In 1951, Sunderland expanded [[Seddon's classification]] to five degrees of peripheral nerve injury:

First-degree (Class I)
Seddon's neurapraxia and first-degree are the same.

Second-degree (Class II)
Seddon's axonotmesis and second-degree are the same.

Third-degree (Class II)
Sunderland's third-degree is a nerve fiber interruption. In third-degree injury, there is a lesion of the endoneurium, but the epineurium and perineurium remain intact. Recovery from a third-degree injury is possible, but surgical intervention may be required.

Fourth-degree (Class II)
In fourth-degree injury, only the epineurium remain intact. In this case, surgical repair is required.

Fifth-degree (Class III)
Fifth-degree lesion is a complete transection of the nerve. Recovery is not possible without an appropriate surgical treatment.