diff --git a/index.js b/index.js
index 5aef4af..f4679d0 100755
--- a/index.js
+++ b/index.js
@@ -14,22 +14,27 @@ const creator = opts => {
 	const exportTo = [].concat(Object(opts).exportTo || []);
 
 	// promise any custom media are imported
-	const customMediaPromise = getCustomMediaFromImports(importFrom);
+	const customMediaImportsPromise = getCustomMediaFromImports(importFrom);
 
 	return {
 		postcssPlugin: 'postcss-custom-media',
-		Once: async root => {
-			const customMedia = Object.assign(
-				await customMediaPromise,
+		Once: async (root, helpers) => {
+
+			// combine rules from root and from imports
+			helpers.customMedia = Object.assign(
+				await customMediaImportsPromise,
 				getCustomMediaFromRoot(root, { preserve })
 			);
 
-			await writeCustomMediaToExports(customMedia, exportTo);
-
-			transformAtrules(root, customMedia, { preserve });
+			await writeCustomMediaToExports(helpers.customMedia, exportTo);
+		},
+		AtRule: {
+			media: (atrule, helpers) => {
+				transformAtrules(atrule, {preserve}, helpers)
+			}
 		}
 	}
-};
+}
 
 creator.postcss = true
 
diff --git a/lib/transform-atrules.js b/lib/transform-atrules.js
index b857588..aab3b92 100644
--- a/lib/transform-atrules.js
+++ b/lib/transform-atrules.js
@@ -1,21 +1,29 @@
-import transformMediaList from './transform-media-list';
-import mediaASTFromString from './media-ast-from-string';
+import transformMediaList from "./transform-media-list";
+import mediaASTFromString from "./media-ast-from-string";
 
 // transform custom pseudo selectors with custom selectors
-export default (root, customMedia, opts) => {
-	root.walkAtRules(mediaAtRuleRegExp, atrule => {
-		if (customPseudoRegExp.test(atrule.params)) {
+export default (atrule, { preserve }, { customMedia }) => {
+	if (customPseudoRegExp.test(atrule.params)) {
+		// prevent infinite loops when using 'preserve' option
+		if (!atrule[visitedFlag]) {
+			atrule[visitedFlag] = true;
+
 			const mediaAST = mediaASTFromString(atrule.params);
 			const params = String(transformMediaList(mediaAST, customMedia));
 
-			if (opts.preserve) {
-				atrule.cloneBefore({ params });
-			} else {
+			if (preserve) {
+				// keep an original copy
+				const node = atrule.cloneAfter();
+				node[visitedFlag] = true;
+			}
+			// replace the variable with the params from @custom-media rule
+			// skip if the variable is undefined
+			if (params != null) {
 				atrule.params = params;
 			}
 		}
-	});
+	}
 };
 
-const mediaAtRuleRegExp = /^media$/i;
+const visitedFlag = Symbol("customMediaVisited");
 const customPseudoRegExp = /\(--[A-z][\w-]*\)/;