Exhibit 4.18

SUPPLEMENTAL INDENTURE

(Merger #1)

SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of December 1, 2016, among NEW AMETHYST CORP., a Delaware corporation (the “Successor Issuer”), the subsidiaries listed on the signature pages hereto (each a “Subsidiary Guarantor”), and U.S. BANK NATIONAL ASSOCIATION, a national banking association organized under the laws of the United States of America, as trustee (the “Trustee”).

W I T N E S S E T H

WHEREAS, AmSurg Corp. (the “Predecessor Issuer”), the Subsidiary Guarantors and the Trustee have heretofore entered into that certain Indenture dated as of July 16, 2014, as heretofore amended and supplemented prior to the date hereof (the “Original Indenture”), governing the Predecessor Company’s 5.625% Senior Notes due 2022 (the “Notes”); and

WHEREAS, on the date of this Supplemental Indenture, the Predecessor Issuer has merged with and into the Successor Issuer, with the Successor Issuer as the surviving entity (the “Merger”); and

WHEREAS, pursuant to Section 5.01(a) of the Original Indenture, the Successor Issuer desires to enter into this Supplemental Indenture expressly assuming all of the Predecessor Issuer’s obligations under the Notes, the Original Indenture and the Registration Rights Agreement; and

WHEREAS, pursuant to Section 5.01(a)(iv) of the Original Indenture, each Subsidiary Guarantor desires to confirm in writing that its Subsidiary Guarantee shall apply to the Successor Issuer’s obligations under the Notes, the Indenture and the Registration Rights Agreement; and

WHEREAS, this Supplemental Indenture is permitted under Section 8.01(a)(ii) of the Original Indenture, without the consent of the holders of the Notes.

NOW, THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt of which is hereby acknowledged, the Successor Issuer, the Subsidiary Guarantors and the Trustee mutually covenant and agree for the equal and ratable benefit of the Holders of the Notes as follows:

1. CAPITALIZED TERMS. Capitalized terms used herein without definition shall have the meanings assigned to them in the Original Indenture.

2. ASSUMPTION OF NOTES AND ORIGINAL INDENTURE. From and after the effective time of the Merger, the Successor Issuer hereby expressly assumes all of the Predecessor Issuer’s obligations under the Notes, the Original Indenture and the Registration Rights Agreement. (The Original Indenture, as amended and supplemented by this Supplemental Indenture, is hereinafter referred to as the “Indenture.”)

3. CONFIRMATION OF NOTE GUARANTEES. Each Subsidiary Guarantor, by its execution of this Supplemental Indenture, hereby confirms that its Note Guarantee shall apply to the obligations of the Successor Issuer in accordance with the Notes, the Indenture and the Registration Rights Agreement.

4. NO OTHER CHANGES. Except as amended and supplemented by this Supplemental Indenture, the Original Indenture shall continue in full force and effect.

5. NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE BUT WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED THEREBY.

6. COUNTERPARTS. The parties may sign any number of copies of this Supplemental Indenture. Each signed copy shall be an original, but all of them together represent the same agreement.

7. EFFECT OF HEADINGS. The Section headings herein are for convenience only and shall not affect the construction hereof.

8. THE TRUSTEE. The Trustee shall not be responsible in any manner whatsoever for or in respect of the validity or sufficiency of this Supplemental Indenture or for or in respect of the recitals contained herein, all of which recitals are made solely by the Successor Issuer and the Subsidiary Guarantors.

(signature pages follow)

2

IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly executed and attested, all as of the date first above written.

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued

Signatures - Continued