The following study incorporates of the use of choke collars and the standard (flat) collars:

A Pauli et al. (2006) study found that compression of the jugular veins caused by a force applied to a collar around a dog’s neck, increases pressure in the veins of the head, including the episcleral veins of the eye which results in an increase in intraocular pressure. Intraocular pressure is the leading cause of glaucoma for dogs and humans, which can permanently damaged vision and cause blindness.

Below is a breed comparison of the intraocular pressure dogs experienced when they were subjected to a force applied their collars.

Am Staff=American Staffordshire terrier

As you can see, Siberian huskies can be subject to a large amount of force to their collars with very little increase in intraocular pressure. However, Cocker Spaniels experience a significant increase in in intraocular pressure when a lesser force is applied to their collars.

Interestingly, some of the dogs bred to pull objects appeared to be somewhat resistant to an increases in intraocular pressure via an applied force to a collar:

A Siberian husky: a working dog breed, bred to pull sleids. Little intraocular pressure effect from a force applied to their collars

An American Cocker Spaniel: a breed of sporting dog. Very susceptible to increasing intraocular pressure from a force onto their collars

The following is the same experiment except harnesses were used instead of collars:

An example of a harness. What differences do you expect to see in the experiment using a harness instead of a collar? Hint: The force from a lead is dispersed across the chest of a dog wearing a harness insead of the jugular veins

Am Staff=American Staffordshire terrier

As you can see the data from the harness is very different to the data generated from the use of collars. This study concluded that dogs with weak or thin corneas, glaucoma, or any condition for which an increase in intraocular pressure could be harmful should wear a harness instead of a collar.