loss and cord atrophy are due to impaired arterial perfusion pressure as a result of venous hypertension.

They are predominantly found in the posterior part of the lower thoracic cord and the conus. 

B) Intradural AVMs: 

Here the nidus is within the piamater or the spinal cord. 

The current concept is, they are the result of maldevelpment during the second stage of vascular formation (around the 6^th week of gestation), leading to persistence of thin walled tortuous vessels with defective tunica media and elastica, primitive capillary and precapillary channels and abnormal arteriovenous shunts. They are frequently associated with other congenital anomalies. 

They are further sub-classified into

                                     a) Intramedullary juvenile, and glomus types. 

                                     b) Perimedullary (direct) AV fistulas

                                     c) Cavernous angiomas (intramedullary & extramedullary) 

The juvenile type has a large intra medullary nidus and contains cord tissue within its interstices. They often occupy the entire cord at the involved level. Multiple medullary branches of the anterior and posterior spinal arteries supply them. They are high flow lesions and often a bruit may be heard at the level of the lesion.

They occur in adolescents and the young and have a poor prognosis. 

The glomus type is a tightly packed mass of blood vessels, supplied by branches of the anterior spinal artery. They are typically located in the anterior half of the cord and are more common in the cervical region. Clinical presentation may be similar to an intra medullary SOL.

They occur equally in both sex and become symptomatic in younger patients. 

The perimedullary AV fistulas are direct communications between spinal arteries and coronal venous plexus. They are less common and found on the surface of the cord with no intramedullary nidus. The flow is rapid with associated venous varices and arterial aneurysms. Arterial steal and resultant cord ischaemia is common. SAH is also a possibility. Usually they are located near the conus and become symptomatic between the 3^rd and 6^th decades.

They are further sub-divided into:

Type 1 – They are simple fistulas with by a single feeder, often by the artery of Adamkiewicz.

Type 2 – They are of intermediate size with more than one feeder, although one major feeder typically originates from the anterior spinal artery. The draining veins are dilated with venous ectasia at the site of the shunt.

Type 3 – They are giant and multipediculated fistulas. The predominant feeder is from the anterior spinal artery and the draining veins are greatly dilated and tortuous. Surgical excision is usually not feasible. 

Cavernomas occur in the vertebrae (commonest), in the extradural space or within the cord substance and represent 5 to 12% of all spinal AVMs. Myelopathy is due to small haemorrhages and cord compression. 

Clinical features: 

Dural fistulas present with slowly progressive myelopathy, usually involving the lower limbs as the lesion is more commonly found in the thoracolumbar region. They are the commonest and usually present in the 4^th or 5^th decades. 

Intradural AVMs frequently cause an apoplectic event with intramedullary haematomas and subarchnoid haemorrhage; upper limbs are also involved since the cervico thoracic cord is commonly involved; they are found typically in younger patients and uniformly distributed along the spinal cord. 

There are no recent studies on the natural history of the spinal AVM. 

The patients demonstrate fluctuation of symptoms against a background of steadily increasing disability; the majority become disabled within three years after the onset of symptoms.

Pain is common and often multiradicular and an increase of pain at nights and after hot bath has been reported; associated anomalies may give a clue. Pregnancy, menstruation, exercise and trauma are found to precipitate or aggravate the symptoms; their significance remains unclear.  Diagnosis:     Lately, MRI scanning and selective angiography are the investigations of choice. MRI scanning (T1) usually reveals a low signal intensity in the cord; rapid flow may produce a signal or flow void. T2 weighted images may show high intensity in the cord due to cord swelling and may be useful in dural AVMs where T1 may be normal. Cavernomas are diagnosed as well defined low intensity areas with high intensity signals.

Selective angiography is a must in every case where active treatment is contemplated.

large Dural AVM-MRI

Dural AVM-angio	Perimedullary AVM-angio	Glomus type AVM-angio

In the dural AVMs the nidus may be visualized at the intervertebral foramen. Sluggish clearence of the contrast is a feature of these lesions.   

Myelography is still the choice in patients with negative MRI as it often happens in small AVMs and the dural AVMs. The accuracy with water-soluble contrast is about 90%. The abnormal mass of sinuous turgid vessels as a ‘bag of worms’ is identifiable. 

Management: 

The aim is to eliminate the transmission of the venous hypertension to the spinal cord and to suppress the arterial steal and the risk of haemorrhage.

Active intervention is delayed to permit lysis and absorption of the clot after an acute event.

The choice is between surgical excision and embolization  and depends on the type of the AVM. 

Surgical interruption of the dural AV fistula between the nidus and the coronal venous plexus is preferred. Stripping of the venous plexus may be harmful. The surgical procedure is simple and less risky than embolization, which may accidentally aggravate venous congestion. 

Dural AVM-before interruption at surgery	Dural AVM-after interruption at surgery

Total surgical excision is possible in glomus types especially in the cervical region because of adequate collateral supply. Embolization may be considered for the thoracic and lumbar lesions.

Surgery is not possible in the juvenile types and embolization may be helpful.

Surgery is indicated perimedullary Type 1 fistula and selected Type 2 fistulas. Embolization is difficult. Type 3 has poor prognosis and embolization may help. 

Presently, surgery is the only available option for cavernomas.  

Prognosis: 

The optimal time for treatment is before the disability is substantial.

A short history (less than 3 months) implies good prognosis for reversal of the deficit.

If the history is longer, minimal or temporary improvement is often the result.

Long-term results of embolization are not yet known.